Auto Electronics Projects

The Maplin ser ies

This book is part of an exc i t ing s e r i e s deve loped by

Bu t t e rwor th -He inemann and Maplin E l e c t r o n i c s P i c .

Books in the se r ies are prac t ica l guides which offer e lec-

t ronic cons t ruc to r s and s tudents c lear in t roduct ions to

key top ics . Each book is written and compiled by a lead-

ing e l ec t ron ics author.

Other books published in the Maplin se r ies include:

Computer Interfacing

Logic Design

Music Projects

Starting Electronics

Audio IC Projects

Video and TV Projects

Test Gear & Measurement

Integrated Circuit Projects

Home Security Projects

The Maplin Approach

to Professional Audio

Graham Dixey 0 7506 2123 0

Mike Wharton 0 7506 2122 2

R A Penfold 0 7506 2119 2

Keith Brindley 0 7506 2053 6

Maplin 0 7506 2121 4

Maplin 0 7506 2297 0

Danny Stewart 0 7506 2601 1

Maplin 0 7506 2578 3

Maplin 0 7506 2603 8

T.A.Wilkinson 0 7506 2120 6

Auto

Electronics

Projects

U N E W N E S

Newnes

An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford 0X2 8DP

- ^ J j A member of the Reed Elsevier group

OXFORD LONDON BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON

© 1995 Maplin Electronics Pic.

All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applica-tions for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers.

The publisher, copyright holder and author have taken all reasonable care to prevent injury, loss or damage of any kind being caused by any matter published in this book. Save insofar as prohibited by English law, liability of every kind including negligence is disclaimed as regards any person in respect thereof.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 2296 2

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Edited by Co-publications, Loughborough

£ ^ Typeset and produced by Sylvester North, Sunderland

all part of The Sylvester Press ^

Printed in Great Britain by Clays L t d , St Ives pic

Preface

This book is a col lect ion of art ic les and projec t s previously

published in Electronics — The Maplin Magazine.

Each project is se lected for publication because of its special

features , b e c a u s e it is unusual, b e c a u s e it is e lectronical ly

c lever, or simply because we think readers will be interested

in it. Some of the devices used are fairly specific in function —

in other words , the circuit is designed and built for one pur-

pose alone. Others , on the other hand, are not specific at all,

and can be used in a number of applicat ions.

This is just one of the Maplin series of books published by

Newnes books covering all a s p e c t s of computing and e lectron-

ics. Others in the series are available from all good bookshops.

Maplin Elec tronics Pic supplies a wide range of e lec tronics

c o m p o n e n t s and o ther p r o d u c t s to private individuals and

trade cus tomers . Telephone: ( 0 1 7 0 2 ) 552911 or write to Maplin

E lec tron ics , PO Box 3, Rayleigh, Essex SS6 8LR, for further

details of product cata logue and locat ions of regional s tores .

VÜ

1 Car electrical systems

The modern motor veh ic le is a precision-buil t highly-

t u n e d m a c h i n e . High s p e e d p e r f o r m a n c e , low fuel

consumpt ion and quiet smooth-running engine all rely

on efficient ignition, ba t te ry charging and general e lec-

t r ical sys tems throughout the car .

The e lec t r ica l sys tem is very complex . One only has to

look behind a dashboard to s ee the hundreds of wires of

all s izes and co lours , in te rconnec t ing the ins t ruments ,

high vol tage and high current c i rcu i t s . Also, the e lect r i -

cal sys tem is very prone to breakdown, whether this is a

blown lamp bulb, a faulty dynamo or badly adjusted con-

tac t breaker points .

1

Auto electronics projects

No two models of cars have identical e lec t r ica l c i rcui t s .

The e lec t r ica l c i rcui ts are, however, similar and fall into

ca tegor i e s such as convent iona l ignition or e lec t r i ca l

ignition, dynamo or alternator, positive or negative earth.

This chap te r desc r ibes the bas ic sys tems: it is left to the

individual car owner to interpret the descr ip t ions and

diagrams to suit their par t icular vehic le .

One word of warning. Car e l e c t r i c c i rcu i t s can cause

damage to e i ther the car or to the user if tampered with.

For ins tance a shor t c i rcui t a c r o s s the ba t te ry can gen-

era te hundreds of amperes and a lot of heat, even a fire:

the ignition circui t genera tes very high vol tages indeed:

tampering with the instrument c i rcui t s , can cause mis-

leading readings and a poss ib l e safe ty hazard to the

dr iver . Befo re embark ing on any c h a n g e s to the ca r

e l ec t r i c s , make every effort to understand how the cir-

cuit works. In this way fault finding should be greatly

simplified.

The ignition circuit

The purpose of the ignition circui t (Figure 1.1) is to sup-

ply the high vol tage required to opera te the spark plugs

in the co r r ec t s equence and so ignite the air /petrol mix-

ture in each cylinder. The explos ions generated push the

pis tons and so turn the engine, causing motion. The cir-

cui t c o m p r i s e s the ca r ba t t e ry , an ignit ion co i l , the

dis t r ibutor and four (or s ix) spark plugs. The principle

of operat ion is descr ibed later .

2

Car electrical systems

Figure 1.1 The ignit ion c i rcu i t

Battery charging

All e lec t r ica l sys t ems draw their power from the 12 volt

ba t te ry (Figure 1.2). If the ba t te ry was not cont inual ly

charged it would b e c o m e exhaus ted very quickly, par-

t icular ly if the lights, wipers and s ta r te r motor were in

cons tan t use. The turning of the engine charges the bat-

tery by connec t ing it to a dynamo, via the fan bel t . A

3


pulley network at the front of the engine cons tant ly turns

the dynamo which genera tes enough power to charge up

the bat tery . A control box cont ro l s the charging rate and

informs the driver via the ignition light if the ba t te ry is

not charging. Some cars use an a l te rnator in preference

to a dynamo. T h e s e are more efficient but genera te a.c.

ra ther than d.c. and so require rect i f icat ion of the a.c.

output. Ba t te ry charging is desc r ibed later .

Figure 1.2 The battery charging c i rcu i t

4


Lighting

The lighting c i rcui t s are the s imples t of all these , com-

prising a simple connec t ion of the 12 volt lamp to the

ba t te ry via the instrument panel swi tches (Figure 1.3).

T h e s e c i rcu i t s are comple te ly independent of the igni-

tion and charging c i rcu i t s , the one connec t ion to each

lamp being taken via a single wire and respec t ive switch

to the bat tery; the o ther connec t ion uses the car chas -

s is . The lighting c i rcu i t s are desc r ibed in more detail

later .

Figure 1.3 The l ighting c i rcu i t

5


Figure 1 .4 The indicator and accessories c i rcu i t

6

Indicators and accessories

Contained within this circui t is the s ta r te r motor which

draws hundreds of amperes from the ba t te ry to turn the

engine until it fires (Figure 1.4). Heavy duty cab le and a

heavy duty solenoid car ry out this operat ion, which is

prone to t rouble for various reasons . Also there is the

fuel pump which is a small solenoid opera ted device to


pump petrol from the tank to the ca rbure t to r , the indi-

ca tor light c i rcui t ry with hazard warning lights, the radio

and c a s s e t t e player c i rcu i t s , the hea te r and wiper mo-

tors , horns , instrument gauges, and heated rear sc reen .

T h e s e c i rcui t s are relat ively s imple and are desc r ibed

toge ther with fault-finding t echn iques later .

Wiring diagram

Car wiring diagrams are often very difficult to read and

interpret . The reason for this is that , in a modern car

with a large number of ins t ruments , l ights, a c c e s s o r i e s

and motors , all are to be in te rconnec ted on one compre-

hensive diagram. Fuses and switches must also be shown,

toge ther with the co lours of the wires and cab le s ; many

manufacturers use an international colour code for easier

identification of the r e spec t ive c i rcui t c ab l e s .

Some of the more popular symbols used in car wiring

diagrams are i l lustrated in Figure 1.5. The cab les are of-

t en c o d e d and c o l o u r e d for i d e n t i f i c a t i o n and a

s h o r t h a n d m e t h o d of s implifying t h e d iagram often

groups all in one bundle (cal led a cable-form) as a single

line. To t r ace the s tar t and finish of one cab le involves

almost m ic ro scop i c analysis of all connec t ions , search-

ing for the required code and colour .

E l e c t r o n i c dev ices such as e l e c t r o n i c ignition or the

dashboard m i c r o p r o c e s s o r are shown as simple b locks .

Fault finding within t h e s e dev ices must be left to the

specia l i s t dealer .

7

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The engine

The most common small to medium car engine is the 4-

c y l i n d e r p e t r o l i n t e r n a l c o m b u s t i o n e n g i n e . M o r e

powerful engines have six cyl inders , some have eight;

motor cyc le s and mopeds have one or two. The arrange-

ment of cylinders varies, some being overhead cam shaft,

some pushrod and rocker , and o the r s with cy l inders

aligned in the shape of a V.

This brief descr ipt ion of the 4-cylinder engine, highlights

the impor tance of a c c u r a t e timing so as to maximise

power and performance. Figure 1.6 shows the arrange-

men t of c y l i n d e r s and t h e four s t r o k e s , i l l u s t r a t e d

separa te ly in Figure 1.7:

induction compression

Crank position {degrees)

Cylinder no. 1

Cylinder no. 2

Cylinder no. 3

Cylinder no. 4

0 - 1 8 0 1 8 0 - 3 6 0 3 6 0 - 5 4 0 5 4 0 - 7 2 0

Power Exhaust Induction Compression

Exhaust Induction Compression Power

Compression Power Exhaust Induction

Induction Compression Power Exhaust

Figure 1.6 4-cy l inder and 6-cyl inder engines

9


Figure 1.7 The four stages of combustion

10


• induction — the petrol /a i r mixture is sucked into

the cylinder,

• compress ion — the piston compresses the mixture,

• power — the spark plug ignites the mixture caus-

ing an explosion which pushes the piston down,

• exhaust — the piston pushes the burnt gases out

of the cylinder.

The four cyl inders opera te in se r ies so that, at any one

t ime, one is being powered. The crank shaft posi t ions

the pis tons in the c o r r e c t s equence , two comple te revo-

lutions (720°) comprising the comple te four-stroke cyc le .

The e lec t r ica l c i rcu i t s have the j ob of supplying each

spark plug with a high voltage pulse to power the piston

in the co r r ec t s equence , and at the t ime when the piston

is at the top of its s t roke ( top dead c e n t r e ) . The distribu-

tor ensures that the pulses travel in s equence to the four

spark plugs and, at the same time, t ime the pulse to top

dead cen t re .

Basic ignition

The main componen t s of the ignition circui t are the igni-

t i on c o i l — a c y l i n d r i c a l t r a n s f o r m e r wi th two

connec t ions SW and CB and a high tension cab le going

to the dis t r ibutor ( s e e Figure 1.8) — and the dis t r ibutor

— a mechanica l device coupled to the engine via skew

gears . This ac t s as a four-way switch to route the high

tension to the spark plugs, and as a means of generat ing

the high tension vol tage.

11

Auto electronics projects liiilllllillfj

Figure 1.8 Basic high voltage generating c i rcu i t

Figure 1.8 shows the bas i c high voltage generat ing cir-

cuit . The operat ion is as follows, assuming the con tac t

breaker points are initially c losed ( see Figure 1.10):

• the piston in one cyl inder ( say number 1) r i ses to

top dead cen t re , compress ing the petrol /a i r mixture,

• the ro tor arm in the dis t r ibutor cap points to the

a p p r o p r i a t e high t e n s i o n c o n n e c t i o n to s p a r k plug

number 1 and,

• the con tac t breaker points open,

• the magnetic field in the primary of the ignition coil

(Figure 1.9) quickly c o l l a p s e s . The turns ra t io of the

t ransformer of about 10,000 to 1 transforms this col lapse

into a vol tage of about 20 ,000 volts a c ro s s the second-

ary,

12


'To distributor • • • •

^ J \ High tension

^ Ι ^ » · ^ ^ Secondary

Figure 1.9 The ignit ion coi l

Θ I Sparking plugs

winding ^ J g n M o n «wteh | I Γ Ι Π Ι Π Ι

winding^ "Jl ^ „. g ^ o ^ ^ ^ ^ a m ^ ^ ^ ^

Figure 1.10 Sparking plugs f i r ing c i rcu i t

13


• the high tension pulse ignites the petrol /a i r mix-

ture in cyl inder 1 causing the engine to ro ta te ,

• the dis tr ibutor shaft ro ta tes to again c lo se the con-

t ac t b reake r poin ts . The c a p a c i t o r a c r o s s the points

suppresses the high voltage pulse genera ted by this c lo-

sure,

• the distr ibutor shaft turns the rotor arm to the next

cyl inder and the procedure repea t s .

The timing of the opening of the points is cr i t ica l . The

dis t r ibutor shaft cam opens the gap as in Figure 1.12,

the posit ioning of the con tac t breaker points a ssembly

is cr i t ica l toge ther with the gap width. The points , after

a period of wear, tend to co r rode and pitting occur s ; a

deposi t which builds up and reduces the effective gap.

The gap is usually about 25 thousands of an inch wide,

opens and c l o s e s s o m e ten mill ion t imes every 1000

miles. One o ther adjustment to opt imise the timing is

the dwell angle. This is the number of degrees that the

points remain c losed; refer to the maker ' s manual for

the recommended value.

Ignition timing is carr ied out in the following sequence :

• c h o o s e cyl inder number 1 — consul t the manual,

Φ loca te the timing marks on the fan belt pulley ( see

Figure 1.13),

• turn the engine crank shaft until the marks align at

top dead cen t re ( t . d . c ) . The engine can be turned by

placing the car on level ground, take out all the spark

plugs, p lace in top gear, r e lease the brakes and move

the car to and fro,

14


• ensure that the dis t r ibutor ro tor arm points to the

high tension lead to cyl inder number 1. If not, turn the

engine through a further 360° ,

• connec t a 12 V lamp between the con tac t breaker

spring ( see point X in Figure 1.12) and a good earth point,

• ro ta te the engine by about 20°, then inch it slowly

backwards until the lamp just l ights,

• if the t .d.c . reading is i nco r rec t , align the t .d.c .

mark, then loosen the dis t r ibutor clamping nut (point Y

in Figure 1.11) and turn the ent i re dis t r ibutor ant ic lock-

wise until the light just goes out. Then turn c lockwise

until it jus t l ights. Clamp the nut,

• c h e c k the t .d.c. set t ing once again,

• rep lace the plugs, put on the brakes and take out

of gear! A faster method uses a s t r o b o s c o p e with the

engine running, a Xenon tube flashing as the points open

and c lose .

Electronic timing

The sys tem so far desc r ibed somet imes fails because of

pitting of the points and wear and tear of the moving

parts of the dis t r ibutor . Two types of e l ec t ron ic sys tem

are found:

• t rans i s to r i sed ignition or capac i to r d ischarge igni-

tion — see Figure 1.14 and,

• con t ac t l e s s (opt ica l or magne t ic ) ignition.

15


Figure 1.11 The distr ibutor

16


Figure 1 .12 Contact breaker assembly

Figure 1.13 Timing marks on fan belt pulley

Trans i s to r ignition uses a power d .c . -d .c . conver te r , a

two t rans i s to r push-pull osc i l la tor , to genera te 400 V or

so , to feed to the ignition coil and produce a higher volt-

age and heal th ier spark. At the same time, the con tac t

b reakers no longer switch the full 12 volt ba t te ry cur-

rent: they merely switch a 12 volt low current signal to

the d .c . -d .c . c o n n e c t o r . T h e points the re fore last far

longer and the sys tem is virtually maintenance-free.

17


Contac t less ignition uses a moving magnet or infra-red

ray to rep lace the cumber some con tac t b reakers , a tran-

sis tor ised d.c.-d.c. conver ter circuit being used as before

to deliver the high tension pulses to the plugs. Both sys-

tems can be installed into an exist ing c i rcui t in a very

small t ime, a number of modern ca rs having such sys-

tems built in when new.

•12 V O-

Figure 1.14 Transistorised and capacitor-discharge ignit ion

circui ts

18


The battery

A car ba t te ry is a real powerhouse and should always be

maintained in prime condi t ion. It is compr ised of a se-

r ies of s ix lead-acid 2 volt ce l l s (Figure 1.15) which,

together , cons t i tu te 12 vol ts at capac i t i e s varying from

about 30 to 100 ampere-hours . A 70 ampere-hour ba t te ry

delivers a cons tan t 70 amps for one hour, or one amp for

70 hours , or on a very cold day, 400 amps for a few s e c -

onds to s tar t the engine.

The negative plates are cons t ruc t ed from spongy lead

plates and the posi t ive plates from lead dioxide. Dilute

sulphuric acid with a specif ic gravity of about 1.2 s ta r t s

the chemis t ry into act ion, current from the ba t te ry turn-

ing the plates into lead sulphate . A ba t te ry charger , by

Figure 1.15 The battery

19


way of the dynamo or a l ternator , r everses this p roces s

by restor ing the ba t te ry plates to their original compo-

sit ion.

Modern ba t te r ies are self maintaining and the e lec t ro-

lyte (ac id ) levels remain cons tan t . Older ba t te r ies are

prone to deter iorat ion and last only 3 or 4 years . The

performance of a ba t te ry falls at low tempera tures , giv-

ing problems on a cold morning and sulphation of the

terminals which causes leakage currents to chass i s ; this

is avoided by smearing petroleum jel ly onto the termi-

nals. A more common cause of bat tery trouble, other than

an old and tired ba t te ry itself, is damp and dirty wiring,

part icular ly around the s ta r te r motor which drains most

of the ba t te ry power.

Ba t te ry charging is carr ied out in one of two ways:

• the dynamo — a d.c. generator , like a motor in re-

verse , which delivers current to the ba t te ry as long as

the engine is running fast,

• the a l ternator — an a.c. genera tor which, although

requiring an a .c . /d .c . rect if ier circui t , has greater effi-

c i ency and charges the ba t te ry even when idling.

Figure 1.16 shows a cut away picture of the dynamo and

the circui t which con t ro l s the charging of the bat tery,

cal led the cut-out or cont ro l box. This unit s enses the

dynamo output vol tage and, if low, cuts the dynamo out

of c i rculat ion. As the voltage r i ses the cut-out connec t s

the dynamo to charge the ba t te ry and if it r i ses beyond

a prese t value, the regulator winding reduces the effec-

tive dynamo output by adjusting the current in the field

winding, excess ive current going direct ly to the car e lec-

tr ical c i rcu i t s .

2 0


Figure 1.16 Dynamo and control box

The a l te rnator is shown in Figure 1.17 toge ther with its

cont ro l c i rcui t ry and rect if ier d iodes . The th ree s ta tor

windings are connec t ed internally to the diodes and a

d.c. output is obta ined. A t rans i s to r i sed cont ro l c i rcui t

maintains a cons tan t ba t te ry charging current by adjust-

ing the current in the ro tor winding.

21


Stator windings in which current is generated

Diodes convert alternating current to d.c.

Rotar turns inside stator assembly

Figure 1.17 Alternator and control c i rcu i t ry

Both sys tems have a built-in ignition warning light with

one side connec t ed to the ba t te ry +12 V terminal , the

o ther to the dynamo or a l ternator output. If the genera-

tor is not working, when the engine is swi tched off for

22


ins tance , or when the fan-belt is slipping or broken, the

12 V bulb has 12 vol ts a c r o s s it and it l ights. Normally

the lamp has 12 vol ts on e i ther side and it goes out.

Lighting

Little needs to be said about the normal lighting c i rcui t s

excep t to say that the headlamp bulbs can consume sev-

eral amperes each and so cab le of the c o r r e c t size must

be used to prevent heating (or melt ing) of the wiring.

Many bulbs , as in Figure 1.18, have two fi laments for

compac tness . Quartz halogen bulbs, with a gas surround-

ing the tungsten fi laments, give off greater br ightness .

Figure 1.18 Dual fi lament bulbs

23


As the headlamps between them consume several am-

peres , the headlamp (or f lasher) switch has to be heavy

duty and high current wires must be sent to the dash-

board. Consequent ly a relay is often posi t ioned near the

headlamps, as in Figure 1.19, this being act ivated via a

(preferred) low current switch and wiring. Operating the

switch act ivates the relay which connec t s the headlamps

direct ly to the ba t te ry terminal .

One final lighting device in common use is the spring

s tee l flasher unit ( s ee Figure 1.20) which turns the indi-

ca to r lamps on and off.

Figure 1.19 Headlamp relay

2 4


While cold, the c o n t a c t s are held toge ther by the dia-

phragm. When current passes through the con t ac t s , by

indicating to turn left or right, the res i s t ance metal heats

up, expands and pushes the con t ac t s apart . They then

cool again, c l o s e and the s e q u e n c e repea t s 60 to 120

t imes a minute. Emergency light units are similar excep t

that heavy duty con t ac t s are used.

Current from A I Current to indicator switch | φ indicator lamps

Indicator lamps on' Indicator lamps off

Figure 1.20 Flasher unit

Starter motor and other accessories

In a similar way to the headl ights being opera ted via a

remote control relay, a s ta r te r solenoid is used as in Fig-

ure 1.21 to switch the 400 amps to the s ta r te r motor .

This wiring is the th ickes t to be seen under the bonnet

and every s tep is taken to minimise any heat generated

25


despi te the c o s t s of the thick copper wire. The s ta r te r

motor engages with the engine via the flywheel to s tar t

the engine, as seen in Figure 1.22. If the ignition circui t

is working well, a few turns of the engine should cause

the engine to fire and cont inue under its own s team. The

s ta r te r motor is then d i sconnec ted from the engine.

Figure 1.21 Starter solenoid

Two methods are used, a pre-engaged motor whose pin-

ion is always linked to the flywheel, a solenoid operat ing

a plunger to engage the s tar ter motor with its pinion (like

a small c lu t ch ) , and the inert ia type whose pinion sl ides

along the shaft to engage with the flywheel as soon as

the s ta r te r motor opera tes . T h e s e are shown in Figure

1.23. Figures 1.24 to 1.28 il lustrate a number of other e lec-

tr ical a c c e s s o r i e s which are essent ia l , and some legally

required, in the modern motor car .

26


Figure 1.22 Flywheel

Petrol pumps ope ra t e e i ther via a mechan ica l rocke r

assembly coupled to the engine forming a small mechani-

cal pump (Figure 1.24), or an e lec t r ica l diaphragm pump,

ra ther like a vibrator , which pumps the petrol from the

tank to the engine, as in Figure 1.25. The petrol gauge

opera tes using a small float coupled to a var iable resis t -

ance unit. As the petrol level r i ses or falls, the current

to the gauge r ises or falls accordingly . This unit, similar

to a WC bal l-cock, is sea led for fire r easons , see Figure

1.26.

27


28

c* «— Ο

Ο

ε

<σ

CO

<ν>

α> »_

β>


29

Fig

ure

1.

23

Co

nti

nu

ed


Figure 1.24 Mechanical fuel pump

Horns come in all shapes and s izes . Figure 1.27 shows a

simple type, working like a v ibra tor whose diaphragm

output is mechanica l ly amplified to warn pedes t r ians to

get out of the way.

Ammeters can be fitted in any car : a s imple means of

installat ion necess i ta t ing a minor change to the wiring

30


Figure 1.25 Electr ic fuel pump

31


Figure 1 . 2 6 Fuel gauge and float

as shown in Figure 1.28. By this means the ammeter does

not record the s ta r te r motor current , but all o ther cur-

rents taken by the car c i rcui t ry .

Figure 1 .27 Horn diaphragm

32


Figure 1.28 Ammeter wiring

Finally, a look into the compute r i sed dashboard now

found in a number of high performance ca r s . Transduc-

ers cons tan t ly read r.p.m., p ressures , t empera tures and

so on; t hese are moni tored and the computer checks and

warns the driver of impending t rouble ( see Figure 1.29).

The day of the J a m e s Bond superca r or the Night Rider 's

Kit looms nearer everyday.

33


Figure 1.29 Computerised dashboard

34

2 Electronic ignition

The e l ec t ro -mechan ica l ignition sys tem that has been

used to fire the fuel/air mixture in an internal combus-

tion engine for severa l decades , and which is familiar to

home mechan ic s everywhere , has prac t ica l ly been re-

placed by e lec t ron ic methods in recen t t imes . Some of

the reasons for this are not quite as obvious as you might

suppose , but cer ta inly, as with everything e lse , a mod-

ern e l e c t r o n i c a l t e r n a t i v e is s u p e r i o r to i t s

e lec t ro-mechanica l ances to r . To be fair though, the lat-

ter has had a lot going for it, it originally rep laced a

method so a rcha ic as to be unbel ievable .

Automotive ignition — a brief history

Earl iest motor ca r s , or in fact anything using the new-

fangled gas engine (many of which were a lso used for

35


powering agricultural machinery) , of slightly over a cen-

tury ago had to make do with a device compr is ing a

thin-walled copper tube with c losed ends, supported in

the middle with a porcelain insulator or some-such simi-

lar item. The insulator sc rewed into the cyl inder head,

like a modern plug — indeed the word plug p robably

or iginates from this t ime.

To s tar t the engine, the outs ide end of the tube is heated

with the flame of a spirit burner until glowing. Then at-

t empts can be made to get the engine going, using a

start ing-handle. When the fuel/air mixture arr ives at the

o ther end of the tube, on the inside, in the right quanti-

t ies (a bit of a juggling a c t ) , it should (hopefully!) burn.

Once the engine is warmed up and running, the spirit

burner can be put out and thereaf ter the tempera ture of

the tube will be maintained by the heat of internal com-

bust ion, in the same way tha t the engine of a model

aeroplane keeps its glow-plug hot.

Not surprisingly, while the gas engine was still only a

few years young, engineers thought hard about improv-

ing this less than ideal s i tuat ion. It was only a quest ion

of t ime before the e lec t r ica l ly powered hot wire type of

ignition, a glow-plug then, was pressed into se rv ice for

the petrol engine. The t rouble with glow-plugs however,

is that the wire burns away quite quickly and a s tock of

spares must be carr ied around at all t imes .

Then, just prior to the turn of the century, a method was

devised which, though it s eems obvious now, must have

taken a good deal of working out at the t ime. It was reli-

able in opera t ion like nothing e l se previously, it was

sophis t ica ted , it was state-of-the-art. It was spark igni-

tion.

3 6

Electronic ignition

37

The advantages included much eas ie r s tar t ing — simply

energise the sys tem and crank the handle. Also, b e c a u s e

the plug was no more than a spark gap at the business

end, and the e l ec t rodes were far more robus t than thin

wire or copper tube, it had a working life h i ther to un-

seen.

From the engine des igners ' point of view it ra ised two

important poss ib i l i t ies :

• the moment of ignition of the fuel/air mixture could

be p r e c i s e l y c o n t r o l l e d . P rev ious ly , the c o m b u s t i o n

chamber had to be designed to prevent the charge ignit-

ing prematurely during compress ion , a shape which did

nothing for efficiency (or performance, if you l ike) ,

• engines with multiple cyl inders could be ca te red

for jus t as easi ly as s ingles . Prior to this engines were

most ly a single cyl inder type — the ignition parapherna-

lia for jus t one was usually quite enough to cope with.

There are basical ly two types of e lectro-mechanical spark

ignition sys tems: the magneto, and what ' s cal led coil ig-

ni t ion. T h e only d i f ference is tha t the magne to a l so

genera tes its own e lec t r i c power to opera te . With coil

ignition the power supply is external . In the beginning,

there was only the magneto. In the 1920s , the Americans

p ioneered coil ignition, which used power h i ther to gen-

erated exclus ively for ancillaries — lights and so forth.

The power supply compr ised a d.c. genera tor in the form

of a dynamo, with a back-up for the per iods when the

dynamo couldn ' t provide the n e c e s s a r y current — an

accumulator (a ba t t e ry ) . In Europe there was great re-

s i s t ance to coil ignition, espec ia l ly among the Bri t ish,

who thought it too gimmicky. Customers wouldn't buy a


car if it had coil ignition — manufacturers had to revert

to the magneto in order to be able to maintain sa les .

Would you bel ieve that such a r e spec ted manufacturer

as Rolls Royce couldn ' t shift their la tes t spor t s tourer

until they had put a magneto back into every car? Such

was the r e s i s t ance to change. Perhaps there is a modern

parallel here , about cus tomers (and m e c h a n i c s ) being

frightened of the complexi ty of fuel in ject ion. . .

Spark ignition — the principles

An e lec t r i c arc is an e lec t r i c current flowing through a

gas, which for the purposes of this d iscuss ion, is air. Air,

as with most insulators r es i s t s the flow of e lec t r i c cur-

rent . If forced, it ionises as e l e c t r o n s begin to move

between molecules . As with any o ther res is tor , this mo-

lecular friction genera tes heat — from the amount of

energy required to cause air to succumb, quite a lot of

heat . The arc is a whi te /blue colour , and hot enough to

s tar t a fire.

It is worth descr ibing how the e lec t ro-mechanica l igni-

tion sys tem opera tes first, s ince there is no substant ia l

difference between it and any e lec t ron ic equivalent —

they all have to do the same thing, make a spark. We

shall s tar t here and work backwards .

Air needs a little persuading in order to ca r ry an e l ec t r i c

current and produce an arc . At normal a tmospher ic pres-

sure it is not all that difficult, but still requires a high

voltage to break down the air be tween a pair of e lec-

t rodes . The narrower the gap, the eas ie r it is . However,

38

Electronic ignition

whilst it is quite easy to bridge a gap of 0.02 inches (a

typical spark plug gap) in open air, it is much more diffi-

cult inside the combus t ion chamber . This is b e c a u s e air

ionises more easi ly the thinner it is ( the typical demon-

st ra t ion is an e lec t r i c a rc in a glass vesse l with a vacuum

pump a t t ached ) , it cor respondingly b e c o m e s more re-

s is t ive the more dense it is, like inside the combus t ion

chamber of an engine. Universally, the fuel/air mixture

is compressed before ignition, the main reason being that

this r e leases more energy on combus t ion (but a lso be-

cause the piston, being a rec iproca t ing part linked to a

revolving part, can ' t help i tself) . The upshot of all this is

that it is more difficult to bridge the gap to produce a

spark in c o n s e q u e n c e , requiring a very high vol tage to

do so , which accoun t s for the 25 to 35 kV HT vol tage

range typical at the plug's live end. I labour on this point

b e c a u s e it causes problems for the design of e l ec t ron ic

ignition amplifiers, as will be seen later .

Obviously it is impract ica l for this sor t of potent ial to

be produced and cont ro l led di rect ly from some engine

driven genera tor , so ins tead a step-up t ransformer is

used, which is where the coil c o m e s in. All the genera-

tion and t imed-switching is done at a more manageable

low voltage, and is conver ted by the coil to the neces -

sary high vol tage.

Actually the sys tem is c levere r than that . The s e q u e n c e

shown in Figure 2 .1 (a ) to 2 .1(d) reveals the sys tem to be

a form of flyback converter. Figure 2 .1 (a ) shows the com-

ponents of a mechanica l sys tem at rest. With switch SI

open, nothing is happening. When S I c l o s e s in Figure

2 . 1 ( b ) , current flows in the primary winding LI of T l ,

39


the ignition coil . T l has a laminated s tee l co re and a fi-

n i t e t ime is t aken for t h i s c o r e to r e a c h m a g n e t i c

saturat ion, by which time the primary current will a lso

be at a maximum. This maximum is set by choos ing a

d.c. impedance for LI by using res is t ive wire, or e l se it

will a t tempt to short-c i rcui t the supply after the co re

sa tura tes! For 12 V sys tems the impedance is chosen for

a maximum current of around 3.5 to 4 A, as a typical

value.

(c) GO

Figure 2.1 Sequence of act iv i ty in contact breaker ignit ion

system

40

Electronic ignition

In Figure 2 . 1 ( c ) , SI opens and unwanted effects take place

in its vicinity, but we'll ignore them for the moment . Suf-

fice to say that as the magnetic field col lapses , it a t tempts

to maintain the current flow in LI in the same direct ion,

and at the same t ime induces a current in L2. B e c a u s e L2

has many more turns than L I , its output vol tage is much

higher. In the cha rac t e r i s t i c manner of flyback conver t -

ers , the coil will a t tempt to output the same amount of

power that went into it. If a path on the primary side is

denied it, then the only r ecou r se is to find an outlet on

the secondary side.

The load is the plug air gap, which basical ly doesn ' t want

to know at first, but the coil will keep pushing the volt-

age up until the gap is bridged. If the total power input

was 50 W and the output r e ached 30 kV then the gap

cu r r en t is ini t ia l ly 1.6 mA. However , o n c e the a r c is

s tar ted , the vol tage level required to maintain it can re-

duce substant ia l ly allowing a grea ter current flow and a

nice heal thy spark. This is indicated in Figure 2 .1 (d ) .

The snag is that a smal ler representa t ion of this act ivi ty

also appears a c r o s s the primary, L I . The effect is an ini-

tial pulse of up to severa l hundred vol ts . At the point of

breaking the circui t , the mechanica l switch SI has a very

narrow gap between its c o n t a c t s which might be meas-

ured in m i c r o n s . Such a gap is e a s y for a coup le of

hundred volts to bridge; the coil expends all its energy

in producing an arc between the switch c o n t a c t s , and

there is none left for the plug. If you want to prove the

effect for yourself t ry it with the coil of a relay, a pair of

tes t leads and a ba t tery .

So this is where the o ther c lever bit c o m e s in, the third

componen t in the set-up, C I . To this day it is still cal led

41


a condenser, a very old-fashioned name for a capac i to r .

Its function is to momentar i ly take over from the switch.

As S I opens , current flow is diverted into CI , charging

it. T h e idea is tha t by the t ime the pr imary vo l t age

reaches a high level, the contac t gap is unattainably wide,

forcing the coil to go for the plug gap instead. This has

two main disadvantages:

• it consumes some power which might o therwise

con t r ibu te to the spark, and,

• it s lows down the ra te at which the HT level can

increase , the output of which takes on more of a milder

ramped pulse shape ra ther than a true pulse. The value

of CI is cr i t ica l : if too small , it will encourage switch arc-

ing; if too large, it will absorb too much power and defeat

the whole ob jec t . A value of 220 nF is usually about right.

Switch arcing and power loss still occur , but at accep t -

able levels .

A third anomaly is that , after the main pulse has oc -

curred , what you are left with is LI and CI , with the

supply as a common terminal , forming a tuned circui t

which rings or r e sona tes slightly. Figure 2.2 shows the

vol tage waveforms a s soc ia t ed with this se r ies of events .

It was ment ioned that the ignition coil has an inbuilt d.c.

impedance to limit current flow while the con tac t break-

ers are c losed . During this t ime the coil is drawing its

maximum power of 45 to 50 watts , to no effect o ther than

that this manifests i tself as heat . Consequent ly an igni-

tion coil has been safeguarded against this , and hence is

a lmost universal ly cons t ruc ted as shown in Figure 2.3. It

is supported in the cen t re of an aluminium can, which is

filled with oil . An ignit ion coi l is , the re fo re , a liquid

cooled component .

42

Electronic ignition

Figure 2 . 2 Voltage waveform from Figure 2 . 1 at coi l primary

Brass HT socket

Terminal Terminal

Moulded insulator

Aluminium can

Oil filled cavity

Synthetic rubber support

Twisted pair of wires.

Primary and HT Common

is · + · terminal

Coil windings

Laminated core as a bundle of steel strips.

HT Ίΐνβ* connects to this, and is

passed via the coil spring at the top

to HT socket.

Figure 2 . 3 Internal construction of a typical ignit ion coi l

43


Advantages of electronic ignition

The first two problems are prac t ica l ly solved by e lec-

t ronic switching, the third by using the coil in a different

way. The re are o ther p rob lems that can be solved at a

s t roke , like mechanica l wear.

The heel of the moving half of a con tac t b reaker wears

on the dis t r ibutor cam. The con t ac t sur faces b e c o m e

damaged, developing a hole or pit in the posi t ive side

and a raised pip on the negat ive surface, as the inevita-

ble arcing causes metal to migrate from one surface to

the other . The lumpy result causes irregular timing and

bad separat ion, but it may be poss ib le to rescue them

with the skilful appl icat ion of a fine s tone .

Then there is the ( somet imes be t te r than dreadful) me-

chanica l auto-advance mechanism, with its centrifugal

bobweights , springs, cam con tours and vacuum ass is t

device. To be fair, in p rac t ice a mechanica l sys tem which

is both well designed and 100% fit is difficult to beat ,

even by an e lect ronic equivalent, but sooner or later wear

takes its toll , affecting engine efficiency, and so it needs

per iodic examinat ion and co r rec t ion or even replace-

ment.

But owners put off having the car serv iced until it des-

perate ly needs it b e c a u s e of exorbitant garage bil ls . In

the meant ime the vehic le is wasting valuable fossil fuel

and polluting the a tmosphere in a way that it wouldn't if

properly tuned. Also of conce rn to car manufacturers ,

under pressure to reduce pollution and fuel consump-

t ion, is the D.I.Y, home m e c h a n i c t inker ing with his

engine. If he knows what he is doing then fine. If he

doesn ' t . . .

44

Electronic ignition

Consequen t ly fac tory se t and m a i n t e n a n c e free e l ec -

t ronic ignition, and ca rbure t to r s with secur i ty blanking

plugs seal ing off the vital b i t s , prevent unauthor i sed

hands fiddling with these and getting it wrong. And you

thought it was all done for your benefit . It a lso explains

the lack of really meaningful information in the modern

owner ' s handbook. Refer servicing to your dealer, or

warranty is void, and that sort of thing. Basical ly it means

s lapped wrist to the potent ial D.I.Y'er.

Electronic ignition — how it works

The good news is that e l ec t ron ic ignition for the average

modern car has boiled down to a recognisab le s tandard

formula, with a long t rack record of rel iabil i ty. The bad

news is that if it does go wrong, you can ' t fix it yourself .

Having a c i rcui t diagram is no help (which you won't be

able to get hold of anyway); both the s enso r and the

amplifier are sealed in resin and you can ' t get inside with-

out destroying them. And assuming you could get into

the amplifier you will most p robably find thick film re-

s i s to rs bonded straight onto a ce ramic base which they

share with o ther micro-mount componen t s and a very

spec ia l i sed cus tom chip, with which you will be able to

do nothing.

The h is tory of t rans i s tor i sed ignition goes back as far as

the 1960s . Unfortunately s e m i c o n d u c t o r s of the t ime,

being made of germanium instead of s i l icon, were some-

what fragile, requiring that spec ia l beefed-up ones be

manufactured to cope . Consequent ly e l ec t ron ic ignition

was expens ive and usually only found a t t ached to simi-

larly unaffordable spor t s ca r s .

45


Timing sensors

In the 1970s , solid s ta te ignition with th ree vers ions of

timing sensor proliferated. The simplest was the so called

transistor assisted ignition, which still required a me-

chanica l switch. The second type had an opto-e lec t r ic

timing sensor , which might use e i ther vis ible light or an

infra-red coupler . Here the beam is interrupted by a ro-

tating shut ter with blades like a fan. The third type uses

a magnet ic sensor .

Many of t hese were available as after-market bolt-on kits

for both ca rs and mo to rcyc l e s . After some twenty years

only one type has c om e out on top as the s implest and

most re l iable — the magnet ic sensor .

The senso r genera tes an e lec t r i c pulse which tr iggers

the amplifier, which in turn drives the coil primary. Fig-

ures 2 .4 (a ) and ( b ) show the now archetypal , s tandard

design in operat ion. Here a permanent magnet couples

to a ferromagnet ic e lement which is mounted on the dis-

tr ibutor shaft and rota tes with it. As this element ro ta tes ,

the s t rength of the field var ies , being largest when the

air gap is smal les t . The t ime varying magnet ic field in-

duces a current in the coil which is proport ional to the

rate of change of the magnet ic field, and which outputs

a vol tage waveform as i l lustrated in Figure 2 . 4 ( c ) . Each

t ime one of the teeth , or r idges, on the ro tor passes un-

der the co i l ' s axis , one of the sawtooth shaped pulses is

generated. The rotor has one tooth for each cylinder and

the voltage pulses cor respond to the spark t ime of the

relevant cylinder. Figure 2 .4(d) shows an advanced ex-

ample of this idea following exac t ly the same principle,

46

Electronic ignition

except that the rotor is a star shaped wheel and the s ta t ic

magnet ic sys tem has a cor responding number of poles ,

in this c a s e six of each , for a s ix cyl inder engine.

Auto advance

One reason why this triggering method has come out on

top over rival designs is simply due to one staggering

implicat ion. B e c a u s e the sys tem is magnet ic ; it is, in ef-

fect, a very simple a.c. genera tor on a small sca le , and

its output is, therefore, proport ional to the driven speed.

What this means is that at slow rotor speeds the output

vol tage is low, while for higher speeds the output is a lso

higher by a proport ional amount. If the tr igger thresh-

old of the amplifier 's input is vol tage dependent , then

triggering can be made to o c c u r at the required point

anywhere on the leading s lope of the output waveform.

Figure 2.5 shows how, from different output levels as

produced by cor respond ing ro tor speeds , the t r igger

level is near the peak of the s lope if the output is low,

and near the beginning if it is high. At a s t roke, what we

have here is, by way of an added bonus , an automat ic

ignition advance mechanism, and this with just one mov-

ing part — the rotor!

The need for ignition advance

While the fuel/air mixture in the combus t ion chamber

burns at a cons tan t rate , the engine as a whole however

47


48

Distributor Rotary shaft ferromagnetic

\ element

W il low reluctance \ J U / /

P e r m a n e nt

and r e s u l t s in • §£ m a g n et

s t r o n g m a g n e t i c I Π . j j

f i e l d f o r c o i l / /

P i c k u p co l l • • ^ 1 ^ / /

( ) ^ 1 N a r r o w Gap

V o l t a g e d u e t o m a g n e t i c M a x i m u m n a r r o w f i e l d c h a n g i n g a s g a p v o l t a g e

r o t o r m o v e s t o w a r d s e n s o r ^ /

V o l t a g e d u e t o m a g n e t i c M a x i m u m w i d e f i e l d c h a n g i n g a s g a p v o l t a g e

r o t o r m o v e s a w a y f r o m s e n s o r

(c)

Figure 2.4 Magnetic timing sensor

Electronic ignition

49

Wide air gap offers ^^^S I/ I I high reluctance / / and results in **^e§> —7/

weak magnetic I Π « I I field for coil Η ^ - Ν / /

( b ) + I W ide G a p

Rotor a r m key

/^^^^^^^^^^^^^^^^^^^^^^^^^^\^^\ R e ' U C t 0 r

I / o V ^ ^ ^ T - | (ts. Li^) / ^ ^ V ^ \ | | Coi l a n t

^ m a g n e t 1 | w f r - - ~ " ^ \ Χ ^ Τ Γ ^ Λ / / « || —j—j-i— s y s t e m u n d e r

rT Λ 1 d f r * ^ \ v ^ X / X / / J / Ii d u s t c o v e r

( j | ^ ^t a

* ' ° Po l es

" I — L D is t r ibu to r

l ι b o d y

V J (D)

Figure 2.4 Continued


Figure 2.5 Auto-advance plot using waveform of Figure 2 .4(c )

is required to opera te over a range of crankshaft speeds .

For this reason the moment of ignition must occu r ear-

lier at higher r.p.m. Full combus t ion of the fuel gas must

occu r during the period where the piston has full lever-

age on the crankshaft , and at high revs the burn actual ly

needs to begin well in advance of this point; at lower

speeds , not so much, at idle, hardly at all. The magnet ic

re luc tance type of ignition timing senso r ach ieves this

auto advance act ion in a much more linear manner than

do compromised mechanica l or e lec t ron ic methods , and

barring the odd rare mishap such as a s c rew coming

loose , once se t it does not need readjustment — for any-

one who has persona l ly endured the long drawn out

p roces s of ignition retiming, the subt le t ies of the opera-

tion do not need rei terat ion!

50

Electronic ignition

Furthermore , s ince this requi rement has already been

taken ca re of by the sensor , it makes the amplifier much

s impler . Otherwise e l ec t ron ic advance might take the

form of f requency sens i t ive switches se lec t ing from a

range of t ime delays, the minimum number of which is

two in the crudes t example of such a sys tem. More than

this requires ra ther more logic gates , or a mic roproces -

sor . Instead the magnet ic re luc tor allows the use of a

compara t ive ly very few t rans i s to rs to produce an ampli-

fier.

The electronic ignition switch

Obviously the hear t of an e l ec t ron ic sys tem which simu-

lates the act ion of a mechan ica l switch to opera te the

coil primary in the traditional way is a t rans is tor , and

you might suppose that any power t r ans i s to r ab le to

ca r ry the maximum on-time current of the primary will

suffice. But oh dear me no. Remember that the primary

potent ial is sufficient to produce an arc a c ro s s the me-

chanica l switch, and that the ignition coil as a whole,

primary included, must be allowed to genera te however

high a vol tage is n e c e s s a r y to bridge the plug gap? We

are therefore obliged to use a high vol tage power tran-

sistor , with a V rating of several hundred volts , and such ' ce ° '

devices are notor iously inefficient, which means to say

that the current gain (H f e) is very small, measured in tens

or less ra ther than hundreds.

The usual biasing method is to use a base bias res i s to r

which typical ly c o n n e c t s di rect ly between the t ransis-

to r ' s base and the supply rail, and this r es i s to r can be

51


formidably beefy to provide the n e c e s s a r y bias current

for the t rans is tor to do its job properly, with the attend-

ant power consumpt ion and heat dissipat ion problems.

I have actual ly seen one design where the base bias re-

s is tor is no more than 9.2 Ω!

No, that wasn' t a printing error . It 's an illustration of how

ex t reme base biasing may have to be to ensure that the

switching t rans is tor ach ieves a sa tura ted on s ta te , es-

sential to get the maximum available vol tage ac ro s s the

primary of the coil and therefore the maximum primary

current . Suppose, in a worst c a s e example, that our tran-

s i s t o r has an Hfe of 3 at 1 A ( y e s , j u s t 3 — al though

fortunately later devices are be t t e r than that now), but

then in order to conduct 4 A this value reduces to say

<2. To ensure adequate biasing we assume a current gain

of 1.5, and c h o o s e a base bias res i s to r with a value of

4 Ω, taking into account a base /emi t t e r forward drop of

1 V. This res i s to r is then sinking 2.6 A and dissipating 28

watts; has to be removed from the res t of the amplifier

to avoid cooking it to death, and be provided with its

own heatsink!

Even in the c a s e of the aforement ioned design using the

9.2 Ω component , the res i s tor is of the high power, metal

encapsula ted type ( see the res i s to r s sec t ion of Maplin's

ca ta logue for examples ) and is sc rewed to the outs ide

surface of the amplifier 's die-cast c a se .

In comparison the power dissipation of the actual switch-

ing t ransis tor is not very much at all, which seems almost

perverse . This is because it performs a switching act ion;

it is e i ther on or off. Which leads us to the next cr i te-

rion, namely ensuring that the t rans i s to r commutâ tes

52

Electronic ignition

( swi tches off) as fast as poss ib le . This is n e c e s s a r y s ince

the coil needs to be swi tched off quickly in order to de-

velop its high tens ion output (a slowly swi tched ignition

coil fails to make a spark) .

High speed switching

Figure 2.6 shows the essen t ia l s of a typical ignition am-

plifier as used with a magnet ic r e luc tance type of timing

sensor . To summarise so far, TR5 is the inefficient, high

vol tage power t rans i s to r switch for the coil , and R9 is

the base bias res i s tor . In this c a s e the bias current origi-

nates from TR4, which is cont ro l led by a Schmit t tr igger

comprising TR2, TR3, and res i s tors R3 to R6. The Schmit t

tr igger is essent ia l to produce the fast edged switching

waveform from the s lower changing input, provided by

T R I .

T R I is the bas is of the input s tage which incorpora tes

the input level th reshold as indicated in Figure 2.5. This

cons i s t s of diode Dl and the base /emi t te r junct ion of TRI

itself, which toge ther will not begin to conduc t until the

applied level is >1.2 V. This signal is of cou r se the ramp

shaped output from the s enso r coil and you can see now

that while the amplitude of the ramp is var iable , the in-

put th reshold is cons tan t . Dl a lso b locks the negative

going part of the input waveform, which is superfluous,

while R l is a current l imiter to p ro tec t Dl and T R I in the

event that for example the input is acc iden ta l ly con-

nec ted to the supply while the power is on.

53


Fig

ure

2.

6 E

ss

en

tia

l ig

nit

ion

a

mp

lifi

er

for

a m

ag

ne

tic

relu

cto

r b

ase

d sy

ste

m

Electronic ignition

Protec t ion for the engine 's mechanica l bi ts can be pro-

vided by including CI , which ac t s as a rev limiter. While

it is charged quickly v i a D l , this charge leaks away slowly

via the base emit ter of T R I due to this dev ice ' s current

gain offering a relat ively high impedance , and in conse -

quence the waveform at T R l ' s emi t ter takes on a more

triangular shape . As engine speed inc reases the mean

average d.c. vol tage drop a c r o s s R2 also inc reases until

a point is r eached where even the lowest level of the

waveform exceeds the low threshold of the Schmit t trig-

ger; the amplifier c e a s e s to opera te and no sparks are

generated.

CI a lso affords some RF filtering, but it might be surpris-

ing to learn that the input leads are rarely s c reened . The

senso r coil is of such low impedance that this is unnec-

essa ry and in any c a s e s ince both these wires are run

toge ther as a pair, any external ly induced current will

be equally present in both, cancel l ing each o ther out.

A real working amplifier

Figure 2.7 shows a c ircui t which is the culmination of s ix

months development including test ing in the field on-

board a real motor vehic le which, for ear l ier vers ions ,

proved to be des t ruc t ive ( to the c i rcui t , not the vehi-

c l e ) . Such is the way of r e sea rch and development , and

t h e s e even t s made defini te ind ica t ions tha t the unit

should be:

• e lec t r ica l ly robust ,

• mechanica l ly robust ; and,

• ut terly weatherproof .

55


56

^

-H2V

te

st

^j4V

LLK

-7

S220n

F

22R

S

IOO

uF

y

w/

T20X

M

lOW

T

35V

ο

1

1—

u—

L

J—

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<fj

/Q\

BU

208A

I

1

1 d

b

e

/ 0

0\

[Ξ N

E55

5 ^

(

be

)

\o/

Fig

ure

2

.7

Re

al

am

pli

fie

r c

irc

uit

d

iag

ram

Electronic ignition

Referring to Figure 2.7, the input s tage is as desc r ibed

for the hypothet ica l amplifier earl ier , with the combined

diode junc t ions of both Dl and TRI forming the input

threshold level, and having Rl as a pro tec t ive current

l imiter. Cl is merely an HF filter in conjunct ion with R l

and does not provide any rev limiting.

To reduce component count , the fast switching act ion

needed to sharpen the pulse produced by T R I is pro-

vided by ICI , a 555 t imer IC used in an unusual way.

Instead of being employed in a convent ional (for the IC)

manner as a monos tab le e t c . both trigger and threshold

inputs (pins 2 and 6) are t ied toge ther to exploit the be-

haviour of the internal b is table , forcing a Schmit t tr igger

act ion. The 555 was chosen b e c a u s e the output s t ruc-

ture can sou rce the driver s tage, TR2, direct ly without

the need for any more t rans i s to r amplifiers.

While there is no input and T R I is off pins 2 and 6 of ICI

are high and the output pin 5 is low, so that TR2 is a lso

off, allowing the bias res is tor , R4, to sa tura te the main

t rans i s tor switch for the ignition coil , TR3, and the coil

is on.

Upon an input ramp vol tage from the timing senso r ex-

ceeding the combined th reshhold levels of Dl and T R I ,

TRI conduc t s and quickly pulls the tr igger input down

to <V3 of the supply level, causing ICI to change s ta te

and switch on TR2, which c lamps R4 to ground and de-

prives TR3 of base drive current . The coil is swi tched

off, ICI is rese t when the ramp is comple ted as TRI col-

l e c t o r g o e s high again , and t h e s y s t e m is r e ady to

genera te another spark.

57


Note that all s tages use the 0 V rail as the so le re ference

and are thus immune to supply rail f luctuations, which

will occu r often in the range of 12 -13 .8 V espec ia l ly if an

e lec t ro -mechanica l regulator is employed, and can be

less than 9 V while the s ta r te r motor is giving the bat-

tery a hard t ime.

Electrical safeguards

The o ther a rea of e lec t r ica l weakness is concen t r a t ed

on TR3. This is b e c a u s e of some horr ib le punishments

that the ignition coil will try to inflict on this device. From

the range of high voltage power t rans i s to rs readily avail-

able the only one to prove itself e lectr ical ly tough enough

to be truly rel iable is the long standing, T 0 3 packaged

B U 2 0 8 d e v i c e d e s i g n e d for u s e in c o l o u r TV l ine

t i m e b a s e s and s w i t c h e d - m o d e power s u p p l i e s . T h e

BU208A vers ion is preferred for its lowest sa tura ted V c e,

essent ial to ensure maximum voltage drop ac ross the coil

and reduce power dissipat ion in the t rans i s tor i tself to a

minimum — it is the more expens ive version, but that

can ' t be helped. The device has a V c e rating of 700 V and

a reasonab le Hf , which reduces bias res i s to r heat dissi-fe'

pation and power loss , as this componen t (R4) has a

conserva t ive value of 22 Ω( ! ) . However TR3 still needs

two essent ia l p ro tec t ion s c h e m e s .

One of t hese must cope with ignition coil back e.m.f.,

which, without a power sapping condense r ( s ee ear l ie r )

is exces s ive . But surely this can only occu r without a

spark plug as a load, e lse how can this happen where

there is an air gap which must s t r ike and conduct and

5 8

Electronic ignition

t h u s l imi t b o t h t h e c o i l ' s p r i m a r y and s e c o n d a r y

vol tages? The truth is that, compara t ive ly speaking, the

air gap takes a long t ime to respond. Until this happens

it is as if there were no load at all and the coil shoves up

the potential enormously . A very simple calcula t ion can

be made to get some idea of the theore t i ca l magnitude

of back e.m.f. from a coil by:

voltage drop across coil

commutation time

where commutat ion time is the t ime taken for the switch-

ing device to swi tch off, which is of c o u r s e not truly

in s t an taneous . Assuming for example a commuta t ion

t ime of 100 ns which even for a BU288 is very much on

the slow side, we get (in theo ry ) :

12V = 120,000 V! 100 ns

This is what we get on the primary side. In p rac t i ce how-

ever it will be prec ise ly 1,400 V. Why so? B e c a u s e this is

the designed c o l l e c t o r to b a s e ( V c b) limit of a BU208,

never mind that this value is double the maximum V ! ce

The b a s e / c o l l e c t o r junct ion is breaking down in the re-

verse direct ion like a Zener diode, and it is not supposed

to be used in this way. Damage is cumulat ive and the

device may fail after even some tens of hours of appar-

ently fault free operat ion.

The voltage limiting protect ion scheme in Figure 2.7 com-

pr i ses ident ica l c o m p o n e n t s SRI and SR2, which are

nothing more e labora te than two mains t rans ient sup-

p r e s s e s in s e r i e s . This c o m p o n e n t is a Metal Oxide

Var is tor (MOV), the r e s i s t ance of which is vol tage de-

pendent . It has a knee vol tage of 340 V ( that is, 1.414 χ

240 V) , which is the peak value of the mains supply. Up

59


to this point its r e s i s t ance is high, but reduces consid-

erably as soon as its knee vol tage is exceeded , and is

normally used to prevent vol tage spikes which would

o therwise exceed the peak mains value from enter ing

mains powered equipment .

Originally it was assumed that two of t h e s e in se r i e s

would be sufficient to limit coil e.m.f. to 680 V (within

the maximum V c e of T R 3 ) on their own, but in real i ty they

are unable to cope . Consequent ly they have to ach ieve

the desired ob jec t ive by the a l ternat ive means of pro-

viding feedback to TR3 base and letting TR3 do the actual

limiting instead. In o ther words, TR3 is made to switch

off up to the 680 V point and then holds this until the

e.m.f. value falls below this level before switching off

properly. Reverse blocking diode D2 detours the current

from SR2 to TR3 base so that it doesn ' t go straight to

ground via TR2.

The o ther pro tec t ion s c h e m e is a provision to prevent

the vol tage ac ro s s TR3 being reversed , i.e. <0 V, which

is inevi table s ince the ignition coil still r e sona tes after

the spark ext inguishes , for while there is no condense r

there is still interwiring capac i t ance , toge ther with that

between TR3 ' s c a s e and its heats ink. The ringing is now

high frequency and very shor t in duration, but still very

much alive and kicking. This is the duty of D3.

Insulation problems

Exper ience has indicated that a g rease less T 0 3 insula-

tor is more rel iable than the tradit ional mica var ie ty for

heats ink mounting. If the mica is not 100% perfect then

60

Electronic ignition

any c racks are weaknesses which can be perforated by

the high vol tage pulses . In the final design the unit was

housed in an extruded modular alloy case ( see Photo 2 .1) ,

with which a slide-in T 0 3 compat ib le heats ink was used.

Although this item comes complete with screws, nuts and

insulator bushes , insulator s leeves were cut from sepa-

rately avai lable T 0 3 bushes and pressed into the holes

before mounting the ent i re a s sembly in the co r r ec t posi-

tion on the s t r ipboard ready to sl ide into the ca se , as

can be seen in Photo 2.2.

Photo 2.1 A complete home-made ignit ion amplif ier in its case

61


Photo 2 . 2 The s t r ipboard assembly of the c i r c u i t of F igure 2 . 7

wi th heats ink in pos i t ion and remote R4 on separate board

Mechanical considerations

Components which are at risk from vibrat ion, e.g. up-

right PCB mounted e lec t ro ly t i cs , should be suppor ted

at their base with b lobs of flexible rubber sealant . ICI

was soldered direct ly without a socke t , or e lse in serv-

ice oxidat ion may cause cont inui ty p rob lems . R4 is a

c e r a m i c b l o c k e n c a p s u l a t e d 10 watt c o m p o n e n t and

should be fitted on a separa te board such that its top

surface is in con tac t with the c a s e and so ldered in this

posi t ion during a tes t fitting. At final fitting this top face

can be smeared with heats ink compound to fill-in the

rough surface. R4 then uses the c a s e as a heats ink.

6 2

Electronic ignition

The reason for the enormous number of external cab le s ,

evident in the example shown in Photo 2 .1 , is that this

unit conta ins an identical pair of t he se amplifiers for a

spec ia l i sed moto rcyc le applicat ion, so there is plenty of

room for one in the case !

Transistor assisted ignition

Trans i s to r ass i s ted ignition simply means that a conven-tional m e c h a n i c a l t iming swi t ch , s u c h as a c o n t a c t

breaker , is not actual ly used to switch the coil d i rect ly

but con t ro l s a solid s ta te switch instead. The ci rcui t of

Figure 2.7 could be used in this role, by merely adding

an ext ra 22 Ω 10 W res i s to r between the input and sup-

ply, as a load for the con tac t breaker . This will greatly

inc rease the life of a pair of normal con tac t b reakers ,

which will consequen t ly require much less frequent tim-

ing readjustment , after which the vehic le will opera te

efficiently for longer per iods with less damage to the

environment . In addition, switching speed is faster mak-

ing more energy avai lable to the spark, although actual

improvement is difficult to measure .

It is worth a mention however that the ignition coil must

be a normal spec type with a r e s i s t ance of 3 - 4 Ω, and

not a high current , high energy type, t hese types will

des t roy the amplifier!

Testing

To be prudent you can c h e c k the operat ion of the ampli-

fier before fitting into the vehic le . A simple tes t requires

a 12 V power supply of up to 4 A output (or a car bat-

63


64

te ry) , and a spare ignition coi l . The amplifier on its own

draws approximately 500 to 600 mA. By wrapping some

tinned copper wire around the + terminal of the coil and

looping the o ther end into the HT socke t , a s imple spark

gap should be formed. This type of sys tem must not op-

era te without a spark gap for a load, or e lse it is likely to

fail.

With the coil wired in, the repeated applicat ion of a 1.5 V

cell to the input should produce crackingly heal thy blue

sparks . For the t rans is tor ass i s ted vers ion, earthing the

input lead for on and re lease for o/ /wi l l have the same

effect. While on, the output ( - terminal on co i l ) will be

0.5 to 1 V.

A more e labora te tes t rig is i l lustrated in Figure 2.8. The

bat tery charger simulates an act ive charging system. The

primary coil voltage can be monitored by an osc i l loscope

using a xlO probe for an effective sensi t ivi ty of 100 V/cm

on the 10 V / c m range . It is very important t ha t the

p robe ' s t r immer be prec i se ly ca l ibra ted for an exac t ly

flat f requency r e sponse using a high quali ty squarewave

signal! The co i l ' s primary winding provides a good rep-

resenta t ion of what ' s going on at the secondary output

end, which can be seen on an 8 cm high grat icule with

the base l ine se t on the bot tom or s econd line.

You may need to turn the br ightness up and shade the

sc reen well, as the whole event is over in less than 3

mi l l i seconds . The t r ace should look like that shown in

Figure 2.9.

Note that the primary 's representa t ion of the gap con-

duction vol tage level is quite low at 80 or 90 V, but this

is because the air gap is at normal a tmospher ic pres-

Electronic ignition

BATTERY CHARGER

12V BATTERY

IGNITION Test COIL gap OSCILLOSCOPE Trigger

' C o Q O O

Ο Ο ο

loi • ο

πι III og

ο

}

X10 Probe

1 αν/cm Y 500uS/Cm Χ

AF SIGNAL GENERATOR

+VS OVE OUT IGNITION

AMPLIRER OVE IN

— c

• ( Ε » f=

sine 20Hz 1V r.m.e.

Figure 2.8 Test rig for monitoring amplif ier output at scope

sure . While providing sparks for a real engine this level

actual ly wanders about all over the p lace in direct pro-

port ion to the gas densi ty in the combus t ion chamber ,

being at its grea tes t while this is high during acce le ra -

tion, and lowest during the over-run while the thro t t le is

c losed . It is for this reason that the upper limit is de-

signed at 680 V and the BU208 chosen in order to provide

plenty of headroom: a different output s tage with a lower

vol tage t rans i s to r will not work properly (as it s tands ,

the design has been found to handle compress ion rat ios

of > 10 :1) . This behaviour a lso explains why any insula-

tion weakness always breaks down during acce le ra t ion .

Such a breakdown is usually total , as I found out the hard

way, leaving me s t randed. So take note!

65


7 0 0 - ,

600

5 0 0 -

ω 4 0 0 -

§ 3 0 0 -

2 0 0 -

1 0 0 -

10_ 0

Limited by protection

scheme #1

© ®

t Limited by protection

scheme §2

5 milliseconds —

+12V

0V

Figure 2.9 Osci l lograph produced by test r ig : (a) in i t ia l e.m.f.

pulse: (b) spark gap ionisation t ime; (c) gap conduction time:

(d) gap extinguishing moment: (e) ringing period

C.D. I .

Who remembers D.I.Y, clip-on ignition b o o s t e r s . At one

point during the late 70s , the popular motor a c c e s s o r y

shops were crawling alive with these things. The selling

point was the third principle ment ioned ear l ier — ca-

paci t ive d ischarge ignition.

CDI employs the ignition coil in a total ly different man-

ne r , n a m e l y as a form of p u l s e t r a n s f o r m e r . T h e

advantage is that the coil is no longer an apprec iab le

part of the e l ec t r i ca l load as in a more conven t iona l

swi tched system; it does not have a heavy current flow-

ing in it for a large part of the t ime and consequen t ly has

6 6

Electronic ignition

an eas ie r life promoting reliabil i ty. In addition, overall

power consumpt ion for the ignition sys tem as a whole is

much lower and is in fact proport ional to engine speed.

As well as by the much reduced power requirement , cold

winter s tar t ing is aided by the very high energy spark

that CDI can genera te , which, if the designer is careful,

is still avai lable even if the ba t te ry vol tage is very low

during start ing.

CDI is e lec t r ica l ly efficient like no o ther a l ternat ive sys-

t em, p r o d u c i n g e n o r m o u s s p a r k s for a m i s e r l y few

hundred milliamps of supply current . Past exper iments

by this author with home grown CDI designs have pro-

duced sparks of V/2 inches! Figure 2.10 shows a typical

sys tem in b lock form, and individual designs do not de-

viate much from this .

The hear t of the sys tem is a d .c . -d .c . conver te r , which

produces a high voltage first (as opposed to the switched

method which derives it at spark t ime by switching the

coil off) d i rect ly from the low tension supply. It is s tored

by capac i to r CI which is in se r ies with the coil primary

winding.

The input stage receives a signal from a magnetic or other

form of timing senso r or a con t ac t breaker , and tr ips a

pulse genera tor , usually a monos tab le . The output pulse

tr iggers on CSR1, which c lamps C l ' s live end to ground.

The coil primary suddenly finds something in the region

of 500 V a c r o s s it, and c o m m e n c e s discharging CI . In the

p roces s , the d ischarge current induces a current in the

secondary winding, where the primary vol tage is multi-

plied by the turns rat io, producing a spark at the HT

output. The counter-e.m.f . from the coil pr imary that

follows turns CSR1 off again. While all this is going on,

67


+ 12V DC IN Ο

σ

DC/DC CONVERTER

+500V DC OUT C1

"II"

INPUT PULSE ) STAGE GENERATOR

Ignition coil

Figure 2.10 Capacitive discharge ignit ion block schematic of

essential parts

the conver te r ' s output is effectively shor t -c i rcui ted to

earth, and it must be designed in such a way that it is

not damaged by this .

The sys tem is that simple, and easy to design, but lat-

ter ly is by and large not taken seriously by most motor

manufacturers . Why should this be? B e c a u s e of two in-

herent , unavoidable flaws in the principle.

One of these is to do with spark conduct ion t ime. The

truth is that this depends on capac i to r d ischarge time,

and as a result can be apprec iably shor te r than that of a

convent ional ly swi tched coi l . This means less gap con-

duction time in the combust ion chamber and, to be blunt,

less than ideal ignition of the fuel gas. In real i ty a be t te r

burn (and less waste and pol lu tants) resul ts from a me-

dium energy long spark than a high energy shor t one —

although this also depends on how the combust ion cham-

ber design can make the bes t use of it; with some older

shapes , which are so inefficient in the first place, it won't

make much difference.

68

Electronic ignition

69

The obvious answer is to inc rease the value of CI to in-

c r e a s e conduct ion t ime, but this aggravates the s econd

problem — which is that the capac i to r should be com-

p l e t e l y r e c h a r g e d p r io r to t h e nex t s p a r k m o m e n t .

Suppose that CI were inc reased to 1 μΡ to provide a 4-

cyl inder engine with r easonab le sparks up to its peak

output speed of 6,000 r.p.m. This requires 200 sparks per

second , further requiring CI to be recharged in the space

of <5 ms. This needs a charging current of 100 mA, which

can be proved by:

100,000 μΑ - < - Λ Π Λ/

- χ 5 ms = 500 V ΙμΡ

and the average power consumpt ion of the conve r t e r

inc reases , by co inc idence , to 50 watts — I say by coinci -

d e n c e b e c a u s e t h i s is a l s o t h e a v e r a g e for a

convent ional ly used ignition coi l . In p rac t i ce the spark

s t rength of CDI always drops off along a s teadi ly wors-

ening curve at h igher r.p.m., aggravat ing i ncomple t e

combust ion , already compromised by gas flow problems

and such. This is not to say that switched ignition doesn ' t

have a similar behaviour , but the roll-off of a swi tched

coil is less acute , and in any c a s e it is eas ie r to se l ec t or

manufacture the coil for the j o b required.

To be fair though, CDI is not a to ta l ly duff idea, but,

should you be toying with the idea of investigating the

principle yourself , be advised that , in order to be able

to deliver the required goods with any semblance of real

usefulness, the conver te r should follow a high frequency

type of swi tched mode power supply principle, using a

ferri te co red t ransformer , and not use a mains t rans-

former in reverse] Mains t ransformers are designed to

tap power from the mains at mains frequency, and are


not very good at doing anything e l se . Given the short-

circuited output p rob lem, the c o n v e r t e r cou ld be a

single-ended flyback conver te r design.

The future

One p o s s i b l e f o r t h c o m i n g i n n o v a t i o n for c a r s is

d is t r ibutor less ignition. Instead of a mechanica l ro tor

delivering the HT current to the required plug as neces -

sary, one i terat ion of the principle is to use high voltage

rect i f iers in a floating secondary circui t to s t ee r HT to

the desired pair of cyl inders in a 4-cylinder engine, the

o ther cylinder, which does not need a spark, is on its

exhaust s t roke and so a spark here is known as a wasted spark. The ignition coi l pr imary is double-ended and

opera ted in push-pull mode by a pair of switching tran-

s is tors ; the direct ion of the secondary pulse determines

which pair of plugs will rece ive the current via the diode

matrix, and the t rans i s to rs will no doubt be under the

cont ro l of an engine management computer .

A variation will use two ignition co i l s , a lso with floating

open-ended s econdary windings but terminated straight

to a spark plug at each end. Again the relevant pair of

p is tons move toge ther but their valve timing is 180° out

of phase, so that while one is on its compress ion s t roke,

the o ther is on its exhaust s t roke .

In actual fact motorcyc les have featured duplicated com-

plete ignition sys tems , and the wasted spark t echn ique

for many years , and it is only a quest ion of t ime before

m o t o r c a r s fo l low su i t and b e c o m e e q u a l l y

d is t r ibutor less .

70

3 Microcontrollers

The microcontro l le r is the workhorse of the modern elec-tronics industry. That s ta tement may be strong, but it is

not an exaggerat ion, for it is becoming increasingly diffi-

cul t to p u r c h a s e any s ign i f ican t p i e c e of e l e c t r o n i c

hardware that does not conta in one or more of t hese

complex ICs.

A microcont ro l le r (μΟ), o therwise known as a single chip

mic rocompute r unit or MCU, is effectively a comple te

compute r cont ro l sys tem integrated onto a single chip

of s i l icon. Referring to Figure 3.1 the main functional

b locks of the mic rocon t ro l l e r are:

• m i c r o p r o c e s s o r co re : with opt imised inst ruct ion

se t for real t ime cont ro l ,

71


R O M - 1 0 2 4 b y t e s

S e l f c h e c k R O M - 2 4 0 b y t e s

R A M - 6 4 b y t e s

R S T C P U

c o n t r o l

A r i t h m e t i c

l o g i c u n i t

( A L U )

M 6 8 H C 0 5 C P U

C P U r e g i s t e r s

A c c u m u l a t o r

I n d e x r e g i s t e r

0 0 0 0 0 1 1 S t a c k p o i n t e r

0 0 0 P r o g r a m c o u n t e r

C o n d c o d e s 1 1 1 H I N Z C

O s c i l l a t o r D i v i d e

b y 2

W a t c h d o g a n d

i l l e g a l a d d r e s s d e t e c t

P O W E R

Figure 3.1 MC68HC05J1 MCU block diagram, showing the

basic functional blocks common to all microcontrollers

72

Microcontrollers

it

σ I Q

P A 7

P A 6

P A 5

P A 4

Ρ A 3

P A 2

PA1

P A O

ö Q

P B 5

P B 4

P B 3

P B 2

P B 1

P B O

1 5 S t a g e

m u l t i f u n c t i o n

t i m e r s y s t e m


73


• memory; usually ROM to contain the cont ro l pro-

gram p lus RAM to ho ld v a r i a b l e s dur ing p r o g r a m

execut ion,

• I/O and on-chip per ipherals ; t hese allow the MCU

to communica te with the hardware of the real world ap-

plicat ion that it is control l ing. T h e s e per ipherals range

from simple digital input/output ( I /O) por ts to complex

analogue-to-digital (A-to-D) and digital-to-analogue (D-to-

A) conver te r s and t imer sys t ems . Tab le 3.1 l ists some of

t h e peripherals t h a t a r e a v a i l a b l e on c u r r e n t

microcont ro l le r families.

Microcont ro l le r s are avai lable in a range of complexi-

t i e s and power (and t h e r e f o r e p r i c e ) , making them

sui table for a very wide range of appl icat ions where they

can rep lace s tandard logic or more complex microproc-

e s so r based solut ions . The advantages of the MCU over

t h e s e t rad i t iona l so lu t i ons a re , r e d u c e d ch ip coun t ,

which br ings c o s t ; r e l i ab i l i ty and s ize b o n u s e s ; and

greater flexibility for the designer — allowing easy modi-

fications to the functionality of the appl icat ion via the

software. T h e s e advantages coupled with the dev ices '

relat ively low cos t ( typical ly from SO.75 in high volume)

have led to m i c r o c o n t r o l l e r s be ing used in a g rea t

breadth of appl ica t ions . With a few excep t ions such as

industrial cont ro l , t hese MCU appl icat ions can be split

into two groups; automotive and consumer .

Tab le 3.2 gives a non-exhaust ive list of mic rocont ro l l e r

appl ica t ions in t he se two a reas . The intent ion of this

chap te r is to give the reader some more insight into a

few of t he a u t o m o t i v e a p p l i c a t i o n s tha t depend on

microcont ro l l e r s , and to highlight the proper t ies of par-

t icular MCUs that make them sui table for each d iscussed

applicat ion.

74

Microcontrollers

MCU peripheral Function

Digital I/O port

Timer

Serial port

VFD port

LCD port

A-to-D

P W M or D-to-A

Watchdog tinner

EEPROM —

in addition to ROM

PLL

RTC

Wake-up port

DTMF

OSD

The basic hardware used by the CPU to access the

outside world (read switches, drive LEDs, etc.).

One off the most common and useful MCU

peripherals — allows timing tasks to be

accomplished while the CPU does something else.

Both synchronous and asynchronous ports are

available allowing fast serial communications over

short or long distances respectively.

Special high voltage output port for driving

vacuum fluorescent displays.

Special low voltage output port for driving LCD

displays. Usually includes multiplexing for large

displays.

Analogue-to-digital converter used to read a

variety of sensors, etc.

A pulse width modulated output that can be

filtered to produce a programmable analogue

voltage, thus acting as a digital-to-analogue

converter.

A special type of timer that guards against CPU

errors and resulting software runaway.

Re-programmable memory that can be used for

calibration purposes or for a non-volatile data store.

Phase locked loop. Used in tuner applications such as

TV and radio.

Real time clock. Special timer designed to count in

real time, i.e. seconds, minutes and hours.

Modified digital I/O port that can generate CPU

interrupts when an input signal changes.

Dual-tone multi-frequency generator, used in tone

dialling telephone applications.

On screen display. A character generator for showing

messages on a TV screen.

Table 3.1 Commonly available on-chip microcontrol ler

peripherals

75


76

The automotive industry is widely recognised by semi-

c o n d u c t o r manu fac tu r e r s as be ing the p e r f o r m a n c e

driver of the mic rocon t ro l l e r market . Originally using

mic rocon t ro l l e r s with 4 and 8-bit buses , the automot ive

des igner ' s ques t for more p rocess ing power for some

appl ica t ions , such its engine management , has pushed

the semiconductor industry into designing first 16-bit and

now 32-bit MCUs. Some ca r s being designed today have

more process ing power under the bonnet than an aver-

age PC!

A well recognised trend in the automotive industry is to

in t roduce new features on up-market ca rs and then mi-

gra te them down on to the i r mass market v e h i c l e s as

rel iabi l i ty and user a c c e p t a n c e are proven, and c o s t s

come down. This explains why many of the features avail-

able on today 's ca r s ( such as e lec t r i c windows) were

yes te rday only avai lable on expens ive luxury models .

However, in many c a s e s t h e s e sys t ems are using yes ter -

day's dumb t echnology and many of the mic rocon t ro l l e r

app l i ca t ions of T a b l e 3.2 a re st i l l the domain of up-

market veh ic les . As the t echno logy migration t rend and

green legislation cont inue, this si tuation will change and

wi th in a few y e a r s al l c a r s will c o n t a i n m o r e

mic rocon t ro l l e r s than wheels! See Figure 3.2.

Interfacing MCUs in the automotive environment

T h e r e is a fundamenta l p r o b l e m with us ing m i c r o -

cont ro l le rs , or digital logic in general , in an automobi le ;

the vehic le e lec t r ica l sys tem is invariably 12 V and logi-

cal devices work at around 5 V, and would be severe ly

Microcontrollers

Automotive Consumer

Engine management

Alarm system

Anti lock braking

Central locking

Trip computer

Dashboard

Electric w indows

ln-car entertainment

Act ive suspension

Multiplexed wiring

Seat adjustment

Electric mirrors

Television

Microwave oven

Telephone

Video cassette recorder

Washing machine

Remote control system

Toys

Fridges and freezers

Alarm system

Radio

Compact disc player

Satell ite receiver

Table 3.2 Typical microcontrol ler applications

Engine lanagement

Dashboard

A.B.S

Alarm system

Central locking

Multiplexed wiring

Active suspension

Figure 3.2 Soon an average car wi l l contain more

microcontrol lers than wheels!

77


damaged if c o n n e c t e d d i rec t ly to a 12 V sys tem. This

means that a supply for the MCU must be derived from

the 12 V supply using a regulator c i rcui t , and that all

inputs to the device must be buffered from the 12 V world

around it. The MCU is a lso incapable of d i rect ly driving

automotive loads, so that external drive c i rcui t s must

be employed to interface the logic outputs to the 12 V

loads. The si tuation is actual ly even worse than this ini-

tial s ta tement implies; the automotive environment is one

of the harshes t known, with ex t remes of temperature and

the sys tem voltage varying cons iderab ly depending on

the condi t ion of the ba t t e ry and whe ther the veh ic le

engine is being cranked (when the vol tage drops consid-

e rab ly ) . The biggest problem however , is the ignition

ci rcui t . When the ignition coil swi tches , large vol tage

impulses (50 to 100 V) can be genera ted on both rails of

the ent i re e lec t r ica l sys tem. Although of shor t duration,

t hese pulses would spell d isas ter for a logic c ircui t in-

put. For th is r e a son great c a r e must be taken when

designing protect ion ci rcui ts for the e lec t ronic hardware

in ca r s . Despite these problems and the assoc ia ted c o s t s

to counte r them, the outlay is justified due to the ben-

efits brought by e l e c t r o n i c s and m i c r o c o n t r o l l e r s , in

particular to the automobile . In the following discuss ions

and examples , the pro tec t ion and drive c i rcui t s may not

always be shown for simplici ty, but the reader should

be aware that these precaut ions have to be taken in all

automotive microcont ro l l e r appl ica t ions .

Electric windows

This is one of the most common electrical goodies to be

fitted to many cars . Figure 3.3 shows the traditional dumb

78

Microcontrollers

Door frame

ι ι

Figure 3.3 Conventional electr ic window c i rcu i t (duplicated for

other doors)

e lec t r ic window circui t that is in common use today. The

swi tches direct ly cont ro l the supply current to the mo-

tors , thus propelling the window in the desired direction.

When the window r eaches the end of its t ravel there is

no cut out, instead the motor simply stal ls and the cur-

rent is limited to a value that does not damage the motor

windings. You can obse rve this by trying to ra ise both

c losed windows in a car when the engine is idling the

engine r.p.m. will drop appreciably due to the heavy load-

ing on the a l ternator . Although this sys tem works quite

well, it does have a couple of problems. The first of these

is quite a major safety conce rn and s tems from the fact

that to deal with icy windows or a dirty mechanism a

powerful motor is deployed. The problem is that if an

obs t ruc t ion is p laced in the way of a c losing window the

motor will exer t a great deal of force before it s tal ls ; that

obs t ruc t ion could be a chi ld 's neck. The second prob-

lem is more of an annoyance than a real problem and it

conce rns the amount of t ime that the driver must keep

his finger on a small but ton to fully open or c lo se the

window.

79


Central locking

That great innovation for the wet Bri t ish c l imate , cen-

tral locking, has t radi t ionally been opera ted via a switch

in the lock mechanism of the front doors , but in recen t

years a new development has made this feature even

80

Both these problems are solved by the intelligent MCU

based system, shown in Figure 3.4. Here the swi tches and

sensor s are connec ted to inputs of the MCU and it in turn

con t ro l s the motors via output por ts that switch exter-

nal dr ivers . The senso r s inform the mic rocon t ro l l e r that

the window has reached the end of its travel and the MCU

can s top the motors . This posi t ional feedback along with

the current s ense means that the MCU can immediately

de tec t when an obs t ruc t ion o ther than the end-stop has

caused the motor to slow or stall ins tead. In t he se c a s e s

the MCU can now take evasive act ion by stopping and

reversing the direct ion of the window for a couple of

inches thus releasing the obs t ruc t ion . The MCU also al-

lows the option of one-touch open or c lose , e i ther via an

additional button, or by counting how long the normal

button is held for — e.g. if the button is p ressed for more

than 2 s econds then the MCU assumes a full motion of

the window is required. Although these features could

be implemented using logic cont ro l , the integrat ion and

very low cos t of a s imple MCU such as the MC68HC05J1

from Motorola make it the ideal c h o i c e . This device is

supplied in a small 20-pin package and has only 1 Κ of

ROM onboard to s to re the program, along with the CPU

and a simple t imer (Figure 3 .1 ) . However, t hese limited

features linked with low cos t make it the ideal device for

displacing c lumsy logic solut ions .

Microcontrollers

81

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more des i rable — remote cent ra l locking. In this set-up

a remote key uses a t ransmiss ion by radio, or more com-

monly infra-red ( IR) , to ac t iva te the cent ra l locking from

a wide angle and cons ide rab le d is tance from the vehi-

cle — Figure 3.5 shows the s chema t i c of such a sys tem.

The t ransmi t te r uses e i ther a very bas i c microcont ro l le r

or, more commonly, a dedicated logic device such as the

MC145026 IC. Instead of using a keypad to de termine

which code to transmit , the device has its inputs fixed

in the factory, into a cer ta in combinat ion of logic levels ,

so that it will always transmit the same code . The number

of inputs allow a large number of different codes to be

configured — just like the number of levers in a padlock.

Although matched pairs of t r ansmi t t e r s / r ece ive r s could

be employed in this applicat ion, the logis t ics of keeping

t rack of which key belongs to which car during produc-

tion are obviously difficult, never mind how you would

handle an owner losing his key and request ing a replace-

ment! For these reasons , intel l igence is employed in the

rece iver to allow it to be cus tomised after product ion.

The microcont ro l le r chosen for the job will include some

on board programmable non-volatile memory (EPROM

or EEPROM) that can be used to s tore the codes of match-

ing t ransmit ters . This customising of the receiver is often

performed by the dealer , just before the new owner gets

his car . The memory size of the MCU allows for several

key codes , so that multiple keys can be used by different

family members . Secure software can be employed to

prevent someone from trying to cyc le through all the

valid codes for the t ransmit ter type until the co r r ec t one

is found. In its s implest form this could just involve ig-

noring incoming IR codes , for a couple of s econds , after

an invalid code has been received — with so many codes

8 2

Microcontrollers

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to cyc le through, this would make the j ob overly t ime

consuming for the potential intruder.

S ince the rece iver must remain powered up at all t imes,

low power consumpt ion is of vital impor tance . For this

reason the MCU will invariably be a CMOS device, with a

specia l low power sleep or stop mode, where the power

consumpt ion will be in the order of microamps . Any in-

coming signal will wake the MCU, via the interrupt pin,

and it will r e c e i v e the c o d e and o p e r a t e the locking

m e c h a n i s m ( e i t h e r s o l e n o i d or m o t o r d r i v e n ) , if it

ma tches one of the valid codes s tored in its memory. A

s u i t a b l e d e v i c e for t h i s a p p l i c a t i o n would b e t h e

MC68HC05P8, which is a c lo se family member to the pre-

viously d iscussed J l device. Its distinguishing feature for

r e m o t e c e n t r a l l ock ing is t h e 32 b y t e s of o n b o a r d

EEPR0M that can be used to s to re several t ransmi t te r

key codes .

Engine management

Engine management in this con tex t means having com-

plete cont ro l over an engine 's ignition timing and fuel

mixture on a cycle-by-cycle bas i s . The t rend in increas-

ing engine management performance has been driven by

the tightening of emissions regulations around the world.

This is the real performance end of the microcont ro l l e r

market, and it has been respons ib le for the growth in

complexi ty of the on-chip t imer sys tems for, as we

will s e e , engine managemen t invo lves a lot of t ime-

cr i t ica l tasks . Before discussing where mic rocon t ro l l e r s

fit into this applicat ion, a brief explanat ion of what is

involved in engine management and how it has been tack-

led in the past would be beneficial .

8 4

Microcontrollers

Figure 3.6 shows the four s tages of a comple te cyc l e of a

four - s t roke in t e rna l c o m b u s t i o n eng ine . In t h e f irst

s t roke , the piston is travell ing downwards with the inlet

valve open, thus drawing in the air/fuel mixture from the

inlet manifold. In the s econd s t roke , the piston r i ses with

both valves c losed , t he reby compress ing the mixture.

As the piston r eaches the top of its t ravel ( top dead cen-

t re or t . d . c ) , the spark plug is fired to ignite the mixture.

The third s t roke is the combus t ion /power s t roke , when

the cyl inder delivers its power; the rapidly combust ing

mixture b e c o m e s very hot and the result ing rapid in-

c r e a s e in p ressure drives the piston down the cyl inder.

In the final s t roke, the piston t ravels upwards again, with

the exhaus t valve open, thus expell ing the remaining

burnt gases . The piston is then ready to s tar t its next

downward intake s t roke , and so ini t iate ano the r four

s t roke cyc le .

The problem for the automot ive designer is that to max-

imise the power and fuel consumption of an engine (while

minimising its pollutants) , the timing of the ignition spark

and the rat io of the air/fuel mixture must vary according

O p e n C l o s e d C l o s e d C l o s e d C l o s e d C l o s e d C l o s e d O p e n

1: I n t a k e 2 : C o m p r e s s i o n 3 : C o m b u s t i o n / 4 : E x h a u s t P o w e r

Figure 3.6 The four strokes of the internal combustion engine

85


to a number of fac tors . The most significant of t hese fac-

tors are engine speed, t empera ture and engine load. The

job of engine management is to cont ro l the ignition and

fuelling of the engine, keeping it as c lo se to its ideal op-

erating condi t ions as poss ib le .

Ignition

On average it takes 2 ms to comple te the combus t ion

p roces s after the ignition spark has been fired. S ince the

aim is to have maximum pressure in the cyl inder just

after the piston has passed the top of its s t roke ( too early

and the pressure would inhibit the pistons upward travel;

too late and power is wasted in the downward s t roke ) , it

is nece s sa ry to fire the spark before the piston r eaches

t .d.c. It is cus tomary to represen t this ignition point as

the number of engine degrees before top dead cen t re

( b . t . d . c ) . As the piston t ravels faster and faster with in-

c reas ing engine speed , and b e c a u s e the c o m b u s t i o n

p roces s takes the same length of t ime, a fixed firing an-

gle for the spark would resul t in maximum p re s su re

occurr ing further and further into the downward s t roke,

so wasting power and increasing fuel consumpt ion. For

this reason the ignition point must be advanced (more

degrees b. t .d.c.) with increasing engine speed. Tradition-

ally th is has been a c c o m p l i s h e d with the centr ifugal

advance mechanism in the dis t r ibutor .

Another factor which influences the ignition timing is the

engine load, which can be shown to be proport ional to

the amount of air inducted by the engine. Historically

this factor was taken into accoun t by connec t ing a pipe

from the inlet manifold to the distr ibutor advance mecha-

8 6

Microcontrollers

nism ( the vacuum a d v a n c e ) . This mechan ica l sys t em

(which has remained virtually unchanged for many years )

is re l iable , but only allows crude cont ro l of the ignition

timing, result ing in compromises in the engine perform-

ance and great difficulty in reaching today 's emiss ion

regulat ions.

Mixture control

For an engine to run well, a specif ic air/fuel rat io must

be maintained. The theore t i ca l rat io of fuel to air for

comple te combust ion (and therefore maximum economy

and lowest emis s ions ) is jus t under 1:15 in weight (or

a l ternat ively 1 L of fuel for every 10,000 L of air in vol-

ume) . In p rac t i ce , maximum fuel e conomy is obta ined

with around 20% e x c e s s air, while maximum power is

ob ta ined with approx imate ly 10% air sho r t age . S ince

engines normally run at part-load, the fuel sys tem is de-

s igned for maximum e c o n o m y at th i s point and the

mixture will be r icher at idle and maximum power. The

task of the ca rbure t to r , or fuel inject ion sys tem, is to

produce the bes t mixture for the engine under its cur-

r e n t o p e r a t i n g c o n d i t i o n s . T h e s i m p l e m e c h a n i c a l

ca rbure t to r again compromises the fuel mixture under

different condi t ions and the t rend today is towards e lec-

t ronic fuel inject ion, where a p rec i se amount of fuel can

be del ivered to the individual cyl inders .

B e c a u s e ignition timing and fuel mixture are both de-

pendent on the same var iables (engine speed, load and

t empera tu re ) , it makes a lot of s e n s e to combine the

cont ro l of both into a single unit — the so-cal led e lec-

t ronic engine management system. With its ability to read

87


sensor s , perform high-speed ca lcula t ions and measure

time, the microcont ro l le r is the ideal device for engine

management .

Figure 3.7 shows a b lock diagram of a typical sys tem

based on a high-performance mic rocon t ro l l e r . Engine

speed and angle are both obta ined from a single induc-

tive senso r that genera tes e lec t r ica l pulses when tee th

on the flywheel pass by. To provide a re ference point

for determining the engine angle, one or more tee th are

omit ted from the flywheel, thus producing a pulse pe-

riod twice or more than the normal. Alternatively, but

less common, an ext ra tooth may be present resulting in

two pulses each of half the normal period. This means

that to determine engine speed and angular posit ion, the

microcont ro l l e r must perform two bas ic tasks :

• it must de tec t the miss ing /ex t ra too th and then

count tee th to determine the engine angle,

• it must t rack the t ime between adjacent tee th , and

from this ca lcu la te the current engine speed.

As there are typical ly 30 to 60 tee th on the engine fly-

wheel and a typical engine has to be designed for an

8000 r.p.m. maximum, the pulse period from the flywheel

sensor can be less than 125 μ 8 . Clearly then, if the μC used software loops to count the periods of the incom-

ing pulses, there would be very little process ing time left

to use the data obtained, even if it didn't have o ther sig-

nals to measure as well. For this reason independent

t imer sys tems on board the microcontrol ler have evolved

to lessen the load on the CPU. T h e s e t imers use an input

capture mechan ism to time tag incoming pulse edges

against a free running counter t imebase , and then inter-

8 8

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rupt the CPU to tell it to read the captured t ime. The

following sec t ion i l lustrates how the t imer sys tem inter-

a c t s wi th t h e CPU on M o t o r o l a ' s M68HC11

microcont ro l l e r in order to determine engine speed and

angle.

The diagram in Figure 3.8 shows a simplified b lock dia-

gram of t h e T i m e r and Pu l se A c c u m u l a t o r s y s t e m s

o n b o a r d the M o t o r o l a M68HC11 MCU, a pa r t i cu la r ly

popular device for current engine management solut ions.

The hear t of the t imer sys tem is a 16-bit free-running

counter that the CPU can read via two 8-bit regis ters ,

TCNTHI and TCNTLO. The main purpose of the counte r

is to ac t as a t imebase for the input capture and output

compare functions. The input capture function allows a

t ransi t ion on an external pin to be timestamped by latch-

ing the value of the free-running counte r at the t ime of

the t ransi t ion. The CPU can then read the la tch at a later

t ime and get an exac t r ecord of when the t ransi t ion oc-

curred.

The output compare , or match, function is the inverse

of the input capture ; it allows the CPU to schedu le a

change in the s ta te of an output pin, at a p rec i se t ime in

the future, by writing a value into the 16-bit compare

register . When that value is matched by the incrementing

free-running counter , the output will change s ta te . The

M68HC11 has various combina t ions of input capture and

output compare pins avai lable on several of its family

members . The pulse accumula tor is an 8-bit counter that

is c locked by a specif ied t ransi t ion on an external input

pin. The CPU can write any value into the counter , and

can read it at any t ime. The pulse accumula tor can gen-

era te an interrupt to the CPU when it overflows.

90

Microcontrollers

The t imer module and pulse accumula tor can be used in

a number of ways to determine engine speed and angle,

and to genera te the n e c e s s a r y output pulses . The fol-

lowing is one such method.

The condi t ioned signal from the flywheel s enso r is con-

nected to both the pulse accumula tor input pin and input

capture pin. Both the input capture pin and the pulse

accumula tor pin are configured to de tec t a rising edge,

and the input capture interrupt is enabled so that the

CPU will be interrupted on every pulse rising edge. The

interrupt software routine will read the captured value

of the free-running t imer, s to re it and then sub t rac t the

last captured value, to obtain the tooth pulse period in

t imer counts . S ince the number of tee th are known, the

engine speed can easi ly be derived from the pulse pe-

riod. S ince the period of every pulse is measured, the

interrupt software can identify the longer period assoc i -

ated with the missing tooth angular re ference . At this

point it can c lear the pulse accumula tor , which will then

s tar t count ing pulses ( t e e t h ) . S ince each tooth cor re -

sponds to a number of engine degrees , the value of the

pulse accumula tor is a represen ta t ion of the engine an-

gle.

To genera te one of the output pulses (e.g. for an injec-

to r ) , the input capture interrupt software can c h e c k the

pulse accumula tor against the desired tooth count (mi-

nus 1 or 2, to allow for interrupt l a t enc i e s ) . When the

pu lse a c c u m u l a t o r m a t c h e s th i s va lue , t he CPU can

schedule the s tar t edge of the pulse by reading the free-

runn ing c o u n t e r , add ing an of f se t and wr i t ing t h e

resul tant value into the output compare regis ter of the

desired pin. The offset is a value in t imer counts that

co r re sponds to a number of engine degrees at the cur-

91


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rent speed . This number of engine degrees is the differ-

e n c e b e t w e e n t h e a n g l e m a t c h e d to t h e p u l s e

accumula tor value, and the exac t number of engine de-

grees at which the pulse must begin — s e e Figure 3.9.

The o ther input parameters of Figure 3.7 are measured

using an analogue-to-digital (A-to-D) conver te r , which is

usually integrated on-chip as part of the microcont ro l le r .

As previously mentioned, the air inducted by the engine

can be used as a measure of the engine load. A value for

this is obta ined, via a vane device in the air intake that

opera tes a potent iometer , or a l ternat ively via a hot-wire

sensor . The lambda senso r is a fairly recen t addition to

engine management s y s t e m s prompted by increas ing

anti-pollution regulations that have led to the use of cata-

lytic conve r t e r s . The ca ta ly t ic conve r t e r is a de l ica te

ob jec t and very str ingent control of the engine emiss ions

must be obta ined if the ca ta ly t ic conver te r is to opera te

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I n p u t c a p t u r e r o u t i n e l o a d s o u t p u t c o m p a r e r e g i s t e r w i t h the c u r r e n t f r e e r u n n i n g t i m e r v a l u e + o f f s e t

In the a b o v e e x a m p l e a n o u t p u t p u l s e m u s t b e g e n e r a t e d s t a r t i n g at a n e n g i n e a n g l e c o r r e s p o n d i n g to 5 t e e t h p l u s a bit ( a s e a c h t o o t h is a n u m b e r o f d e g r e e s ) f r o m the r e f e r e n c e m a r k

Figure 3.9 An example of output pulse timing

94

Microcontrollers

efficiently. The lambda senso r is bas ica l ly a hot plati-

num/ce ramic dev ice that p roduces an output vol tage

which var ies , depending on the oxygen con ten t of the

gas it is surrounded by. By insert ing such a s enso r into

the exhaust manifold, it is poss ib le to determine the air/

fuel compos i t ion current ly being burned in an engine.

This effectively t ransforms the engine management sys-

tem, from an open-loop control sys tem into a closed-loop

one, where def ic iencies in the desired output ( c o r r e c t

air/fuel mixture) can be de tec ted and the input var iables

(ignition timing/fuel quant i ty) adjusted to compensa te .

This means that much c lose r control of the exhaust emis-

s i o n s can b e m a i n t a i n e d , he lp ing to m a x i m i s e t h e

e f f ic iency of the c a t a l y t i c c o n v e r t e r mounted down-

s t ream in the exhaust sys tem.

Having m e a s u r e d all t h e s e p a r a m e t e r s , t h e

microcont ro l l e r must de termine the cor responding out-

puts — i.e. the timing of the spark ignition pulses , and

the t iming/duration of the pulses which fire the fuel in-

j e c t o r s . Th i s is a c h i e v e d by a c c e s s i n g the so -ca l l ed

engine maps t h a t a r e s t o r e d in t h e m e m o r y of t h e

microcont ro l le r . T h e s e maps are, in fact, t ab les of data

that hold the ignition and fuelling cha rac t e r i s t i c s of a

par t icular engine type against a number of input vari-

ab les . B e c a u s e it is impract ica l to try and s to re all the

pos s ib l e c o m b i n a t i o n s of output t iming ve r sus input

cha rac t e r i s t i c s , a number of points are held in the map

table , and the μC must then perform an ar i thmet ic cal-

culat ion to in terpolate between the two c lo se s t points

given, to the exac t input condi t ions obta ined from the

various s enso r s .

As there are a number of var iables to be taken into con-

siderat ion, t he se interpolat ion ca lcu la t ions are complex

95


and require a lot of p rocess ing power to be comple ted

quickly, in t ime to set up the output timings for the next

engine cyc le . This is the reason why 16 and now 32-bit

mic rocon t ro l l e r s are replacing older 8-bit sys tems for

engine management . They allow more complex calcula-

t ions to be comple ted quickly so that c lo se r cont ro l can

be maintained on a cycle-by-cycle bas i s .

When the microcont ro l l e r has obta ined the desired out-

put t imings, it must actual ly genera te the pulses to fire

the spark plugs and in jec tors . This is done via the out-

put match facility of the t imer sys tem, where the CPU

writes a value into a specia l regis ter . When the value of

the incrementing t imer-counter r eaches the same value

as that in the regis ter , the hardware of the t imer sys tem

automat ical ly changes the output pin s ta te to a desired

level. This mechanism allows very accu ra t e p lacement

of the various pulses required in the engine cyc le , as we

have seen from the descript ion of the Motorola M68HC11.

The method desc r ibed above, using the input capture

and output match t imer functions, is used in virtually all

of today 's production engine management sys tems . How-

ever, this sys tem is not perfect as the CPU still has to

respond to a large number of interrupts generated by

the t imer, thus slowing down its cont ro l ca lcu la t ions .

This interrupt overhead has se t the performance limits

of today 's sys tems , and so a new approach will be re-

quired for the even more complex cont ro l a lgori thms

required for tomorrow's emiss ion regulat ions.

Motorola has been the first microcontrol ler manufacturer

to address this problem by introducing the innovative

MC68332 device . Not only does this device have a pow-

erful 32-bit 68000-based CPU, but is unlike any o ther

9 6

Microcontrollers

microcontro l le r in that it a lso has a second on-board CPU

d e d i c a t e d to con t ro l l i ng t imer func t ions . Th i s T ime

Process ing Unit, or TPU, is in effect a mic rocont ro l l e r

within a microcont ro l le r ! The TPU is used to handle al-

mos t all of the in te r rup t s a s s o c i a t e d with the t imer

channels , thus freeing the main CPU to spend more t ime

on complex cont ro l ca lcu la t ions . At sui table points in

the cont ro l cyc le , the main CPU obta ins new input read-

ings from the TPU and presen ts new data for the TPU to

ca lcu la te and schedule the output pulse t imings.

Vehicle alarms

The huge inc rease in car-related c r imes in the 1980 /90s

has been paral leled by an equally large inc rease in the

demand for car a larms. Originally based on simple logic

c i r c u i t s and t r i g g e r e d d i r e c t l y from i n t e r i o r l ight

swi tches , the complexi ty of a larms has grown to try and

match the skill of the potent ia l in t ruder . Figure 3.10

shows the s c h e m a t i c of a typical soph i s t i ca t ed MCU-

based alarm sys t em. Using a m i c r o c o n t r o l l e r in th is

application provides a great deal of sophis t icat ion within

a very low componen t count , allowing the alarm to be

small and thus easi ly concea led .

An MCU chosen for this j ob should have a low power mode s ince the alarm must be powered up for long peri-

ods of t ime without the engine running. It should also be

poss ib le to wake the device from this mode via several

sou rces , so that a number of c i rcu i t s can trigger the de-

vice into sounding the alarm. A simple 8 or 16-bit on-chip

t imer is a lso des i rable to t ime the output audio/visual

warning p u l s e s , and to r e s e t t h e a la rm af ter it has

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sounded for a se t t ime — this is a legal requirement . The

t imer can also be used to arm the alarm after a defined

period, if it is not armed via a remote cont ro l .

A.B.S.

The increased performance of everyday ca r s , along with

their increasing numbers (and therefore greater densi ty

on the roads ) , has resul ted in a cont inual improvement

in braking performance. This t rend has included the pro-

gress ion from all-drum braking, drum/disc braking and

vent i la ted d isc /drums, through to the all-disc braking

sys tems found on today 's higher per formance ca r s . The

most recen t improvement has been the introduct ion of

ABS.

The Antilock Brake System does not i tself inc rease the

braking capac i ty of the vehic le , but improves safety by

maintaining optimum braking effort under all condi t ions .

It does this by preventing the vehic le wheels from lock-

ing, due to over -app l ica t ion of the b r akes , and thus

maintains s teerabi l i ty and reduces stopping d i s tances

when braking on difficult surfaces such as ice .

ABS allows shor te r s topping d i s tances than with locked

wheels , due to the friction or mu-slip cha rac t e r i s t i c of

the tyre-to-road interface; as a wheel brakes , it slips rela-

tive to the road surface producing a friction force . A

typical mu-slip curve is dep ic ted in Figure 3 .11 . This

shows that peak friction occu r s at about 10 to 20% slip,

and then falls to approximately 30% of this value at 100%

slip ( locked whee l ) .

100

Microcontrollers

mu 0.5H

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Fully locked

Figure 3.11 A typical mu-slip characterist ic for the tyre-to-road

interface

The aim of the ABS sys tem is to cont ro l the braking force

so as to s top the slip for any wheel exceeding this opti-

mum value by more than an a c c e p t a b l e window.

At t h e h e a r t of al l ABS s y s t e m s ( e x c e p t t h e a l l -

mechan ica l sys tem implemented by Lucas ) is an e lec -

t r o n i c c o n t r o l unit (ECU) b a s e d a round a powerful

microcont ro l le r . Figure 3.12 shows a b lock diagram of

such a sys tem. The solenoid valves that form part of the

hydraulic modulator allow cont ro l of the p ressure avail-

able to the individual wheel brake cylinders, independent

of the force supplied by the driver via the brake pedal.

T h e s e three-way valves can connec t the brake cyl inders

to:

• the normal master cylinder circuit , so that the brak-

ing pressure will be di rect ly cont ro l led by the driver,

• the return pump and accumula to r in the hydraulic

modulator , so that the p ressure in the brake cyl inders

will fall as the fluid returns to the mas te r cylinder,

101


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• nei ther of the above two c i rcui ts , thus isolating the

brake cyl inder so that the p ressure will be maintained

at the value immediately preceding the move to this po-

sit ion.

The cont ro l for t hese valves is supplied via drive cir-

cui ts from the output por ts of the microcont ro l le r .

T h e b a s i s for all e l e c t r o n i c ABS s y s t e m s is t h e

mic rocon t ro l l e r ' s abil i ty to determine the speeds of the

individual wheels (al though some front-wheel drive ve-

h i c l e s s h a r e a c o m m o n s p e e d s e n s o r for b o t h r ea r

whee l s ) . It does this via an inductive senso r and too thed

ring that produce an output waveform, the f requency of

which represen t s the speed of the wheel . This arrange-

ment is a lmos t ident ica l to the engine speed s e n s o r

d i scussed earl ier , excep t that s ince no angular posi t ion

information is required the re are no missing or ex t ra

teeth. It follows from this that the explanation previously

given on determining engine speed a lso applies to deter-

mining wheel speeds in an ABS sys tem.

In this ca se , there are around 50 to 100 tee th on the en-

coder ring, and this could result in a pulse f requency of

6000 Hz when the vehic le is travelling well in e x c e s s of

100 mph. As the re can be a speed senso r on each of the

4 wheels, a total of 24,000 pulse edges have to be resolved

every second . The solenoid valves in an ABS sys tem typi-

ca l l y have a r e s p o n s e t ime of 10 to 20 ms , and t h e

mic rocon t ro l l e r must be able to sample the inputs at

least twice that often, to reso lve lock-ups in 5 to 10 ms.

Put another way, the mic rocon t ro l l e r must be able to

de te rmine 4 independent wheel speeds from 6000 Hz

103


signals within a 5 ms window, and still have t ime to carry

out p rocess ing on this data to determine the new valve

s t a tes . T h e s e str ingent timing requi rements mean that

ABS sys tems are the domain of high performance 16-bit

mic rocon t ro l l e r s that can respond quickly to interrupts

from the t imer sys tem which is capturing the speed sen-

sor edges .

So far it has been s ta ted that the mic rocon t ro l l e r in an

ABS sys tem must prevent the wheel-slip value from ex-

ceeding the optimum, and we have d i scussed how the

μC measures the wheel speeds (angular ve loc i ty ) . How-

ever, it may not be c lea r how these wheel speeds are

related to the slip values that the sys tem is at tempting

to cont ro l . The slip of any wheel can be defined as the

difference between the angular ve loc i ty of the slipping

and non-slipping wheels , divided by the angular ve loc-

ity of the non-sl ipping wheel . Th i s makes s e n s e and

sounds quite simple, but for one problem; how to find a

non-slipping wheel? The ABS algorithm s e a r c h e s for the

fastest spinning wheel and uses this as the re fe rence for

calculat ing the slip values of the o ther wheels . If the slip

value of a wheel is greater than the peak friction value

by a cer ta in margin ( i .e . the wheel is heading towards a

locked condi t ion) , then an ABS cont ro l cyc le is execu ted

on that wheel .

First the mic rocon t ro l l e r will i so la te the wheel brake

cyl inder from the brake c i rcui t to prevent further pres-

s u r e i n c r e a s e . It will t h e n r e c h e c k t h e s l ip and

acce le ra t ion values to determine if the wheel is still de-

celerat ing, and whether the slip value is still exceeding

the desired value. If so , then the valve posi t ion is moved

104

Microcontrollers

momentar i ly to the return posi t ion, reducing the brak-

ing effort on that wheel . This pulsed re lease of p ressure

is cont inued until the mic rocon t ro l l e r de tec t s that the

wheel acce le ra t ion is posi t ive, at which point it s tops

reducing the pressure , and r e c o n n e c t s the wheel cylin-

der to the b rake c i rcu i t to prevent o v e r s h o o t of the

acce le ra t ion . This ent i re cont ro l cyc le of holding/reduc-

ing/ increasing brake pressure is repea ted until the slip

value for the wheel has been brought back into the ac-

cep tab le window.

This is obviously a simplified explanat ion of how ABS

works and the algori thms are in fact very complex and

will vary from one ABS implementat ion to another . When

you remember that this algori thm must be execu ted on

all wheels in jus t a few mil l i seconds , it is not surprising

that ABS is among the most demanding mic rocon t ro l l e r

appl ica t ions .

An important point worth discussing about ABS is that it

is one of the most safety cr i t ica l p r o c e s s o r appl icat ions

in ex i s t ence . The c o n s e q u e n c e s of a faulty ABS sys tem

could be potent ial ly d isas t rous if the brakes were pre-

vented from operating, or were applied er roneously . For

this reason ABS manufacturers take great ca re in the

safety a spec t s of the sys tem design. It is not uncommon

for two identical mic rocon t ro l l e r s to be implemented,

running the same software in parallel and cont inual ly

checking each o ther via a communica t ion pro tocol for

any e r roneous operat ion.

Another solution to this problem is to have a s impler

( lower c o s t ) s lave ( that ac t s as a watch-dog for the

main ABS m i c r o c o n t r o l l e r . Th i s s lave dev i ce is pro-

105


grammed to monitor the major act iv i t ies of the mas te r

and it has the abil i ty to shut down the ABS sys tem if

a fault is de tec ted , thus revert ing full braking cont ro l to

the driver.

A subjec t worth mentioning here is t ract ion control . Trac-

tion cont ro l is a fairly recen t development and can be

thought of as applying ABS in reverse . The idea of t rac-

tion cont ro l is to prevent wheel-slip due to e x c e s s power

on loose surfaces by applying a braking force to the slip-

ping w h e e l ( n o t e t h a t t r a c t i o n c o n t r o l is o n l y

implemented on the driven whee l s ) . This feature is a

natural progress ion for ABS, as all the n e c e s s a r y com-

ponents and measurements required for t rac t ion cont ro l

are inherent in the ABS sys tem — excep t some means of

applying a braking force when the driver is not depress-

ing the b rake pedal . Th i s is usual ly a c h i e v e d via an

e lec t r i c pump arrangement .

With the cons ide rab le improvement in safety provided

by ABS, there can be little doubt that the next few years

will s ee this sys tem becoming more popular, poss ib ly

becoming a s tandard feature on all but the lowest co s t

ca r s .

The future

Hopefully this chap te r will have given the reader some

insight into the fascinating and challenging appl icat ions

for mic rocon t ro l l e r s in automotive appl ica t ions . It has ,

of course , been imposs ib le to cove r all of the applica-

t ions l isted earl ier in this chapter , or even to cover some

106

Microcontrollers

of t hose we have in great t echn ica l depth (engine man-

agement or ABS themse lves could each fill a text book) ,

but the se lec t ion chosen has shown just how varied in

complexi ty the automot ive mic rocon t ro l l e r applicat ion

can be .

As a finishing thought, it may be worth pondering what

t h e fu ture h o l d s for e l e c t r o n i c s , and p a r t i c u l a r l y

mic rocon t ro l l e r s , in ca r s .

Perhaps the next major advance, one which all the ma-

j o r v e h i c l e manufac tu re r s and s t anda rds b o d i e s a re

working on, is the multiplexed wiring system. As the e lec-

t r i ca l c o n t e n t of v e h i c l e s e s c a l a t e s even higher , the

weight and cos t of all the in te rconnec t ing cab le s is be-

coming a major concern , and the number of e lec t r ica l

c o n n e c t o r s poses a rel iabil i ty problem — most veh ic le

breakdowns are due to e lec t r ica l faults. The concep t of

the mult iplexed wiring sys tem is to use a very high per-

f o r m a n c e s e r i a l c o m m u n i c a t i o n s n e t w o r k b e t w e e n

intelligent and semi-intelligent modules s ta t ioned at stra-

tegic points around the veh ic le . This means that only

power and the serial link need be dis t r ibuted about the

car — all the loads have shor t connec t ions to the near-

est intell igent sub-module.

The poss ib i l i t ies of this sys tem are enormous; the en-

gine management sys tem could talk to the e l ec t ron i c

gearbox cont ro l le r and to the ABS/ t rac t ion cont ro l sys-

tem. No longer would turning on your l ights s imply

connec t power direct ly to the bulb — it would signal one

unit to send a command to another unit, instruct ing it to

turn on the bulb using a Smart Power device .

107


This scenar io is not fantasy, it is going to happen and

because the microcont ro l le r has a p lace at the hear t of

every one of t hese intelligent modules, it is safe to say

that its future in the automotive market is very secu re

indeed.

108

4 Car battery monitor

Any number of things from a faulty a l te rnator to left-on

headlights (or s idel ights , even) can result in a flat bat-

tery — and the first you are likely to know about it is

when you turn the key one morning and the car won't

start! This car ba t te ry monitor is a useful little unit de-

signed to warn you in advance by displaying the bat tery ' s

s ta te of charge with a row of ten LEDs.

The moni tor consumes a miser ly 20 mA (it would take

2000 hours to d ischarge a 40 Ah ba t t e ry ) , so it can be

left permanent ly connec t ed to the ba t te ry if required.

Alternatively, it could be connec ted to the ancillaries side

of the ignition switch.

109


The car ba t tery monitor will even reveal faults like a slip-

ping fan-belt: a problem which prevents the ba t te ry from

charging proper ly , but l eaves the da shboa rd ba t t e ry

warning light off. It will even show how the ba t t e ry is

handling the s t renuous work of s tar t ing the ca r (did you

know it takes some twenty minutes of driving to put back

what a five-second s tar t takes ou t? ) .

Circuit

The heart of the monitor circuit (Figure 4 .1) is the LM3914

bar-graph driver IC, used to drive a row of red, orange

and green LEDs which toge ther indicate a magnitude of

the ba t te ry charge vol tage in ten s teps , approximately

V2 V each s tep from 9 V to 14 V. The IC conta ins an input

buffer, a potential divider chain, compara to r s , and an

accura t e 1.2 V reference source . Logic is a lso included

which gives the c h o i c e of bar or dot-mode operat ion —

the la t ter is used in this appl icat ion. The compara to r

causes the LEDs to light at 0.12 V intervals of the input

voltage. TRI ac t s as an amplified diode, raising the lower

end of the divider chain and the negative terminal of the

re ference source (ICI pins 4 and 8 ) to 1.9 V. The upper

end of the chain at ICI pin 6 is connec t ed to a re ference

sou rce output vol tage of approximate ly 3.1 V from pin 7.

The potential divider formed by R l and RV1 a t tenuates

the supply voltage and produces the signal input to the

compara tor , such that a supply range of 9 - 1 4 V covers

the span of the divider chain and is indicated over the

whole of the ten segment LED display. The LED bright-

ness is held cons tan t by an internal cons tan t current

source .

110

Car battery monitor

Construction

Component pos i t ions and printed c i rcu i t board t rack

layout is shown in Figure 4.2. Construct ion of the projec t

is s traightforward: first fit the res i s to r s R l to R3 (so lder

and crop as you go) ; next insert the two prese t s , then fit

both printed c i rcui t board pins from the t rack side us-

ing a hot soldering iron to push them home. Now fit the

IC socke t and t rans i s to r T R I . Note that the t rans i s tor

package is not the same as the legend on the PCB — see

Figure 4.1 for pinout detai ls . Next, identify the polari ty

of each LED. Hold the LED with the ca thode towards you

( the ca thode is the shor t e r lead and adjacent to the flat

on the lower side of the package) , then with the aid of

long-nosed pliers bend the leads downwards through 90

degrees at a point approximate ly 5 mm from the body

TR1 B C 5 4 8

TOP view

H t

i e. c i

LM3914

PIN VIEW

D1-3 red D4-7 orang* 08-10 green

TIL 209

RV1

BOARD OUTLINE

Figure 4.1 Ci rcu i t diagram

111


Figure 4.2 PCB

( s ee Figure 4 . 3 ) . Each LED is inser ted from the compo-

nent side of the PCB then soldered in posi t ion to c r ea t e

a line of LEDs at the same d is tance from the edge of the

PCB. Fit in the following order: D1-D3 red, D4-D7 orange,

D8-D10 green. Lastly insert ICI into its socke t .

112

Car battery monitor

Figure 4.3 LED lead forming

The next j o b is to drill the holes in the box. Cover the

box with masking tape, as this helps with marking out

the holes and prevents sc ra tch ing the box, and it a lso

provides a non-slip surface to prevent the drill bit mov-

ing. See Figure 4.4, for hole posi t ions . After having drilled

all the holes , the PCB can be fitted into the box using

two M3 χ 16 mm sc rews , with two M3 χ V4 inch space r s

under the PCB to posi t ion it at the c o r r e c t height, and

the PCB secured with M3 nuts ( s ee Figure 4 . 5 ) . The zip

wire should now be soldered to the veropins ; fit the Ρ

clip to the zip wire and secure it to the M3 χ 16 mm screw

using a s econd M3 nut. Having fitted the zip wire, insert

the fuse holder in the posi t ive (+) supply line as c lo se to

the ba t te ry as poss ib le ; s ee Figure 4.6 for a s sembly in-

s t ruc t ions . The fuse is included to pro tec t the ba t te ry

wiring in the event of a shor t c i rcui t . The unit is now

comple te and ready for ca l ibra t ion.

113


Zip wire exit hole'

12 ,

γ — I

Bock

20

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(insiovTbox)

Hole dato: A - # 3mm Β *• # 3.5mm C « 0 4.5mm

Figure 4.4 Box dr i l l ing details

12

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In mm

IS

Back Track Component

side side

Lid Screws

Figure 4.5 Box and PCB assembly

<-Solder C U M to wires - »

Λ Λ - φ H=»HHHH |frJ«=fttttttt

t t Fuse Spring

Figure 4.6 Fuse holder assembly

114

Car battery monitor

Calibration

Connect the ba t te ry moni tor to a fully charged bat tery ,

or preferably use a var iable vol tage bench PSU set to

14 V. If this is not poss ib le c o n n e c t a ba t te ry charger to

the charged ba t te ry and switch it on to ensure that a

real 14 V level is achieved (maximum output from a ca r ' s

charg ing/ba t te ry sys tem while running is 14 V, not 12 V ) .

Take note — Take note — Take note — Take note

Connecting the battery monitor to the supply the

wrong way round will result in permanent damage

to ICI!

Set your mult imeter to the 2 V range, connec t the com-

mon (b l ack ) lead to 0 V, and the posi t ive ( red) lead to

pin 8 of ICI . Using a screwdriver , adjust RV2 until the

vol tage on the mult imeter reads 1.9 V. Remove the me-

ter leads, then adjust RV1 until the top end green LED

lights. The ba t te ry moni tor is now ca l ibra ted . All that is

left to do now is to sc rew the back cover on to the box,

and fit it into your car . Quickst ick pads are supplied with

the kit to mount the box onto the dashboard if required,

and r emember to s e c u r e the wiring away from hot or

moving parts using cab le t ies (order code B F 9 1 Y ) as ac-

c i d e n t s c an be e x p e n s i v e if no t d a n g e r o u s . Happy

motoring!

115


T a k e n o t e — T a k e n o t e — T a k e n o t e - T a k e n o t e

I n c e r t a i n i n s t a n c e s t h e r e i s a n a p p a r e n t p o s -

s i b i l i t y o f d a m a g e t o t h e L M 3 9 1 4 I C i n s o m e

v e h i c l e s , d u e t o h i g h v o l t a g e s p i k e s b e i n g

p r e s e n t o n t h e s u p p l y l i n e * T h e o p t i o n a l 1 5 V

z e n e r d i o d e m a y b e f i t t e d a c r o s s t h e s u p p l y

p r i n t e d c i r c u i t b o a r d p i n s a s s h o w n i n F i g u r e 4 . 7

t o m a k e s u r e t h i s d o e s n o t o c c u r .

Figure 4.7 Adding zener diode protection to the module

L O O O O O O o> m no Γι% Γ\Λ ηκ ne 2D1 D2 D3 D4 D5 D6

PCB

116

Car battery monitor

Car battery monitor parts list

Resistors — 0.6 W 1% metal film

117

RI 27 k 1 (M27K)

R2 lk2 1 (M1K2)

R3 15 k 1 (M15K)

RV1 10 k hor e n d prese t 1 (UH03D)

RV2 47 k hor e n d prese t 1 (UH05F)

Semiconductors

D l - 3 mini LED red 3 (WL32K)

D4-7 mini LED orange 4 (WL34M)

D 8 - 1 0 mini LED Green 3 (WL33L)

T R I BC548 1 (QB73Q)

ICI LM3914 1 (WQ41U)

Miscellaneous

batt mon PCB 1 (GA19V)

DIL socke t 18-pin 1 (HQ76H)

box 301 1 (LL12N)

zip wire 3 mtrs (XR39N)

Ρ clip 78 in 1 ( JH21X)

M3 χ V4 in s p a c e r 1 pkt (FG33L)

M3 χ 16 mm sc rew 1 pkt ( JD16S)

M3 nut 1 pkt (BF58N)

quicks t ick pad 1 strp (HB22Y)


in-line fuse holder

1 74in 100 mA fuse

c o n s t r u c t o r s ' guide

1 mm PCB pins

instruct ion leaflet

15 V 1.3 W zener diode

(RX51F)

( W R 0 8 J )

(XH79L)

pkt (FL24B)

(XK10L)

(QF57M)

All of the above avai lable as a kit

ba t te ry moni tor kit (LK42V)

118

5 Car digital tachometer

In t hese days of ever-higher motoring c o s t s the unit de-

sc r ibed here will help the driver to change gear at the

most advantageous point to save fuel and extend engine

life. Anyone using a car to tow a t rai ler or caravan will

a lso benefit by being able to make the bes t use of the

torque avai lable from the engine.

Conventional t achomete r s give a display of engine speed

on a mil l iammeter, usually with a s ca l e of about 270° a rc .

Pulses produced by the act ion of the con tac t b reakers

are integrated and fed to the meter to give an analogue

display of engine revolutions. The disadvantages are that

(1 ) an average reading is displayed, which can easi ly lag

behind rapid speed changes , and ( 2 ) meters tend to be

somewhat fragile.

119


The t achomete r descr ibed here ove rcomes both of these

disadvantages by counting pulses and displaying engine

revolut ions over a very shor t t ime, as the digital display

is cont inuously updated. Two digits display the number

of revolut ions χ 100, hence a display of 99 would cor re-

spond to 9,900 r.p.m. On the o ther hand, as the unit only

has a two digit display, the reading could be in er ror by

as much as 100 r.p.m. compared with true engine speed.

The unit is designed for negative earth ca r s , and if you

are not sure of the polari ty of your car , a glance at the

owners manual or even at the ba t te ry connec t ions will

tell you.

Circuit

The comple te c i rcui t is shown in Figure 5 .1 . Pulses pro-

duced by the make-and-break action of the engine contac t

breaker points are fed to ICla , which is a dual Schmit t

t r igger m o n o s t a b l e , via a r e s i s t o r / c a p a c i t o r ne twork

composed of R l , R2 and CI . This network helps to smooth

out any high vol tage spikes which may be present on the

con tac t breaker pulses . The zener diode ZD1 limits the

input pulse at IC la to 4.7 vol ts , to avoid any damage to

the device . To prevent any false triggering due to con-

tact point bounce (produced when the points do not open

and c lose c leanly) the monos tab le period is se t to 3 mil-

l i seconds by R3 and C2, after which it is ready to be

retr iggered by the next pulse, and this t ime period al-

lows for the maximum count for a 4-stroke, 4-cylinder

engine of 10,000 r.p.m. — a speed not often at tained on

normal road cars! The maximum count of 10,000 r.p.m.

(- 100 r.p.m.) co r responds to 20 ,000 pulses /minute and

the t ime for 1 pulse is 60 /20 ,000 s econds or 3 ms. A high

120

Car digital tachometer

121

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engine speed would the re fo re not al low enough t ime

between pulses for triggering the monos tab le . This de-

sign is for 4-cylinder ca r s only and anyone using it on a 6

or 8-cylinder car would have to modify the count period

accordingly, or use a compensa t ion factor on the read-

ings — not easy while driving!

The output pulses from ICla , pin 12, are fed to the count

input, pin 11, of IC2. This is a 4 digit coun te r with both

latch and rese t . It drives a mult iplexed 2 digit display

directly, with t rans is tors TRI and TR2 select ing the digit,

and res i s to r s R 4 - R 1 0 limiting the segment current . The

counte r requires latch pulses in order to give sens ib le

readings and these are provided by IC3, TR3 and their

a s soc ia t ed componen t s . IC3 is the ever useful 555 , used

as an osc i l la tor whose f requency is cont ro l led by RV1.

The osci l la tor output waveform, arranged such that there

is a long high and a shor t low period, is inverted by TR3

so that a shor t high period is achieved . This shor t pulse

is used to cont ro l the latch of the coun te r device IC2, so

that when this input goes high the information in the

counter is transferred to the internal latch and displayed.

The trailing edge of this shor t pulse is used to tr igger

the monos tab le IClb , whose output pulse is used to re-

set the counter so that it will s tart counting from 00 again.

The use of a separa te monos tab le to rese t the counter

ensures that the rese t pulse always occurs after the latch

pulse and a true reading displayed.

B e c a u s e the supply vol tage of a car , nominally 1 2 - 1 4 V,

varies between these limits with engine speed, integrated

circui t IC4 is used to derive a regulated 5 V supply from

this , which is then used to supply ICI , IC2 and IC3 and is

important for the s tabi l i ty of the osc i l la tor ( IC3) . Diode

D2 and capac i to r C5 remove any noise on the supply.

122


Construction

T h e Digital T a c h o m e t e r is cons t ruc t ed on two PCBs; the

main board and the display board. The display board is

mounted at 90° to the main board by being soldered to

Veropins , and this holds the display so that it can be

viewed through the cut-out display window at the end of

the ca se . Figure 5.2 shows the cons t ruc t iona l detai ls .

First j ob , however, is to build up the main printed cir-

cuit board. Referring to Figure 5.3, begin by fitting the

smal les t componen t s first. Check the polari ty of C3, C5,

and the direct ion of Dl, D2 before fitting, then work your

way through the componen t s by size fitting C5, the larg-

est , last . Insert the ICs into the appropr ia te socke t s only

Hain Board

Figure 5.2 Preparing the veropins for attaching main and

display PCBs together

123


Figure 5.3 Digi ta l tachometer main PCB and legend layout

after all o ther cons t ruc t ion of the t achome te r module is

comple ted , taking the usual precaut ions with CMOS de-

v ices . Note that the negative end of C5 must be c lo se to

the PCB or you may find that adjusting RV1 is difficult

during cal ibrat ion!

Display board

Refer to Figure 5.4. First mount res i s to r s R4, R5, R9, RIO,

and the veropins from the component s ide, being care-

ful not to s t r ip the pads off the PCB in the p rocess ! A hot

soldering iron will help to push the pins home. Don't

124


forget to fit the wire link ( this can be made from an off-

cut from a res is tor , or with single-strand wire) . Solder

and crop the res i s to r s and the wire link. Next fit the dis-

play to the PCB; pins 1 to 9 are on the side where the

decimal point will be found, and pins 1 and 18 are marked

on the PCB. Solder and c rop pins 1 to 18. Now measure

the required length for the display board pins by offer-

ing the display board up to the main PCB, 3 - 4 mm is

about right; s ee Figure 5.2. If the pins are too shor t , the

connec t ions won't be mechanica l ly s trong. After trim-

ming the pins down you can solder the display board to

the main PCB. The pins on the display board also require

soldering, and if this has a l ready been done, you may

find that the two boards do not marry snugly to each

other .

Figure 5.4 D isp lay board layout and legend

Use m a i n s c o n n e c t i o n wi re for p o w e r s u p p l y and

sc reened wire for the input signal so ldered to the three

pins. Sc reened cab le is used to s top the emiss ion of RF

interference, and the outer screening must be connec ted

to earth, preferably at the HT coil end. Label the func-

tion of each wire at the end that will connec t to the car

e l ec t r i c s . If you are going to use the optional box, the

front panel of the c a s e is rep laced by a p iece of red filter

125


cut to size (using the original panel as a t empla te ) with a

pair of s c i s s o r s or craft knife. This s lo ts neat ly into the

case , which is moulded in two sec t ions . As you may have

not iced, there is no method of mounting the t acho mod-

ule into the suggested box, so the a l ternat ive is to use

qu icks t ick pads. The sugges ted box a lso needs to be

modified by removing part of the ba t te ry compar tment ;

only the top and front part i t ions of this need to be re-

moved, the s ides will help to keep the PCB centra l in the

box, and the sc rew holes must remain intact or e lse the

box cannot be fastened toge ther ( s e e Figure 5 .5 ) .

12mm, 30mm

Cut out shaded area of battery compartment wall to leave box fixing holes.

This will allow the PCB, pins and large capacitor

to pass through.

Figure 5.5 Box modification details

126


Setting up

One advantage of a digital t a chome te r over an analogue

type is the ease of setting-up and cal ibra t ion. Only one

adjustment to RV1 needs to be made and, barring acc i -

dents , will prevail for the life of the unit. This set t ing

ensures that the osc i l la tor runs at the c o r r e c t f requency

and the method of cal ibrat ion depends on the equipment

available. Calibration against another t achomete r is pos-

s i b l e , s e t t i ng RV1 to give a d i sp lay of 30 when the

s tandard t achome te r reads 3000 r.p.m. If you have ac-

c e s s to a signal genera tor , se t the frequency to 100 Hz

(s ine or squarewave) and the output level to maximum

(more than 4.7 V ) . Connect this signal to the I/P pin on

the PCB, and again this will s imulate an ignition pulse

train input of 3000 r.p.m.

Alternatively cal ibra t ion can be carr ied out against the

mains frequency by using a t ransformer and bridge rec-

tifier to provide a 100 Hz signal as shown in Figure 5.6,

Signal to Tachometer

OV

Figure 5.6 Mains frequency doubler for cal ibrat ion

127


and a ba t te ry charger is very effective in this role . In

e i ther c a s e RV1 is adjusted to give a display reading of

30 . Calibration should include a tes t run for up to half an

hour or so for warming up and s tabi l isa t ion, whereafter

it might be noted that RV1 requires further fine tuning.

When you are satisfied with the ca l ibra t ion of the coun-

ter, RV1 should be fixed in posi t ion with wax, paint or

nail varnish.

Fitting the unit into the car

After cal ibrat ion, the unit is ready to be fitted to the car .

It is imposs ib le to give detai led ins t ruc t ions for every

car but the following notes may be helpful.

• it is a good idea to try the unit in various posi t ions

for bes t readabil i ty, using adhes ive tape, until you are

satisfied,

• having decided on the bes t posi t ion use double-

sided tape, adhesive pads or two p ieces of velcro- tape,

one glued to the unit and one glued to the car dashboard .

All of t hese methods , of course , mean that the unit can

be removed easi ly and the dashboard c leaned and left

unmarked,

• al ternatively, use self-tapping sc rews through one

half of the c a s e into the dashboard . This works well, but

unless you can util ise exist ing sc rew holes you will be

left with holes in the dashboard if you decide to remove

the unit.

128


The th ree leads must pass into the engine compar tment

and it is important that they are p ro tec ted by a rubber

or plas t ic grommet . It may be poss ib le to squeeze them

through an exist ing cab le entry or you may have to drill

a new hole, but e i ther way make sure they are p ro tec ted .

Connect ion to the car e l ec t r i c s is fairly straightforward;

the t acho input lead is connec t ed to the CB terminal of

the HT coil , which can be identified by the lead from the

points and dis t r ibutor to the HT coi l . CB s tands for con-tact breaker, often marked with a ( - ) minus sign. The

supply would bes t be taken from the ignition switch via

a 100 mA fuse, so that the unit is swi tched off when the

ca r is not running. The eas ies t way of doing this would

be to follow the o ther lead from the HT coil (marked with

a (+) plus s ign) up to the bal las t r e s i s to r (if f i t ted), and

make the connec t ion to the o the r s ide of it, s e e Figure

5.7.


Not all ignition systems are the same so consult

your workshop manual before attempting to fit

the tacho. Also please remember that a car en-

gine compartment is a hazardous area — never

attempt to fit the tachof or anything else for

that matter, while the engine is running! Also,

secure all cables away from hot or moving parts !

Anchor them to existing wiring looms using ca-

ble ties.

129


To starter motor

solenoid switch

Γ 2 ί

Tachometer +/N connection here

Contact To breaker

distributor points

Tachometer CB connection here

Ballast resistor

Figure 5.7 Connecting tachometer to a typical ignit ion system

with contact breaker

130


Car digital tachometer parts list

Resistors — All 0.6 W 1% metal film

R l , 2 , 5 6 0 Ω

R3 100 k

R 4 - 1 0 150 Ω

R H 390 k

R12 1 k

R 1 3 - 1 5 10 k

RV1 100 k ver t encl p rese t

2 (M560R)

1 (M100K)

7 (M150R)

1 (M390K)

1 (M1K)

3 (Ml OK)

1 (UH19V)

Capacitors

CI 100 nF polyes te r 1 (BX76H)

C2 47 nF poly layer 1 (WW37S)

C3 1 μΡ 35 V tantalum 1 (WW60Q)

C4 2n2F mylar 1 (WW16S)

C5 1000 μΡ 16 V axial 1 (FB82D)

C6 10 nF 50 V disc 1 (ΒΧ00Α)

Semiconductors

ICI 74LS221

IC2 74C925

IC3 NE555

IC4 LM78L05ACZ

TR1-3 BC549

ZD1 BZY88C4V7

D2 1N4001

DY3 DD display type C

1 ( Y F 8 6 T )

1 ( Q Y 0 8 J )

1 (QH66W)

1 (QL26D)

3 (QQ15R)

1 (QH06G)

1 (QL73Q)

1 ( B Y 6 8 Y )

131


Miscellaneous

8-pin DIL socke t

16-pin DIL socke t

dig t acho main PCB

dig t acho display PCB

1 mm PCB pin

red display filter

cab le grommet

twin mains DS black

cab le single black

quickst ick pads

c o n s t u c t o r s ' guide


1 ( B L 1 7 T )

2 (BL19V)

1 (GA26D)

1 (GA27E)

1 pkt (FL24B)

1 (FR34M)

1 (LR47B)

3 mtrs ( X R 4 7 B )

3 mtrs (XR12N)

1 (HB22Y)

1 (XH79L)

1 (XK03D)

All of the above available as a kit

car digital t a chome te r kit 1 (LK79L)

Optional (not in kit)

small remote cont ro l box 1 (LH90X)

in-line fuse holder 1 (RX51F)

1 74 in 100 mA fuse 1 (WR08J )

ve lc ro mounts as reqd (FE45Y)

cab le tie-wrap 100 as reqd ( B F 9 1 Y )

132

6 Car lights-on warning indicator

If your car is not fitted with some kind of lights-on warn-

ing, the c h a n c e s are that you will at some t ime (if you

have not a l ready done so!) leave your lights swi tched

on. Murphy's law d ic ta tes that when you do so , your

a b s e n c e from the car will be of sufficient duration to

ensure that the ba t t e ry will be well and truly flat. Of

cou r se Murphy, not con ten t to do things by halves , will

ensure that it happens when you are late for some im-

portant occas ion and that the re is no one e lse around to

give you a push or a jump start!

Modern ca rs further aggravate the si tuat ion as many of

them, being fitted with e l ec t ron ic ignition or e l ec t ron ic

engine management sys tems, just plain refuse to be push-

star ted!

133


It is amazing that such mechanica l ly advanced cars of-

ten do not have a lights-on warning indicator of some

kind. To i l lustrate this , the pro to type was instal led in a

2.0 li tre petrol- inject ion Ford Sier ra Es ta te — despi te

being a Ghia, there was no lights-on warning device!

Various warning devices are avai lable, however, some

b e c o m e a nu i sance b e c a u s e they sound cont inuous ly

when the lights are de l ibera te ly left on. For ins tance ,

while the driver is waiting in the car at night, with the

engine swi tched off.

Some more sophis t i ca ted devices will not sound if the

lights are swi tched on again after the ignition has been

switched off, i.e. for parking lights. However, this fails

to warn the driver if he inadvertent ly knocks the light

switch on when leaving the car — as could be the c a s e

with many ca r s having the light switch stalk on the driv-

er ' s door side of the s teer ing column.

This lights-on warning indicator will emit a c lear ly audi-

ble buzzing sound when the car lights are left on, the

ign i t ion s w i t c h is tu rned-of f and t h e d r i v e r ' s d o o r

opened. In this manner the buzzer will only sound when

the driver is genuinely about to leave the car .

Now that you are thoroughly convinced that for the sake

of a few pounds, you need not be caught out in the fu-

ture, why not build this handy a c c e s s o r y (which the

manufacturer should have included as s tandard) and fit

it into your car? Enterprising readers may wish to offer

this add-on to friends, re la t ives and neighbours for a

sui table fee (don ' t forget to tell the tax man!). A personal

tale of woe and the a s su rance that, " / V e got one and it has stopped me from getting caught out againF is sure

to win a few favourable r e sponses .

134

Car lights-on warning indicator

Circuit description and operation

The circui t of the lights-on warning indicator is very sim-

ple , as can be s e e n from Figure 6 . 1 . However , it is

worthwhile to know how the c i rcui t ope ra tes as this will

help, should problems occur .

P I of the unit is connec t ed to the sidelight c i rcui t of the

car and provides power to the circuit only when the lights

are swi tched on. The sidelight c i rcui t is live when e i ther

s idel ights or headlights are swi tched on.

P2 is connec t ed to the a c c e s s o r y c i rcui t and when the

ignition switch is off, P2 is pulled low via res i s to r R3 (P3

is c o n n e c t e d to OV) . Diode Dl is forward b iased and

turns on t rans i s to r T R I via res i s to r R2. Note that the

internal r e s i s t ance of a c c e s s o r i e s ( i .e . r ad io -casse t t e )

may be sufficiently low to make the connec t ion to P3

unnecessary; this can be determined by experimentat ion.

P6 is connec t ed to the dr iver 's door switch, thus when

the door is opened, a comple te path to 0 V is provided

by the door switch, allowing the buzzer to sound.

P I

ο

J L LN4001 ^ R3 ^

Γ Ρ" P3 P6


135


When the ignition switch is on, P2 is pulled high, reverse

biasing diode D l . Res is tor R3 ensures that t rans is tor TRI

is held in the off s ta te . The posi t ive supply to buzzer

BZ1 is removed and thus prevents it from sounding, re-

gardless of whether the driver 's door is open or shut.

When the lights are off and the car doors are c losed , the

polari ty of the supply to the unit is effectively reversed .

Diode D2 prevents damage to the circui t under this con-

dition.

Construction

Assembly of the unit is s implici ty itself. Referring to Fig-

ure 6.2, it is advised that the PCB pins are fitted first,

followed by the res i s to r s and the diodes and finally the

t rans is tor . Make sure that the t rans i s to r is fitted fairly

c lo se to the PCB otherwise the PCB will not fit into the

ca se .

Next solder the buzzer 's wires to the PCB pins, red (+V)

to P4, black ( - V ) to P5. Attach the connec t ing wires to

the PCB pins and label the free ends so that you can iden-

tify the wires after the PCB has been fitted into the case !

The PCB simply lies in the case , the wires protruding

through the aper ture provided. Screw the c a s e toge ther

Figure 6.2 PCB legend and track

136


and affix the buzzer onto the lid of the c a s e using one of

the double-sided adhes ive pads. The o ther pads can be

placed onto the unders ide of the c a s e ready for fitting

into the car .

Although it is unlikely that there will be any problems

with the unit, it is advisable to tes t it before fitting into

the car . It is eas ie r to take remedial ac t ion on the work

bench than underneath the car dashboard! Using a 9 to

14 V supply ( i .e . PP3-sized bat tery , ba t te ry el iminator ,

e t c . ) connec t P3 and P6 to 0 V, then connec t PI to +V,

the buzzer should sound. Connect P2 to +V as well, this

should s i l ence the buzzer.

Refer to Figures 6.3, 6.4, 6.5 and 6.6. It is n e c e s s a r y to

gain a c c e s s to the ca r ' s wiring, which will undoubtedly

involve removing the unders ide of the dashboard , trim

Installation

D r i v e r s d o o r s w i t c h

i n l i n e f u s e

• C h a s s i s c o n n e c t i o n

m a y n o t b e r e q u i r e d

S e e t e x t

Figure 6.3 PCB connections

137


panels , e t c . It is advisable to refer to a workshop manual,

e.g. of the Haynes variety; if you do not have one, e i ther

buy one — as it will be useful anyway, or borrow one

from your local library. A workshop manual will a lso help

you to ascer ta in the c o r r e c t wires to connec t to — oth-

erwise it will be a c a s e of t racing the c o r r e c t wires with

a mult imeter .

Take note - Take note - Take note - Take note

Disconnect the car battery before making con-

nections to the wiring* Connections to existing

wiring can be made using snap lock connectors or

terminal blocks of adequate current rating —

remember the lights-on unit draws very little

current, but two 55 W headlamp bulbs draw con-siderably more ! Ensure that the new wiring will

not become entangled with any controls, espe-

cially the brake pedal and steering column. To

prevent short circuits, make sure that all con-

nections are properly insulated, use adhesive

electrical tape.

Connect P I , via a fuse and fuseholder, to a point in the

wiring which b e c o m e s l ive when t h e s i de l i gh t s a re

swi tched on (Figure 6 .4 ) .

Connect P6, to the driver 's door switch (Figure 6 .5 ) . To

prevent o ther doors from operat ing the buzzer, install

an MR751 diode in se r ies with the wire to the cour t e sy

light.

138


139

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Connect P2 to a point in the wiring which b e c o m e s live

when the ignition switch is turned to accessory, i.e. +V

supply to the radio (Figure 6 .6 ) . Alternatively, if there is

no accessory posit ion, connec t P2 to a point in the wir-

ing which becomes live when the ignition switch is turned

to ignition.

Connect P3 to the ca r ' s chass i s (0 V) or to a point in the

wiring which is permanently connec ted to the ca r ' s chas-

sis . Note that this connec t ion may be unnecessa ry if the

internal r e s i s t ance of any a c c e s s o r y is sufficiently low.

This may be ascer ta ined by test ing the unit with P3 left

unconnec ted and all a c c e s s o r i e s switched off. If in doubt

connec t P3 as previously desc r ibed .

Double-check connec t ions , r e connec t the car bat tery .

COURTESY LIGHT

+ 1 2 V Permanent

supply O -Fuse

- 0 — o ^ c Door

Of f r N.C. ι Ο η Ο η

Drivers I — ° °~ door switch

Other d o o r switches

4 Γ

Cut wire and insert diode

^ i ( M R 7 5 1 )

To P6 of Lights on PCB

Figure 6.5 Typical ignit ion switch c i rcu i t and connections

140


IGNITION SWITCH

fc Permanent (~"^+12V supply

Start To starter solenoid

Fuse Ignition Ignition switched 5 + 12V supply

V Off

Accessory To P2 of lights on PCB

Car 12V

Battery

I JCTJ Radio

Figure 6.6 Typical ignit ion switch c i rcu i t and connections

Φ switch lights on, leave ignition swi tched off and

open the driver 's door; the buzzer should sound,

• with the driver 's door shut, opening any other door

should not cause the buzzer to sound,

• with the ignition swi tched to accessory or ignition, opening the driver 's door should not cause the buzzer

to sound,

• with lights turned off, the buzzer should not sound

with any combina t ion of ignition swi tch pos i t ions or

doors open or c losed .

Assuming the unit is working cor rec t ly , refit underside

of dashboard and trim panels . Happy motoring.

Testing

141


Lights-on warning parts list

Resistors — all 0.6 W 1% metal film

R l 3k9 1 (M3K9)

R2 10 k 1 (M10K)

R3 100 k 1 (M100K)

Semiconductors

Dl 1N4148

D2 1N4001

T R I BC327

MR751

1 (QL80B)

1 (QL73Q)

1 (QB66W)

1 (YH96E)

Miscellaneous

BZl 12 V buzzer

P l - 6 1 mm PCB pins

1 V4 in in-line fuseholder

1 74in 100 mA fuse

PCB

mini box and base

quickst ick pads


c o n s t r u c t o r s ' guide

All the above available as a kit

lights-on warning kit

1 (FL40T)

1 pkt (FL24B)

1 (RX51F)

1 ( W R 0 8 J )

1 (GE88V)

1 ( J X 5 6 L )

1 strp (HB22Y)

1 (XT11M)

1 (XH79L)

1 (LP 7 7 J )


16/0.2 wire as req (FA26D-

FA36P)

snap lock cab le c o n n e c t o r as req ( J R 8 8 V )

5 A terminal b lock as req (HF01B)

142

7 Courtesy lights extender

How many t imes have you got into your car of a night

only to find the ignition switch has gone for a walk around

the dashboard , as your a imless efforts to s tar t the ca r

only result in the ignition key gouging severa l grooves

into the p las t ic?

This pro jec t keeps the interior light on after the car door

has been c l o s e d , allowing t ime to find keys , ignit ion

switch, or even your way out of the garage!

Circuit description

Figure 7.1 shows the circui t diagram of the cour tesy light

extender. PI and P3 connec t directly ac ross a door switch

143


PI

o-

TR1 TIP122

D Rl 4k7 Dl

1N4148

R2 560k

D2

9 1N4148

4k7 1

TR2 MPSA14

9 cb ci •i 47uF

P3 O-


control l ing the inter ior light and P2, to a sou rce that has

power while the ignition is on.

With a door open, PI and P3 are effectively shorted, caus-

ing capac i to r CI to d ischarge through diode D l . As soon

as all doors are c losed , as capac i to r CI is d ischarged,

t rans i s tor TR2 is turned off. Res i s to r R l pulls the base

of T R I high, turning it on and causing current to flow

through the cour tesy light. Capaci tor CI now s ta r t s to

charge through res i s tor R2 until t rans is tor TR2 turns on,

pulling the base of t rans i s tor TRI low and halting the

flow of current through the inter ior light. B e c a u s e ca-

paci tor CI charges through res i s to r R2 and the cour tesy

light, it can be seen that the wattage of the inter ior light

plays quite an important role in the t ime delay given.

144

Courtesy lights extender

Figure 7.2 shows typical t ime delays given at var ious

values of R2 for 5, 10, 15 and 20 watt cou r t e sy l ights.

If, during the t ime delay given by the unit, the ignition is

turned on, capac i to r CI charges up very quickly through

res i s to r R3, turning the inter ior light off a lmost immedi-

ately. This avoids the poss ibi l i ty of driving away at night

with the cour t e sy light still on.

Construction

The pos tage s tamp sized printed c i rcui t board is of the

high quality, single-sided glass fibre type, s e e Figure 7.3.

The s e q u e n c e in which the componen t s are fitted is not

cr i t ical ; however, the following ins t ruc t ions will be of

use in making these tasks as straightforward as possi-

ble.

145

50-1 1 j , 1 1 1 1 1 j 1

0" ^^^ —ι » ι ι ι ι ι ι ι ι ι ι ι ι — τ- I t \ I I I I I I ' I I I

0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1M R2/0

Figure 7.2 Graph of R2 against time delay for various wattage

courtesy lamps


Insert and solder the PCB pins using a hot soldering iron.

If the pins are heated, very little p ressure is required to

press them into posi t ion. Once in place , the pins may

then be soldered. It is now easier to start with the smaller

components , such as the res is tors , work upwards in size,

and t rans i s tor TRI is fitted last .

The diodes should be inser ted such that the band at one

end of the diode co r re sponds with the white b lock on

the PCB legend. When fitting the e lec t ro ly t ic capac i to r ,

it is essent ia l that the co r r ec t polari ty is obse rved . The

negative lead of the capac i to r , which is usually marked

by a full-length s t r ipe and a negative ( - ) symbol , should

be inser ted away from the hole marked with a posi t ive

(+) sign on the PCB legend. Insert and solder the two

t rans i s tors , matching the shape of each c a s e to its out-

line on the legend.

Lastly, so lder lengths of wire to the veropins and mount

the PCB inside the box, as shown in Figure 7.4.

Installation

The cour t e sy light ex tender is ex t remely simple to fit.

However, for someone who is not familiar with automo-

tive e lec t r ica l installation it is advised that they seek the

advice of a qualified person before proceeding.

The re are two methods of switching the inter ior cour-

tesy light:

146


Figure 7.3 PCB legend and track

î Figure 7.4 Fit t ing unit into box


When carrying out any form of electrical work on

a vehicle always disconnect the battery and

never work inside the engine compartment with

the engine running!

147


• door swi tches are fitted to the 0 V side of the cour-

tesy light, for instal lat ion follow Figures 7 .5(a) or 7 .5 (b ) ,

• door swi tches are fitted breaking the +12 V supply

to the cour tesy light, for installation follow Figures 7 .5 (c )

or 7 .5(d) .

In its s implest configuration, the unit c o n n e c t s direct ly

ac ros s a door switch; PI connec t ing to the more posi-

tive side of the switch and P3 to the more negative.

If ignition override is required then P2 must be connec ted

to a sou rce that has power while the ignition is on (for

example, + SW terminal of the ignition co i l ) . If no easy

connec t ion can be made to the ignition circui t then P2

can be connec t ed into the accessory c i rcui t .

As the comple te unit is small and unobtrus ive it can eas-

ily be mounted inside a door-post , behind an exist ing

door switch. The box can be held in p lace using a self-

adhesive pad (such as HB22Y) or bol ted down using the

two mounting holes provided in the b a s e of the box .

Check behind panels before drilling any holes and en-

sure that no wiring harness or o ther componen t s are

located behind panels that would otherwise be damaged.

Min Typ Max Units

Operating voltage 10 12 15 V

Quiescent current at 12 V 3 mA

Maximum switching current 5 A

Table 7.1 Specif icat ion of prototype

148


Courtesy +12V

ü3

ht

Fueebox ® I I I I ι lp1 .

y S S J Extender

Courtesy

+ 1 2v LÎQht To ignition switch From QO * j · » 1 P1 (or other source of

Fueebox r-—if-1—ι +12V when engine

(b) P2 {

« running). / / / / o.S°°u Extender

+ 12V From f f 1 1 1 P 1

Fueebox ι „ 'r' ι

Courtesy Ught

β »ιΓ / / / / Extender

( ο m l _ V J L Courtesy ® Light

+12V To ignition swttch From • · 1 τ 1 P1 ( o r other source of

Fueebox r - — i f ' ι +12V when engine

°Ϊ5|Γ P ? "running).

<d) 1 1 1 1 _ r JL Courtesy ® Ught

Figure 7.5 Λ*Γ

(a) Simple connection for vehicles with negative switched

courtesy l ight

(b) Connection for negative switched courtesy l ight with ignit ion

override

(c) Simple connection for vehicles with posit ive switched

courtesy l ight

(d) Connection for positive switched courtesy l ight with ignit ion

override

149


Courtesy light parts list

Resistors — All 0.6 W 1% metal film

R l , 3 4k7 2 (M4K7)

R2 560 k 1 (M560K)

Capacitors

Cl 47 μΡ 16 V minelec t 1 ( Y Y 3 7 S )

Semiconductors

T R I TIP122

TR2 MPSA14

Dl ,2 1N4148

1 (WQ73Q)

1 (QH60Q)

2 (QL80B)

Miscellaneous

PI ,2 ,3 1 mm PCB pins 1 pkt (FL24B)

PC board 1 (GE81C)

min box and b a s e 1 ( J X 5 6 L )

cour t e sy light leaflet 1 (XK96E)

c o n s t r u c t o r s ' guide 1 (XH79L)

All of the above are avai lable as a kit

cou r t e sy light kit 1 (LP66W)


16/0.2 b lack hook up wire 1 (FA26D)

16/0.2 red hook up wire 1 (FA33L)

150

8 Car audio switched-mode psu


This project requires a fair degree of expertise and pa-

tience to build. Please read through the article thoroughly

before undertaking construction. The power supply design

described in the following article is intended to power two

Maplin 50 W bipolar amplifiers and a stereo pre-amplifier.

It is possible for the power supply to be used to supply

other amplifiers, however, it is outside the scope of this

article to detail the necessary modifications . The supply

is capable of delivering instantaneous power levels much

higher than the continuous rating, which is ideal for au-

dio applications where the peak current requirement, due

to transients, is much higher than the average current re-

quirement. Higher levels of power may be drawn as long as

the average power is maintained at 120 W. The figure of 120 W

is based on maintaining a heatsink temperature at less than

65°C.

151


For many years the motor is t has not been able to ben-

efit from hi-fi quality sound while travelling in the car .

For the long-distance traveller , bus iness execut ive or hi-

fi buff o n - t h e - m o v e , t h e c a r is a far from idea l

env i ronmen t for l i s ten ing to mus ic ; th i s is due to a

number of r easons . First, the ca r ' s inter ior is designed

for conveying passengers and not for ideal locat ion of

c o n v e n t i o n a l box design l o u d s p e a k e r s . S e c o n d , the

sound replay/receiving equipment has to be miniatur-

i s e d and c a p a b l e of o p e r a t i o n in a v e r y h a r s h

environment. Dashboard temperatures often exceed 60°C

in hot weather (yes , even in the English cl imate!) and

fall to several degrees below zero in cold weather . Vi-

brat ion and humidity also add to the s t r e s s e s that the

equipment must endure. Third, the low, noisy and some-

what v a r i a b l e supp ly vo l t age makes life even m o r e

difficult for the e lec t ron ic c i rcui t ry .

The environmental and size problems of the car envi-

ronment have largely been solved by c lever ly designed

equipment . Car loudspeakers are opt imised for opera-

tion in rear parcel she lves and door panels instead of

conven t iona l sea led or por ted e n c l o s u r e s . Car radio,

ca s se t t e , CD (compac t d i sc ) and DAT (digital audio tape)

equipment is very compac t . Such equipment is designed

for e i ther mounting in the dashboard /cen t re conso le or

remote mounting in the boot or under a seat , with just

the cont ro l s loca ted within the driver 's easy reach .

It is however, the third point that is the main reason for

this projec t , the vehic le e lec t r ica l supply. The 12 V e lec-

tr ical sys tem is far from ideal when it c o m e s to powering

audio amplifiers. The e lec t r ica l sys tem itself, although

general ly referred to as being 12 V, usually opera tes at

152

Car audio switched-mode psu

around 1 3 - 1 4 V when the engine is running. By conven-

tion, the vol tage when the engine is running is assumed

to be 13.8 V.

A singled-ended amplifier operat ing from a supply volt-

age of this ( low) level is capab le of delivering around

7 W r.m.s. into a 4 Ω load. If a BTL (br idge tied load)

amplifier is used the power output can be increased to

around 22 W r.m.s. into a 4 Ω load. Most high power ra-

d i o / c a s s e t t e players have an output power of around

22 W r.m.s., regardless of how many watts the advert is-

ing b rochures boast!

For hi-fi quali ty sound reproduct ion in a car it is neces -

sary to have the capabi l i ty of higher power levels . This

not being required for blowing out the windows (although

often used as such by drivers of ageing Ford Cort inas

with pink fluffy dice hanging from the inter ior rear-view

mir ror ) , but simply b e c a u s e a high power amplifier op-

erating at modes t power levels will in t roduce far less

dis tor t ion and handle t rans ien ts far be t t e r than a me-

dium power amplifier running a lmost flat out. This is

espec ia l ly true if the sound sou rce is CD, where the dy-

namic range of the recording is often very wide.

The re are two ways in which the output power can be

increased , by e i ther decreas ing the loudspeaker imped-

a n c e o r i n c r e a s i n g t h e s u p p l y v o l t a g e . T h e main

disadvantage of the former method is that ca r speakers

are not commonly produced with impedances below 4 Ω

and that power losses in cab le s are increased . The lat-

ter method of increasing the supply voltage is commonly

used in high power car boosters and in hi-fi ca r audio

amplifiers — this is the method that is desc r ibed here .

153


Circuit description

Figure 8.1 shows a b lock diagram representa t ion of the

power supply and Figure 8.2 shows the full c i rcui t dia-

gram.

The supply input to the power supply is via PI (+V) and

P2 (0 V ) . The power supply is connec t ed direct ly to the

vehic le ba t te ry via high current cab les , therefore the off-

board supply fuse FS1 is e s sen t i a l in c a s e of a fault

causing a shor t c i rcui t d i rect ly a c r o s s the bat tery . Re-

mo te power swi tch ing is a c h i e v e d by T R I , RL1 and

a s s o c i a t e d c o m p o n e n t s . T h e con t ro l input P3, when

taken to +V, b i a s e s T R I on and o p e r a t e s RL1 , thus

powering-up the rest of the supply. LD1 serves to indi-

ca t e power on. The cont ro l signal is provided by the

electric aerial output found on most radio-casset te units.

Diode Dl clamps the voltage spike produced by RL1 when

it de-energises . Diode D2 provides polari ty pro tec t ion

by blowing fuse FS2 and preventing the remote power

switch from operating.

Capaci tors CI , C2, C3, C4, C5 and toroid LI form the in-

put ji-filter, the output of which supplies the push-pull

output s tage. The power MOSFETs are arranged in two

pairs (TR2 and TR3) and (TR4 and T R 5 ) , each driving

one half of the t ransformer primary. Res i s to r R8 and ca-

p a c i t o r C6 form a s n u b b e r ne twork to i n c r e a s e t he

rise-time of switching spikes . Components ZD1, D3 and

TR2 and ZD2, D4 and TR5 form an ac t ive spike clamp,

employed to pro tec t the MOSFETs' d ra in / source junc-

t ions from high voltage switching spikes . This opera tes

by feeding the spike back into the gate of the relevant

154


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156


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MOSFET thus turning it on and clamping the spike. Gate

res i s to r s R4 to R7 help to ba lance current flow through

each MOSFET pair and a lso help to r educe switching

noise .

T l is a s tep up t ransformer compris ing six windings, two

connec t ed to form a cen t re tapped primary winding and

four are connec ted in two pairs to form two cent re tapped

secondary windings.

Components R36, C27 and C28 form a simple R-C filter

for ICI which a t tenuates supply borne noise . C29 and

R20 se t the soft-start t ime period for ICI . At switch on,

C29 is d ischarged and IC l ' s outputs are inhibited. As C29

charges via R20, the pulse width of the PWM drive sig-

nals are allowed to increase from zero. D14 prevents ICl ' s

soft-start input from being pulled negative at switch-off

and a lso se rves to d ischarge C29 more quickly. TR6 dis-

charges C29 and inhibits IC l ' s outputs in r e sponse to a

thermal shutdown condi t ion or a s tandby input ( low)

from P19. D18 and D13 form a d i sc re te AND circui t . When

the shutdown condition and s tandby inputs are removed,

TR6 allows C29 to charge again and the power supply

res ta r t s .

Res i s to r R21 and capac i t o r C31 se t the osc i l l a to r fre-

q u e n c y , P24 may be used to m o n i t o r t h e o s c i l l a t o r

waveform. Care should be exe rc i sed to ensure that this

pin is not sub jec t to undue capac i t ive loading, o therwise

the osc i l l a tor f requency will shift.

Res i s to rs R17, R18, R19 toge ther with capac i to r C30 form

a phase se lec t ive network that se t s the gain of the over-

vol tage amplifier. Phase compensa t ion is n e c e s s a r y to

ensure good loop stabi l i ty , o therwise the power supply

159


could break into osci l la t ion. Res i s to rs R15 and R16 form

a potential divider which is used to apply over vol tage

feedback to ICI, with the values as shown, the maximum

output vol tage is ±30 V.

T r a n s i s t o r s TR7 to TRIO and a s s o c i a t e d c o m p o n e n t s

form two high speed driver c i rcui ts which are able to

charge and discharge the gate capac i t ance of each of the

MOSFETs very quickly. Circuit operat ion for one of the

(two ident ica l ) drivers is as follows: R23 is the pull-up

res is tor for the open co l lec tor output of ICI (pin 8 ) . When

pin 8 goes low (output on) TR7 is b iased on by R25 (C34

se rves to inc rease switching speed) , D15 conduc t s and

TR2, TR3 turn on quickly. At this t ime TR8 is swi tched

off. When ICI pin 8 goes high (off) TR7 swi tches off and

TR8 base is pulled low; as the gates of TR2 and TR3 are

charged to a posit ive potential , D15 is reverse biased and

TR8 conduc t s . This act ion rapidly swi tches off TR2 and

TR3.

Integrated c i rcui t IC2 is a compara to r with its inputs

connec t ed to two potential dividers. Res i s to r s R31 and

R32 form a reference potential divider and thermis tor

TH1 and R30 form a tempera ture sensing network. R33

and D17 provide a large degree of hys te res i s when the

output changes s ta te . Normally the output from IC2 (pin

7) is high and the vol tage on pin 2 is around l/ 2 supply.

The vol tage on pin 3 is dependent on the r e s i s t ance of

TH1, governed by the heats ink tempera ture with which

it is in con tac t . As the tempera ture of the heats ink r ises ,

the r e s i s t ance of TH1 reduces and the vol tage on pin 3

inc rease . When the vol tage on pin 3 exceeds the vol tage

on pin 2, the output of IC2 goes low. LD2 il luminates in-

dicating thermal shutdown and the power supply shuts

160


down. At this point D17 conduc t s , this adds R33 to the

lower half of the re ference divider reducing the refer-

ence potential on pin 2 to around V3 supply (ignoring

D17 vol tage drop and sa tura ted output vol tage of IC2).

The vol tage on pin 3 will now have to fall below V3 sup-

ply before the circui t will rese t and the supply allowed

to res tar t . Correspondingly the r e s i s t ance of TH1 will

have to r ise and its t empera ture fall before supply op-

erat ion is resumed. With the circui t values as shown, the

trip tempera ture is 80°C and the rese t t empera ture is

60°C.

Diodes D5 to D8 form a bridge rect if ier (main output) ,

the devices used are high speed types , essent ia l for use

in switch mode appl ica t ions . Capaci tors C7 to CIO help

to reduce t rans ients and switching noise . Components

C i l , C12, L2, L3, C13, C14, C15 and C16 form jt-filter net-

works for the main outputs . Res i s to r s R9 and RIO serve

to provide a minimum load lor the power supply and also

d ischarge the filter capac i to r s quickly after switch-off.

Fuses FS3 to FS6 provide pro tec t ion against shor t cir-

cui ts and over loads . Posi t ive 30 V outputs are avai lable

from P4, 5, 6 and 7. Negative 30 V outputs are avai lable

from P12, 13, 14 and 15. Pins 8, 9, 10 and 11 provide a

zero volt return.

Diodes D9 to D12 form a s econd bridge rect if ier (auxil-

iary output) , again high speed types are used. Capaci tors

C17 to C20 help to reduce t rans ients and switching noise.

Capaci tors C21 and C22 are the reservoi r capac i to r s for

the auxiliary output. Res i s to r s R l l and R12 serve the

same purpose as R9 and RIO in the main output c ircui t ry .

Voltage regulators RG1 and RG2 regulate the supply rails

and a t tenuate switching noise on the auxiliary output.

161


Capaci tors C23, C24, C25 and C26 are decoupling capaci -

tors and ensure supply s tabi l i ty . Posi t ive and negative

12 V auxiliary outputs are avai lable on P16 and P18 re-

spect ively . P I 7 provides a 0 V return.

Construction

The PCB is of the single-sided glass fibre type, with a

printed legend to ass is t insert ion of the componen t s . To

inc rease the current rating of some of the t racks it is

n e c e s s a r y to tin the exposed areas of t rack on the un-

ders ide of the PCB. T h e s e t racks will be c lear ly seen as

they are not covered by the so lder res is t layer. Tinning

of the t racks should actual ly be the final a s sembly task.

Removal of misplaced componen t s can be very difficult,

espec ia l ly on a densely populated board such as this , so

p lease double check component type, value and orienta-

tion (where appropr ia te ) before insert ing and soldering

the component .

Referring to the following const ruct ional notes , the parts

list and Figure 8.3, begin const ruct ion. It is recommended

tha t t he following c o n s t r u c t i o n o rde r is adhe red to

c lose ly , o therwise it will be found ex t remely difficult, to

fit some of the componen t s .

Start by insert ing the th ree 22 SWG wire links, t hese are

indicated on the PCB by a single s t ra ight line and an

adjacent LK mark.

Next insert the 1N4148 signal diodes, ensuring co r r ec t

or ientat ion.

162


Insert 0.6 W metal film res i s to r s , but do not insert the

3 W wire wound res i s to r s at this s tage .

Bend and insert the four 16SWG wire links, t he se are

indicated on the PCB by a single s t ra ight line and an

adjacent LK number.

Next insert the 1N4001 diodes and the two 39 V zener

diodes .

Referring to Figure 8.4, loose ly fit the M3 power input

connec t ion hardware and solder the M3 nuts to the PCB

pads.

Insert the polystyrene capaci tors and the ceramic capaci-

tors .

Next insert the DIL socke t s , but do not insert the ICs at

this s tage.

Insert the 45 PCB pins into the holes for TR2 to TR5 and

D5 to D8; and pos i t ions marked with a c i rc le and a Ρ number. Do not insert pins into posi t ions marked with a

c i rc le and a W number.

Next insert the fuse c l ips . You should find that by care-

fully bending over the two legs on the t rack side of the

PCB before soldering, the fuse cl ips will remain straight .

Insert the BC337 and BC559 t rans is tors , ensuring co r r ec t

or ientat ion.

Next insert the tantalum capac i to r s , ensuring that the

co r r e c t vol tage rating capac i to r is inser ted in the cor-

r ec t loca t ion . Tanta lum c a p a c i t o r s are polar i sed and

must be co r r ec t l y or ienta ted, the plus (+) sign on the

body must be inser ted into the hole neares t that marked

with a plus sign.

163


Figure 8.3 PCB track and legend

164



165


M 3 I s o s h a k e

M 3 S o l d e r T a g

/ i M 3 N u t !

(Ttn S o l d e r

P C B

M 3 N u t t o P C B p a d .

Figure 8.4 Power input connection assembly

Form the l eadouts of the B Y W 9 8 rec t i f ie r d iodes , as

shown in Figure 8.5 and insert t hese into the PCB. En-

sure that the ca thode lead, which is indicated by a band

around the componen t body is inser ted into the hole

neares t that marked with a k sign.

C a t h o d e

b a n d

Figure 8.5 Lead formation for BYW98 rect i f iers

166


Insert the 0.1 μΡ poly layer capac i to r s and the small e lec-

t r o l y t i c c a p a c i t o r s . T h e e l e c t r o l y t i c c a p a c i t o r s a re

polar ised and must be co r r ec t l y or ienta ted, the negative

( - ) s t r ipe on the capac i to r can must be inser ted into the

hole furthest away from the hole marked with a plus (+)

sign.

Drill the heats ink as shown in Figure 8.6. Form the leads

of the BUZ11 MOSFETs and the BYW80 rectifiers as shown

in Figures 8.7 and 8.8. Assemble the heats ink a s sembly

81 m m

3 . 5 m m D i a .

51 m m

Γ Τ

E X I S T I N G H O L E S - U N U S E D E X I S T I N G ^ H O L E S -D R I L L — O U T T O D R I L L - O U T T O

3 . 5 m m D i a . 3 . 5 m m D i a .

Figure 8.6 Heatsink dr i l l ing information

Figure 8.7 Lead formation for BUZ11 MOSFETs

167


Figure 8 .8 Lead formation for B Y W 8 0 r e c t i f i e r s

using the M2.5 hardware as shown in Figure 8.9 and Photo

8.1. Solder the leadouts of the t rans i s to rs and rect if iers

to the PCB pins. Referring to Figures 8.10 and 8.11 and

Photo 8.1, connec t the sc reened cab le to the thermis tor

and the PCB pins, use heat shrink sleeving where neces -

sary to avoid shor t c i rcu i t s . Glue the thermis tor to the

heats ink using some epoxy resin. Hold the thermis tor in

place whilst the resin se t s .

M2.5 χ 12mm Bolt

Heatsink

Insulat ing b u s h

BUZ1 1

c

Insulat ing w a s h e r

M2.5 Nut

Figure 8 . 9 Assembly of heatsink components

168


Phote 8.1 Close-up of heatsink assembly

Twist a n d s o l d e r c o n n e c t i o n s

s c r e e n C P 16 Heat—shr ink T h e r m i s t o r

P C B Pins

Figure 8.10 Thermistor connection

169


Figure 8.11 Wiring to switching PSU

Insert the two regulators , ensuring that the c o r r e c t type

is fitted in the co r rec t locat ion and that the package lines

up with the outline marked on the legend. Ensure that

the two metal t abs do not touch, see Photo 8.2.

Referring to Figures 8.12 and 8.13 extend the leadouts of

the 3 W res i s to rs and axial inductors and insert t hese

into the PCB, see Photo 8.2.

Referring to Figure 8.14 wind 2l/2 turns of two lengths of

16 SWG EC wire wound bifilar (s ide by s ide) around the

toroid co re . Prepare the ends of the EC wire to facil i tate

soldering and insert this inductor into the PCB at the

170

R R R R P P P P © ™ 1

I II 11 ~|I I I II 11 ΙΓΊ P26

Main s^J1-*) L J \P27 Outputs ( V ] C K LeJ^ ' _J

to power >—»* V - > * r _ I _a β P « C

. Γ^ρ^ <B Q J^T^K^^ Thermal shut down

H o £ Tp P " 9 | P 3 & 2 " | d P o w e r 0 n

+ 12V - 1 2 V Standby Π 15A OV

, nP

ut LJFuse

Auxiliary Oscillator I er ι ι Outputs test OV 13.8V

to pre—amp point » , > O n / O f f Battery control supply Input Input


Photo 8.2 Close-up of the regulators, resistors and inductors

posit ion marked L I , s ee Photo 8.3. It is helpful to smear

the windings and toroid co re with si l icon rubber sealant

to prevent the a s sembly from rattling.

C u t o f f

e x c e s s w i r e

W r a p w i r e a r o u n d

l e a d o u t Sc s o l d e r

2 0 S W G

T C w i r e

Figure 8.12 Extending leadouts of axial inductors

171


T C w i r e

Figure 8.13 Extending leadouts of axial resistors

2 x 2 / 2 t u r n s o f 1 6 S W G

E C w i r e w o u n d b i f i l a r

o n F X 4 - 0 5 4 t o r r o i d

S m e a r s i l i c o n

r u b b e r s e a l a n t

o v e r w i r e s a n d

t o r r o i d t o h o l d

i n p l a c e

R e m o v e e n a m e l f r o m

w i r e e n d s u s i n g

e m e r y p a p e r t o

f a c i l i t a t e s o l d e r i n g

Figure 8.14 Li winding information

172


Photo 8.3 Close-up of the toroid inductor f i t ted into the PCB

Next insert the power relay.

Insert the large SMPS e lec t ro ly t ic capac i to r s , ensuring

that the co r r ec t vol tage rating capac i to r s are fitted in

the c o r r e c t loca t ions and are c o r r e c t l y or ien ta ted as

previously desc r ibed .

Referring to Figure 8.15 wind the t ransformer , this is

p robably the most difficult part of the cons t ruc t ion pro-

cedure and should not be rushed. Note that the diagrams

do not figuratively show the required number of turns

per layer. When winding the t ransformer take ca re not

to over s t r e s s the bobb in o the rwi se pins may break

173


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Photo 8.4 The component parts of the ferr i te transformer

Cover each layer with a single layer of masking tape.

Starting at pins 13 and 11, wind bifilar two 9 turn windings

of 18SWG EC wire first, finish at pins 3 and 5 respec-

tively, as shown in Figure 8 .15 (a ) .

S ta r t ing at pins 16 and 15, wind bif i lar two 26 turn

windings of 20 SWG EC wire in two layers ; first wind

bifilar 13 turns, as in Figure 8 .15 (b ) . Wind bifilar a fur-

ther 13 turns and finish at pins 10 and 9 respect ively , as

in Figure 8 . 1 5 ( c ) , note the wires c ros s over. Check for

cont inui ty between pins 16 and 10, and 15 and 9.

176

off — use pliers to carefully bend the wire around the

pins. It will be nece s sa ry to remove the enamel coat ing

from the wire to allow soldering, emery paper is ideal

for this . Photo 8.4 shows an exploded view of the com-

ponent parts of the t ransformer .


Starting at pins 1 and 2, wind bifilar two 15 turn windings

of 20 SWG EC wire in two layers; first wind bifilar 13 turns,

as shown in Figure 8 .15(d) . Wind bifilar a further 2 turns

and finish at pins 7 and 8 r e s p e c t i v e l y , as in Figure

8 .15 (e ) , note the wires c r o s s over . Check for cont inui ty

between pins 1 and 7, and 2 and 8.

Solder all of the leadouts to the t ransformer bobbin pins,

fit the c o r e s and clip into p lace the sprung s tee l co re

re ta iners .

Insert the t ransformer into the PCB, ensuring that pin 1

aligns with the number 1 on the PCB.

Referring to Figure 8.11 and using the 32 /0 .2 power con-

nect ion wire, link Wnumber holes ; W l to W l , W2 to W2,

W3 to W3.

Finally tin the exposed lengths of PCB t racks with a thick layer of solder . Take ca re not to splash solder e l sewhere

which may cause shor t c i rcu i t s .

Double-check your work and remove e x c e s s flux from the

underside of the PCB using PCB c leaner . Photo 8.5 shows

the assembled PCB.

Connect the two LEDs to the PCB via lengths of insulated

wire as shown in Figures 8.11 and 8.16.

Testing

Figure 8.11 shows the locat ion of the input and output

connec t ions referred to in this sec t ion .

177


Photo 8.5 The assembled PCB

Fit ICI, IC2 and the fuses.

Using a mult imeter on a sui table r e s i s t ance range, meas-

ure the r e s i s t ance between FS7 and P2, the r e s i s t ance

should be greater than 2 kQ. Check also the r e s i s t ance

between FS2 and P2, the r e s i s t ance should be greater

than 2 kQ. If significantly lower readings than s ta ted are

measured, r echeck all of your work as there is likely to

be a shor t c ircui t or a misplaced component .

178


F l a t o n p a c k a g e

S h o r t l e a d

C a t h o d e A n o d e

C P 3 2 H e a t - s h r i n k s l e e v i n g

\

S l i d e o v e r c o n n e c t i o n s a n d s h r i n k .

R e s i s t o r

T w i s t a n d s o l d e r Λ

c o n n e c t i o n s

C a t h o d e

A n o d e

Figure 8.16 LED leadout ident i f icat ion and connections

Connect a 12 V supply capab le of delivering 5 A to the

input pins PI (+V) and P2 (0 V) via a 5 A fuse (for FS1)

and a mult imeter on 5 A or higher range. The qu iescen t

current should be less than 1 mA.

Link P3 and PI with light duty wire, whereupon the relay

should energise and the power-on LED (LD1) should illu-

minate. The current indicated on the meter should be

approx ima te ly 400 mA. If an o s c i l l o s c o p e and /or fre-

quency counte r are avai lable, then t he se may be used to

confirm that a 50 kHz (approximate ly) sawtooth wave-

form is avai lable on P24. Avoid undue capac i t ive loading

o therwise the f requency of the osc i l l a tor will be shifted.

179


Unlink P3 and P I , d i sconnec t the supply and d i sconnec t

the mult imeter .

Reconnec t the supply and relink P3 and P I . Measure the

voltage on the output pins, using a suitable voltage range.

P4 to P7 should read +30 V with r e spec t to P8. Pins P12

to P15 should read - 3 0 V with r e spec t to P8. P16 should

read +12 V with r e s p e c t to P17 and P18 should read

- 1 2 V with r e spec t to P17.

The thermal shutdown circui t may be tes ted by carefully

hea t ing t h e t h e r m i s t o r with a h a i r d r y e r . When the

thermis tor r eaches a tempera ture of approximately 80°C

the thermal shutdown LED (LD2) will i l luminate and the

power supply will shutdown, this can be confirmed by

measuring one of the supply vol tage outputs . When the

thermis tor temperature drops to approximately 60°C the

power supply will res tar t and the thermal shutdown LED

will extinguish.

This comple tes test ing of the power supply.

As previously s ta ted, the power supply is specif ical ly

intended for use with two Maplin 50 W bipolar power

amplifiers. In most appl icat ions the audio output power

a t ta inable from these amplifiers when used in conjunc-

t ion with t h i s p o w e r s u p p l y s h o u l d b e m o r e than

sufficient for in-car use. However the purist may wish to

use separa te power suppl ies for each amplifier to in-

c r ea se the power available per channel . Similarly, if a

single channel subwoofer amplifier is required, a single

amplifier may be driven from one power supply.

It is strongly recommended that the power supply is fully

cased and provided with an additional external heatsink,

type 2E is suggested. Metal c a s e s are ideal for this pur-

180


pose, and a lso provide a degree of shielding against ra-

diated radio frequency emiss ions . The audio amplifiers

may also be housed in the same c a s e , which could be

convenient ly mounted in the ca r boot or under a seat .

The audio amplifiers should a lso be heats inked, again

type 2E is suggested.

To connec t the 50 W bipolar amplifiers to the power sup-

ply, t r e a t t h e s w i t c h e d - m o d e p o w e r s u p p l y as a

convent ional power supply (as shown in the amplifier

cons t ruc t iona l de ta i l s ) and c o n n e c t accord ing ly (HT1

and HT2 are posi t ive, HT3 and HT4 are negat ive) . Refer

to Figure 8.11 for connec t ions to the power supply. The

amplifier set-up procedures should be followed in the

same way as for the convent ional power supply. Connec-

t ions from the power supply to the amplifiers should be

made using 32/0 .2 wire.


Loudspeakers should be suitably rated for high

power use. Beware — many car loudspeakers are

given misleadingly high power ratings, so try

and find out what the true r.m.s. ratings are

before you use any loudspeaker. Often car loud-

speaker ratings are given in peak power or total

peak power, so be prepared to divide the rating

by 1.414 or even 2.828! Loudspeaker wiring

should also be sufficiently rated for the pur-

pose .

181


Connect ions from the power supply to the car e lec t r ica l

sys tem should be made using very heavy duty cab le . It

is advisable to connec t the power supply direct ly to the

car ba t te ry via its own in-line fuse at the car ba t te ry end.

Assuming a negative earth car , the chas s i s may be used

to provide the 0 V connec t ion , which saves on wire.

Take note — Take note - Take note - Take note

It should be pointed out that excessive sound

pressure levels may lead to long term, irrevers-

ible hearing problems · High levels of sound may

also blot out other external sounds, which could

be dangerous when on the move · Please use common

sense when using a high power in-car entertain-

ment system.

Input: Input current (P 0= 116 W ) : Output power: Outputs Main: Auxiliary: Continuous output current ±30 V ±12 V Efficiency: Thermal shut-down temperature: Thermal shut-down hysteresis: Standby input: Remote switch-on input: Thermal shut-down output: Input noise (P 0 = 120 W ) : Output noise (P 0 = 120 W ) Main: Auxiliary: Switching frequency: Converter mode:

11 to 15 V d.o, nominally 13.8 V 10.7 A (Vs = 11.3 V) 120 W continuous, see note below

±30 V ±12 V

2 + 2 A 50 mA + 50 mA >90% 80°C 20°C Active low Active high Active low 140 mV

60 mV 40 mV 25 kHz Push-pull

Table 8.1 Specif icat ion of Prototype

182


Car audio switched-mode PSU parts list

Resistors — All 0.6 W 1% metal film (unless specified)

R l 6k8 1 (M6K8)

R2 68 k 1 (M68K)

R3.35 lk2 2 (M1K2)

R4,5,6,7 56 Ω 4 (M56R)

R8.36 1 0 Ω 2 (Ml OR)

R9.10 1 k 3 W 2 ( W I K )

R11.12 470R 3 W 2 (W470R)

R13,17 ,22 ,

23 ,26 ,27 ,

28 ,29 ,34 1 k 9 (M I K )

R14 ,19 ,21 ,

31 ,32 ,33 1 0 k 6 (Ml OK)

R15 24 k 1 (M24K)

R16,20 ,

24 ,25 4k7 4 (M4K7)

R18 1 M 1 (M1M)

R30 3k3 1 (M3K3)

TH1 15 k bead the rmis to r 1 (FX22Y)

Capacitors

C l , 5 , 1 5 ,

16 ,25 ,26 ,

28 ,30 100 nF polyes te r

C2.13.14 220 μ Ρ 5 0 ν 8 Μ Ρ δ

C3,4 , l 1,12 1000 μΡ 50 V SMPS

C6.31 2n2F 1% polys tyrene

C7,8 ,9 ,10,17,

18,19,20 560 pF ce ramic

8

3

4

2

(BX76H)

( J L 5 1 F )

( JL57M)

( B X 6 0 Q )

8 (WX65V)

183


C21.22 1000 μΡ 25 V SMPS 2

C23.24 10 μΡ 25 V tantalum 2

C27 100 μΡ 25 V PC e lec t 1

C29 22 μΡ 25 V PC e lec t 1

C32 10 μΡ 16 V tantalum 1

C33,34 150 pF polys tyrene 2

Dl ,2 1N4001 2

D3.4.13,

14,15,16,

17,18 1N4148 8

D5,6,7,8 BYW80-150 4

D9.10,

11,12 BYW98-150 4

ZD1,2 39 V BZX61C/BZX85C 2

T R I BC337 1

TR2,3 ,4 ,5 BUZ 11 4

TR6,7 ,8 ,

9,10 BC559 5

LD1,2 Red LED 2

RG1 μΑ78121Κ: 1

RG2 μΑ79121Κ: 1

ICI TL494 1

IC2 LM311 1

Miscellaneous

LI FX4054 ferrite toroid 1

L2,3 3 A RF suppressor 2

T l ETD39 ferrite co re 2

ETD39 former 1

ETD39 clip 2

RL1 12 V 16 A relay 1

( JL56L)

(WW69A)

(FF11M)

(FF06G)

(WW68Y)

(BX29G)

(QL73Q)

(QL80B)

(UK63T)

(UK65V)

(QF67X)

( Q B 6 8 Y )

(UJ33L)

(QQ18U)

(WL27E)

(QL32K)

(WQ93B)

(RA85G)

(QY09K)

( J R 8 4 F )

(HW06G)

( JR81C)

( JR82D)

( J R 8 3 E )

(YX99H)

184


FSl

FS2.7

FS3,4 ,5 ,6

15 A 1 V 4in AS fuse 1 (UK13P)

100 mA 20 mm QB fuse 2 (WR00A)

2 A 20 mm AS fuse 4 (WR20W)

174·η chass i s fuse holder 1 (RX50E)

fuse clip 12 (WH49D)

6mm M3 isobol t 1 pkt (BF51F)

12 mm M2.5 i sobol t 1 pkt (BF55K)

M3 isonut 1 pkt (BF58N)

M2.5 isonut 1 pkt ( B F 5 9 P )

M3 i soshake 1 pkt ( B F 4 4 X )

M2.5 i soshake 1 pkt ( B F 4 5 Y )

M3 isotag 1 pkt (LR64U)

TO220 insulator 8 (QY45Y)

T0220 bush long 1 pkt (UL69A)

50 W heats ink 1 (HQ69A)

16-pin DIL skt 1 (BL19V)

8-pin DIL skt 1 ( B L 1 7 T )

1 mm PCB pins 1 pkt (FL24B)

PCB 1 (GE61R)

0.9 mm 20 SWG TC wire 1 (BL13P)

1.6 mm 16 SWG TC wire 1 ( B L U M )

3202 green wire 1 mtr (XR35Q)

1.6 mm 16 SWG EC wire 1 ( B L 2 4 B )

1.25 mm 18 SWG EC wire 1 (BL25C)

0.71 mm 22 SWG EC wire 1 (BL27E)

lapped pair 1 m t r ( X R 2 0 W )

CP 32 heat shrink 1 mtr (BF88V)

CP 16 heat shrink 1 mtr ( B F 8 6 T )

c o n s t r u c t o r s ' guide 1 (XH79L)

ins t ruct ion leaflet 1 (XK50E)

fast-setting adhes ive 1 (FL45Y)

All of the above are avai lable as a kit

switched-mode PSU kit 1 (LP39N)

185



car fuse holder

15 A 1 V4 in AS fuse

HC wire b lack

HC wire red

32/0 .2 wire red

32 /0 .2 wire b lack

32 /0 .2 wire blue

zip wire

50 W power amp

2E heats ink

1 (RX51F)

1 (UK13P)

as r eq (XR57M)

as req (XR59P)

as req (XR36P)

as req (XR32K)

as req (XR33L)

as req (XR39N)

2 (LW35Q)

2 (HQ70M)

186

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