28-1 CHAPTER 28 DIRECT SENSING (A2) Learning outcomes: a)showanunderstandingthatanelectricsensorconsistsofasensingdeviceanda circuit that provides an output voltage b)showanunderstandingofthechangeinresistancewithlightintensityofalight-dependent resistor (LDR) c)sketchthetemperaturecharacteristicofanegativetemperaturecoefficient thermistor d)showanunderstandingoftheactionofapiezo-electrictransducerandits application in a simple microphone e)describe the structure of a metal-wire strain gauge f)relate extension of a strain gauge to change in resistance of the gauge g)show an understanding that the output from sensing devices can be registered as a voltage h)recall the main properties of the ideal operational amplifier (op-amp) i)deduce,fromthepropertiesofanidealoperationalamplifier,theuseofan operational amplifier j)showanunderstandingoftheeffectsofnegativefeedbackonthegainofan operational amplifier k)recall the circuit diagrams for both the inverting and the non-inverting amplifier for single signal input l)show an understanding of the virtual earth approximation and derive an expression for the gain of inverting amplifiers m) recallanduseexpressionsforthevoltagegainofinvertingandofnon-inverting amplifiers n)show an understanding of the use of relays in electronic circuits o)showanunderstandingoftheuseoflight-emittingdiodes(LEDs)asdevicesto indicate the state of the output of electric circuits p)show an understanding of the need for calibration where digital or analogue meters are used as output devices 28-2 28.1Sensing devices Whenyouapproachanautomateddoor,anelectronicsensordetects yourpresenceandcausesthedoortoopen.Sensorsareusedinany electronic system that responds to an external change. Examplesofsuchelectronicsystems:smokealarm,sensorsinside incubator,triplesensorwatch,roboticmachinesinvehicleassembly plants, and etc.

Asensorsuppliesasignaltoaprocessingunit,whichcanoperatean outputdevicesuchasanindicatorlamporarelay.Theprocessingunit may receive signals from more than one sensor and it is programmed to respond according to the signals it receives. Sensing deviceProcessing unitOutput device 28-3 Anelectronicsensorconsistsofasensingdevice,whichrespondstoan external change by producing: oachangeofp.d.becauseitisinapotentialdividerandits resistancechanges(asinathermistororalight-dependent resistor or a strain gauge); oa p.d. directly as in piezo-electric transducer. Using a potential divider oconsistsoftworesistorsinseriesofresistancesR1andR2, connected to a source of fixed p.d. Vs as shown in Fig. 1.

Fig. 1 Fig. 2 oBecausethecurrentthroughtheresistorsisequalto thesource p.d. divided by the total resistance of the two resistors,

oThe p.d. across each resistor is equal to its resistancethe current. Therefore,the p.d. across R1,

the p.d. across R2, 28-4

oOne of the resistors is a sensing device such as thermistor. oThe other resistor is a fixed resistance. oWhen

increases,thecurrentdecreasessothep.d.across

decreases. oTherefore, p.d. across the sensing device increases. A temperature sensor uses a thermistor as the sensing device, as shown in Fig. 1. oThermistorsthathavearesistancethatfallswithincreaseof temperaturearesaidtohaveanegativetemperaturecoefficient andarereferredtoasNTCthermistors.Fig.2showhowthe resistanceofanNTCthermistordecreasesasthetemperature increases. Notice that: the variation of resistance with temperature is non-linear the rate of change of resistance with temperature decreases as the temperature increases oInFig.1,whentemperatureofthethermistorincreases,its resistancedecreases.Asaresult,thecurrentinthecircuit increasessothep.d.acrossthefixedresistance (=currentresistanceofthefixedresistance)musttherefore increase.Thep.d.acrossthethermistorthereforedecreases because it is equal to the source p.d minus the p.d. across the fixed resistor. Since the output p.d. in Fig. 1 is across the thermistor, the output p.d. therefore decreases. oNotice that if the output p.d. had been across the fixed resistor, the output p.d. would have increased. 28-5 Alightsensorusesalight-dependentresistor(LDR)asthesensing device.TheresistanceofanLDRdecreasesnon-linearlywhenthe intensityof light incidentonit increases.Typically,theresistanceofan LDR decreases from 1 M in darkness to about 100 in sunlight. oFig. 3 shows an LDR in a potential divider. Fig. 3Fig. 4 oWhenthelightintensityisincreased,theresistanceoftheLDR decreases(Fig.4)sothecurrentinthecircuitincreases.Asa result, the p.d. across the fixed resistor increases. This means that the p.d. across the LDR decreases and so the output p.d. decreases. oWhenthelightintensityisdecreased,thecurrentinthecircuit decreases so the p.d. across the fixed resistor decreases. Hence the p.d. across the LDR increases so the output p.d. increases. Atransducerisanydevicethatisdesignedtoconvertenergyfromone form to another. oA piezo-electric transducer generates a p.d. when it is squeezed.oThepiezo-electricmaterialcontainspositiveandnegativeions, which are held together by the electrostatic forces they exert on each other.28-6 Fig. 5 Tetrahedral silicate unit. Fig. 6 Quartz complex structure made up ofa large number of repeated tetrahedal silicate units oInthenormal unstressedstateofthe crystal,the centreof chargeof thepositiveionscoincideswiththecentreofchargeofthenegative ions. oWhen pressure is applied to the crystal, the crystal changes shape by asmallamountandthecentresofthepositiveandthenegative chargenolongercoincide.Avoltageisgeneratedacrossthecrystal. This is known as the piezo-electric effect. oThemagnitudeofthevoltagethatisgenerateddependsonthe magnitude of the pressure on the crystal, and its polarity depends on whether the crystal is compressed or expanded. oThegreaterthepressureis,thelargerthep.d.thatisgenerated.A piezo-electric transducer can therefore be used directly as a pressure sensor or a force sensor. Piezo-electric ceramics are used in pressure sensors. Force sensors, microphones and gas lighters. A piezo-electric rod, about 1 mm in diameter, of length 10 mm squeezedbyaforceof10Nbetweenitsends,cangeneratea p.d. in excess of 100 V. 28-7 Fig. 7 Piezo-electric effect in quartz oA sound wave consists of a series of compressions and rarefactions. If asoundwaveisincidentonapiezo-electriccrystal,thenavoltage will be produced across the crystal that varies in a similar way to the variation in pressure of the sound wave.oTo detect the voltages, opposite faces of the crystal are coated with a metal and electrical connections are made to these metal films. oSincethevoltagesgeneratedaresmall,theyareamplified.The crystalanditsamplifiermaybeusedasasimplemicrophonefor converting sound signals into electrical signals.

Fig. 8 Microphone and its symbol Astraingaugetakesadvantageofthechangeinresistanceofametal wire as its length and cross-sectional area change.oWhen stretched, a metal wire becomes narrower and longer; both these changes increase the electrical resistance. oWhencompressed,ametalwirebecomesshorterandwider;as longasitdoesnotbuckle,thesechangesdecreaseitselectrical resistance. 28-8 Fig. 9 Metal wire strain gauge (made by sealing a length of very fine wire in a small rectangle of thin plastic) oTheresistanceRofthewireinthestraingaugeisgivenbythe equation:

where L is the length of the wire, A is its area of cross-section is its resistivity. Suppose the wire is stretched so its length increases fromto , causing its resistance to increase fromto. Therefore,

assumingtheareaofcross-sectiondoesnotchange significantly. Hencetheincreaseofitsresistance

,whichmeans that its increase of resistance is proportional to its change of length . Asthestrainonwiresinthestraingaugeisdefinedas

,itfollowsthattheincreaseofresistanceof the strain gauge is directly proportional to the strain. Thestraingaugecouldbeconnectedasthesensingdevice inacircuitsuchasFig.10.Theoutputp.d.couldbe recorded using a data logger. Iftheoutputp.d.isfoundtochangeovertime,thestrain gaugemusthavebeengraduallystretchedmoreifits resistanceincreasedorstretchedlessifitsresistance decreased. 28-9 Fig. 10 oStrain gauge is used by an engineer to study crack line of building. Bystickingastraingaugeoverthecracktheengineercan measuretheresistancemanydaysorevenyearslaterandseeif there has been any movement. Example 1: MJ2010 P42 Q10(a) (a)Statethenameofanelectricalsensingdevicethatwillrespondto changes in (i)length, strain gauge (ii)pressure. transducer/piezo-electric/quartz crystal Example 2: ON09P41Q9(a)&(b) A metal wire strain gauge is firmly fixed across a crack in a wall, as shown in Fig. 9.1, so that the growth of the crack may be monitored. 28-10 (a) Explain why, as the crack becomes wider, the resistance of the straingauge increases. Solution: The resistance of wire,

as crack widens,increases anddecreases so resistance increases. (b)The strain gauge has an initial resistance of 143.0 and, after being fixed in position across the crack for several weeks, the resistance is found to be 146.2 . The change in the area of cross-section of the strain gauge wire is negligible. Calculate the percentage increase in the width of the crack. Explain your working. Solution:

..

100 ..

1002.24 28.2The ideal operational amplifier Once the output of the sensing device is in the form of a voltage, it may requirefurtherchange(processing)beforeitisusedtocontrolan output device. The basis of many circuits used for processing the sensor voltage is the operational amplifier or op-amp. Anoperationalamplifierisanintegratedcircuitthatcontainsabout20 transistors together with resistors and capacitors. It is referred to as an integratedcircuitbecauseallthecomponentsareformedonasmall sliceofasemiconductorintoacircuit.Thewholeoftheintegrated 28-11 circuitisencapsulated.Thesymbolforanop-ampandsomeofits connectionsareshowninFig.11.Itisreferredtoasanoperational amplifierbecausethecircuitcaneasilybemadetocarryoutdifferent operations. These operations include: oacting as a switch when a voltage reaches a certain level oamplifying direct voltages oamplifying alternating voltages ocomparing two voltages and giving an output that depends on the result of the comparison. The op-amp has two inputs,oinverting input (-) onon-inverting input (+) For many applications, the positive and the negative power supplies are 6 V or 9V.

Fig. 11 An operational amplifier and its symbols. Properties of an ideal op-amp oInfinite input impedance (no current enters or leaves either of the inputs). Connectingtheop-amptothepotentialdividerwould changethep.dacrossthecomponentsinthepotential divider.Iftheop-ampconnection (the input) hasinfiniteresistance (or,morestrictly,foralternatingvoltages,infinite impedance)thenconnectingittothepotentialdividerwill notaffect thepotentialdifferences.Theinput impedanceof an op-amp may be as high as 1012 , but 1016 is typical. 28-12 oZerooutputimpedance(thewholeoftheoutputvoltageisseen across the load connected to the output).Iftheoutputconnectionhadsomeresistance,thenthe voltage produced in the op-amp (the output voltage) would bedividedbetweentheoutputandtheresistorconnected to the output load.Zerooutputresistance(orzerooutputimpedance)means that all of the output voltage will be seen across the resistor connected to the output load.i.e.if the op-amp haszerooutputresistanceitisactingjust like an electrical battery with zero internal resistance. oInfinite open-loop gain (this means that when there is only a very small input voltage, the amplifier will saturate and the output will have the same value as the supply voltage). The voltage gain, or simply gain of an amplifier is a measure ofhowmanytimestheoutputvoltageisgreaterthanthe input voltage. Whentherearenocomponentsconnectedbetweenthe outputand the inputoftheop-amp(nofeedbackloop), the gainthatismeasuredissaidtobetheopen-loopgain.The outputoftheop-ampcannotbegreaterthanthesupply voltage(fromenergyconservation)andso,ifaverysmall voltageisappliedtotheinputandthegainisinfinite,then theoutputwillbeatthesupplyvoltage.Theoutputcannot beanygreater,evenforalargerinput signal.Theamplifier is said to be saturated. 28-13 oInfinite bandwidth (all frequencies are amplified equally). Ifanalternatingvoltageisappliedtotheinput,thenthe output will have the same frequency but larger amplitude. Therangeoffrequenciesthatareamplifiedbythesame amount(theinputsignalsofdifferentfrequenciesthatall have the same gain) is known as the bandwidth. Note:thebandwidthofanop-ampistherangeof frequenciesthatwillproduceanoutputvoltagewithouta big attenuation. oInfinite slew rate When the input signal is changed, then the output signal will also change.Theslewrateisameasureofthetimedelaybetweenthe changestotheinputandoutput.(orhowfasttheop-amp can respond to changes in the input) A high slew rate implies a short time delay. With an infinite slew rate there is no delay. The operational amplifier as a comparator oWhenanop-ampisincorporatedinanelectricalcircuit,itis usuallyconnectedtoadualpowersupplysothattheoutput voltagecanbeeitherpositiveornegative.Suchapowersupply can be represented as two sets of batteries, as shown in Fig. 12, Fig. 12 The op-amp used as a comparator 28-14 oThe link between the two batteries is referred to as the zero volt lineorearthanditprovidesthereferencefromwhichthe voltagesontheinputsandtheoutputaremeasured.Theoutput Vout of this circuit is given by

where

is the voltage at the non-inverting input,

is the voltage at the inverting input,

is the open-loop gain of the amplifier. oConsider case 1, where, non-inverting input,

is 0.95 V, inverting input,

is 0.94 V,

10

supply voltagesare 6 V Bysubstitutingintotheequation,theoutputvoltage

,

10

0.95 0.941000 V. Fromenergyconsiderations,theseanswersareimpossiblesince theoutputvoltagecanneverexceedthepowersupplyvoltage. The amplifier is saturated and the output voltage will be +6 V. oConsider case 2, where,

3.652 V

3.654 V supply voltages are 6 V

10

3.652 3.654200 V

oFromenergyconsiderations,theseanswersareimpossiblesince theoutputvoltagecanneverexceedthepowersupplyvoltage. The amplifier is saturated and the output voltage will be -6 V. So, If

, the output is

28-15 If

, the output is

thepolarityoftheoutputdependsonwhichinputisthe larger. oThecircuitofFig.12iscalledacomparatorbecauseitcompares the voltages applied to the non-inverting and the inverting inputs andthengivesanoutputthatdependsonwhether

or

. Example 3: ON07P4Q8 (a)Fig. 8.1 shows a circuit incorporating an ideal operational amplifier (op-amp). The voltage applied to the inverting and the non-inverting inputs are

and

respectively. State the value of the output voltage

when (i)

,-9 V (ii)

. +9 V (b)The circuit of Fig. 8.2 is used to monitor the input voltage

. 28-16 AtpointA,apotentialof5.0Vismaintained.AtpointB,apotentialof 3.0 V is maintained. Complete Fig. 8.3 by indicating with a tick the light-emitting diodes (LEDs) that are conducting for the input voltages

shown. Also, mark with a cross (X) those LEDs that are not conducting.

Red LEDGreen LED +2.0XX +4.0X +6.0 Fig. 8.3 (c)The input voltage

in (b) is provided by a sensor circuit. (i)CompleteFig.8.4toshowasensorcircuitthatwillprovidea voltageoutputthatincreasesasthetemperatureofthesensor decreases. Show clearly the output connections from the circuit.

Output V 28-17 (ii)Explain the operation of the sensor circuit. as temperature decreases, thermistor resistance

increases. p.d. across thermistor =

as

increases, output increases Whenacircuitincorporatinganop-ampisusedasacomparator,itis usual to connect each of the two inputs to a potential divider, as shown in Fig. 13.oOnepotentialdividerprovidesafixedvoltageatoneinputwhile theotherpotentialdividerprovidesavoltagedependentonlight intensity. oIn Fig. 13, the resistors of resistance R will give rise to a constant voltage of

at the inverting input. oTheLDR,ofresistance

isconnectedinserieswithafixed resistor of resistance F. Fig. 13 The op-amp used as a comparator to monitor illumination oIf

(that is, the LDR is in darkness), then

and the output is positive. oIf

(that is, the LDR is in daylight), then

and the output is negative. oThe output can be made to operate an output device such as LED. 28-18 oItcanbeseenthatbysuitablechoiceoftheresistanceF,the comparatorgivesanoutput,eitherpositiveornegative,thatis dependentonlightintensity.Thelightintensityatwhichthe circuit switches polarity can be varied if the resistor of resistance F is replaced with a variable resistor. oTheLDRcanbereplacedbyothersensorstoprovidealternative sensingdevices.Forexample,useofathermistorcanprovidea frost-warning device. Example 4: Forthecircuitshownbelow,theresistanceofthethermistorTis8kata temperature of 15 C. What are

and

, the potentials at the inverting and non-invertinginputs?Andwhathappenswhenthetemperaturefallssothat the resistance of T rises above 10 k? Solution: p.d. at A

95.0 V p.d. at B

94.5 V theop-ampactsasacomparatorand,since

islargerthan

,theoutput will be the highest voltage that the op-amp can produce, in this case +9 V. ThethermistorTisanegativetemperaturecoefficientthermistorandsoits resistance rises sharply and eventually becomes larger than 10 k.Suppose it becomes 12 k. Then, p.d. at A

94.1 V Now

smallerthat

andtheop-ampoutputvoltageisthelowestitcan provide, near the negative supply voltage, in this case -9 V. Theswitchfrom+9V to9 Visquitesuddenbecauseofthelargeopen-loop voltage gain. The value of the temperature when the output voltage switches from +9V to -9V can be altered by adjusting the resistance of the resistor in series with the thermistor. 28-19 Operational amplifiers and feedback oFeedbackisaprocesswherebyafractionoftheoutputofany device is fed back to the input, so as to assist in the control of the device.Much of the movement of humans is controlled by feedback.If a person wishes to pick up an object, for example, then the person stretched out a hand while, at the same time, looking at the hand and the object. The visual signal from the eye is fedbacktothebraintoprovidecontrolforthehand.This feedback is a continuous process refining the position of the hand relative to the object. oFor an amplifier circuit, the basic arrangement is as shown in Fig. 14. Fig. 14 An amplifier circuit with feedback oAfractionoftheoutputsignal

isfedbackandaddedtothe inputsignal

.Theamplifierhasopen-loopgain

andit amplifies whatever voltage there is at its input. So,

the amplifier input

and this gives

1

theoverallvoltagegain(orsimplygain)

oftheamplifier is given by

1

28-20 oNote:ifthefractionisnegative,then 1

isgreaterthan unityandtheoverallgainoftheamplifiercircuitislessthanthe open-loop gain of the operational amplifier itself.oThis is known as negative feedback. Negativefeedbackseems,atfirstsight,todefeattheobject of an amplifier. However, the reduction in amplification is a small price to pay for the benefits.These benefits: 1) increasedbandwidth(therangeoffrequenciesfor which the gain is constant) 2) lessdistortionoftheoutput(outputsignalisexactly the same as input signal) 3) greaterstability(thegainismorestable,notaffected by changes in temperature, etc) 4) the output resistance (impedance) can be low and the input resistance (impedance) high. oNegative feedback can be achieved by feeding part of the output of the op-amp back to the inverting input, see Fig. 15. Fig. 15 Negative feedback with an op-amp For higher frequency, the voltage gain is small. This means that the amplification signal of 10 Hz to 105 Hz is unstable. The lower frequency amplification is very much higher than the high frequency. 28-21 28.3Operational amplifier circuits The inverting amplifier oAcircuitforaninvertingamplifierincorporatinganop-ampis shown in Fig. 16. Fig. 16 An inverting amplifier oAn input signal

is applied to the input resistor

. oNegative feedback is applied by means of the resistor

. oTheresistors

and

actasapotentialdividerbetweenthe input and the output of the op-amp. oIn order that the amplifier is not saturated, the two input voltages must be almost the same.oTounderstandhowtheinvertingamplifierworks,weneedto understand the concept of the virtual earth approximation. In this approximation, the potential at the inverting input (-) is very close to 0 V. 1) Theop-ampmultipliesthedifferenceinpotential betweentheinvertingandnon-invertinginputs,

and

,toproducetheoutputvoltage

.Because the open-loop voltage gain is very high, the difference between

and

must be almost zero. 2) Thenon-invertinginput(+)isconnectedtothezero volt line so

0. Thus

must be close to zero and the inverting input (-) is almost at earth potential. oPoint P is known as a virtual earth. It cannot actually be 0 V but it is very close to 0 V.28-22 oTheinputimpedanceoftheop-ampitselfisverylargeandso thereisnocurrentineitherthenon-invertingortheinverting inputs. So all the current from, or to, the input signal to the circuit mustpassthroughthefeedbackresistortotheoutput,asshown in Fig. 17. Fig. 17 Feedback current oBecause the invertinginput isatzerovolts,apositiveinputgives risetoanegativeoutputandviceversa.Thisiswhythe arrangement is given the name inverting amplifier. oReferringtoFig.17,sincetheinputresistanceoftheop-ampis infinite, current in

current in

and ..

..

The potential at P is zero (virtual earth) and so

0

0

The overall voltage gain of the amplifier circuit is given by, voltage gain

oThenegativesignshowsthatwhentheinputvoltageispositive then the output voltage is negative and when the input is positive the output is negative. oIttheinputvoltageisalternatingthentherewillbeaphase differenceof180orradbetweentheinputandtheoutput voltages.

28-23 The non-inverting amplifier oThe circuit for a non-inverting amplifier incorporating an op-amp is shown in Fig. 18. Fig. 18 A non-inverting amplifier oThe input signal

is applied to the non-inverting input.oNegativefeedbackisprovidedbymeansofthepotentialdivider consisting of resistors

and

. oAslongastheop-ampisnotsaturated,thepotentialdifference between the inverting (-) and non-inverting (+) is almost zero. So

. oSincethenon-invertinginput(+)isconnectedtotheinput voltage,

. Thus,

. oThetworesistors

and

formapotentialdivider.Thetotal voltageacross

and

is

andthevoltageacross

aloneis

. oThe current in the two resistors can be written as:

voltage gain

1

oThenon-invertingamplifierproducesanoutputvoltagethatisin phasewiththeinputvoltage.Whentheinputvoltageispositive, so is the output voltage.28-24 Example 5: MJ10P42Q9(a)Negative feedback may be used in amplifier circuits. State (i)what is meant by negative feedback. Fraction of the output (signal) is added to the input (signal); Out of phase by 180 between input and output voltages. (ii)twoeffectsofnegativefeedbackonanamplifierincorporatingan operational amplifier (op-amp) Less distortion/increases bandwidth/gain is more stable/reduces gain (b)Fig. 9.1 is a circuit for an amplifier that is used with a microphone. The output potential difference

is 4.4 V when the potential at point P is 62 mV.Determine, (i)the gain of the amplifier Gain

.. 71(ii)the resistance of the resistor R. 1

71 1

71 1.710

(c)ThemaximumpotentialproducesbythemicrophoneatpointPonFig. 9.1 is 95 mV. Thepowersupplyfortheoperationalamplifiermaybeeither 5 Vor 9 V.

28-25 Statewhichpowersupplyshouldbeused.Justifyyouranswer quantitatively. For the amplifier not to saturate,Maximum output = 719510

6.7 V Power supply should be 9 V. Example 6: MJ09P4Q10 (a)By reference to an amplifier, explain what is meant by negative feedback. (part of) the output is added to/returned to /mixed with the input and is out of phase with the input/fed to inverting input. (b)Anamplifiercircuitincorporatinganidealoperationalamplifier(op-amp) is shown in Fig. 10.1. The supply for the op-amp is 9.0 V. The amplifier circuit is to have a gain of 25. Calculate the resistance of resistor R. 251

5 k (c)Statethevalueoftheoutput

oftheamplifierin(b)forinput voltages

of(i)-0.08 V-2 V (ii)+4.0 V+9 V 28.4Output devices Circuitsincorporatingop-ampsproduceanoutputvoltage.Thisoutput voltage can be used to operate warning lamps, digital meters, motors etc.However, the output of an op-amp cannot exceed a current of more than 25 mA. Otherwise, the op-amp would be destroyed. In fact, op-amps generally contain an output resistor so that, should the output be shorted, the op-amp will not be damaged. 28-26 The sensing circuit may be required to switch on or off an appliance that requires a large current; e.g. an electric motor. The switching on or off of alargecurrentbymeansofasmallcurrentcanbeachievedusinga relay. The relay oIt is an electromagnetic switch that uses a small current to switch on or off a larger current. oThe symbol for a relay is shown in Fig. 19. Fig. 19 A relay oWhenacurrentpassesthroughthecoiloftheelectromagnet,the iron armature is attracted on to the iron core of the electromagnet. oThearmatureturnsaboutthepivotandclosesthegapbetween the switch contacts. oInthisway,asmallcurrentcanbeusedtoswitchonamuch greater current. For example, when the ignition switch of a car is turned on, a small current passes through the electromagnet coil so the relayswitchcloses.Thisallowsamuchgreatercurrentto pass through the starter motor. oTheconnectionofarelaytotheoutputofanop-ampcircuitis shown in Fig. 20. 28-27 Fig. 20 The connection of a relay to an op-amp The diode D1 conducts only when the output is positive with respecttoearthandthustherelaycoilisenergizedonly when the output is positive.When the current in the relay coil is switched off, an e.m.f. is induced (back e.m.f.) across the terminal in the coil.Thise.m.f.couldbelargeenough(becauseofaveryrapid fall in magnetic flux within the coil) to damage the op-amp. AdiodeD2isconnectedacrossthecoiltoprotecttheop-amp from this back e.m.f. Whentheoutputoftheop-ampispositive,thediode D2 is reverse-biased and will not conduct any current. Whentheop-ampisswitchedoff,thediodeD2is forward-biased and will conduct current. The induced voltageisthecoilcausesthebottomofthecoiltobe morepositivethanthetopofthecoil.(D2createsan easypathforthecurrenttocirculateuntiltherelays inductancehaslostallitsenergy,throughwireand diode. Example 7: MJ10P42Q10 (a)Statethenameofanelectricalsensingdevicethatwillrespondto changes in (i)length,strain gauge (ii)pressure. transducer/quartz crystal/piezo-electric 28-28 (b)A relay is sometimes used as the output of a sensing circuit. The output of a particular sensing circuit is either +2 V or -2 V. OnFig.10.1,drawsymbolsforarelayandanyothernecessary componentsothattheexternalcircuitisswitchedononlywhenthe output from the sensing circuit is +2 V. The light-emitting diode (LED) oTheLEDisasemiconductordevicethatisrobust,reliableand dissipates much less power than a filament lamp; commonly used as indicators because they have a low power consumption. oThey are available in different colours including red, green, yellow and amber. Fig. 21 Symbol for a light-emitting diode (LED) oThe LED emits light only when it is forward-biased. oAresistorisfrequentlyconnectedinserieswithanLEDsothat, whentheLEDisforwardbiased(thediodeisconducting),the current is not so large as to damage the LED. oAtypicalmaximumforwardcurrentforanLEDis20mA. Furthermore, the LED will be damaged if the reverse bias voltage exceeds about 5 V. 28-29 Fig. 22 Circuit using two diodes to indicate whether the output from an op-amp is positive or negative with respect to earth. oWhentheoutputispositivewithrespecttoearth,diodeD1will conductandemitlight.DiodeD2willnotconductbecauseitis reverse biased. oIf the polarity of the output changes, then D2 will conduct and emit light and D1 will not emit light. oThestateoftheoutputcanbeseenbywhichdiodeisemitting light. The diodes can be chosen so that they emit light of different colours. Example 8: ON09P41Q10 The circuit of Fig. 10.1 may be used to indicate temperature change. 28-30 The resistance of the thermistor T at 16C is 2100 and at 18C, the resistance is 1900. Each resistor P has a resistance of 2000. Determine the change in the states of the light-emitting diodes R and G as the temperature of the thermistor changes from 16C to 18C. Solution: At 16C,

1.00V,

0.98V (

) So, at 16C, output is positive. Diode R is on and diode G is off. As temperature rises, diode R goes off and diode G goes on.

) Example 9: MJ08P4Q9 A block diagram for an electronic sensor is shown in Fig. 9.1. (a)Complete Fig. 9.1 by labeling the remaining boxes. (b)A device is to be built that will emit a red light when its input is at +2V. When the input is at -2V, the light emitted is to be green. (i)On Fig. 9.2, draw a circuit diagram of the device. (ii)Explain briefly the action of this device. correct polarity for diode to conduct identified hence red LED conducts when input (+) or vice versa. 28-31 Digital and analogue meters as output devices oAnLEDmaybeusedtoindicatewhetheranoutputispositiveor negative.Iftheoutputisfromacomparator,thenLEDscangive information as to, for example, whether a temperature is above or belowasetvalue.However,theLEDdoesnotgiveavalueofthe temperature reading. oMany sensors, for example, a thermistor or an LDR, are non-linear. Itwasseenthatthesensorcouldbeconnectedintoapotential dividercircuitsothattheoutputofthepotentialdividervaried withsomeproperty,forexampletemperatureorlightintensity. Thisvariablevoltagecouldbemeasuredusingananalogueora digital voltmeter. oThe reading on the voltmeter would vary with the property being monitored. However, the reading on the voltmeter would not vary linearlywithchangeintheproperty.Inorderthattheproperty can be measured, a calibration curve is required. oThe reading on the voltmeter is recorded for known values of the property X (i.e. the physical property that changes its resistance). For example: a thermistor can be used to measure temperature an LDR can be used to measure light intensity a strain gauge can be used to monitor strain oA graph is then plotted showing the variation with the property X of the voltmeter reading. The value of the property X can then be read from the graph for any particular reading on the voltmeter. Fig. 23 A calibration curve 28-32 Example 10: MJ12P41Q10 Astudentdesignsanelectronicsensorthatistobeusedtoswitchonalamp when the light intensity is low. Part of the circuit is shown in Fig. 10.1. (a)State the name of the component labeled X on Fig. 10.1.[1] light-dependent resistor (allow LDR) (b)On Fig. 10.1, draw the symbols for (i)two resistors to complete the circuit for the sensing device, [2] two resistors in series between +5V line and earth midpoint connected to inverting input of op-amp (ii)a relay to complete the circuit for the processing unit.[2] relay coil between diode and earth switch between lamp and earth (c) (i) State the purpose of the relay. [1] switch on/off mains supply using a low voltage/current output (ii)Suggest why the diode is connected to the output of the operational amplifier (op-amp) in the direction shown. [2] relay will switch on for one polarity of output (voltage) switches on when output (voltage) is negative 28-33 Example 11: MJ12P42Q9 An operational amplifier (op-amp) may be used as part of the processing unit in an electronic sensor. (a)State four properties of an ideal operational amplifier. [4] infinite input impedance/resistance zero output impedance/resistance infinite (open loop) gain infinite bandwidth infinite slew rate (b)A comparator circuit incorporating an ideal op-amp is shown in Fig. 9.1. The variation with time t of the input potential VIN is shown in Fig. 9.2. On the axes of Fig. 9.2, draw a graph to show the variation with timeof the output potential VOUT.[3] square wave 180 phase change amplitude 5.0V 28-34 (c) The output potential VOUT is to be displayed using two light-emittingdiodes(LEDs).AdiodeemittingredlightistoindicatewhenVOUTis positive and a diode emitting green light is to be used to indicate when VOUT is negative. Complete Fig. 9.3 to show the connections of the two LEDs to the output of the op-amp. Label each LED with the colour of light that it emits. [3] correct symbol for LED diodes connected correctly between

and earth diodes identified correctly Example 12: ON13P43Q9 (a)State three properties of an ideal operational amplifier (op-amp).[3] (b)An amplifier circuit is shown in Fig. 9.1. (i) Calculate the gain of the amplifier circuit.[2] gain1 .. 10 (ii) The variation with timeof the input potential

is shown in Fig. 9.2. 28-35 On the axes of Fig. 9.2, show the variation with timeof the output potential

.[3] straight line from (0, 0) toward

1.0V,

10V horizontal line at

9.0V to

2.0V correct +9.0V -9.0V (and correct shape to

0) Example 13: ON10P43Q10 (a)State three properties of an ideal operational amplifier (op-amp).[3] (b)A circuit incorporating an ideal op-amp is to be used to indicate whether a door is open or closed. Resistors, each of resistance R, are connected to the inputs of the op-amp, as shown in Fig. 10.1. The switch S is attached to the door so that, when the door is open, the switch is open.The switch closed when the door is closed. (i)Explainwhythepolarityoftheoutputoftheop-ampchanges when the switch closes. [3] with switch open,

is less (positive) than

output is positive withswitchclosed,

ismore(positive)than

sooutputis negative 28-36 (ii)A red light-emitting diode (LED) is to be used to indicate when the door is open. A green LED is to indicate when the door is closed. On Fig. 10.1, 1.draw symbols for LEDs to show how they are connected tothe output of the op-amp.[1] diodes connected correctly between output and earth 2.identify the green LED with the letter G.[1] green identified correctly Example 14: ON11P42Q9 (a)State two effects of negative feedback on the gain of an amplifier incorporating an operational amplifier (op-amp).[2] reduced gain/increase stability/greater bandwidth or less distortion (b)An incomplete circuit diagram of a non-inverting amplifier using an ideal op-amp is shown in Fig. 9.1. (i)Complete the circuit diagram of Fig. 9.1. Label the input and the output.[2]

connected to midpoint between resistors.

clear and input to

clear (ii)Calculate the resistance of resistor R so that the non-inverting amplifier has a voltage gain of 15.[2] gain1

151

860

(c)On Fig. 9.2, draw a graph to show variation with input potential

of the output potential VOUT. You should consider input potentials in the range 0 to +1.0 V. 28-37 (d)The output of the amplifier circuit of Fig 9.1 may be connected to a relay. State and explain one purpose of a relay.[2] relay can be used to switch a large current/voltage output current of op-amp is a few mA/very small