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Counting circuits 2

1 Switching machine off for5 s in every 20s● The binary counter and timed

bistables provide an output of 0 for 15out of every 20 second period. This isinverted by the NOT gate whichactivates the relay, turning the motoron.

● For the remaining 5 seconds theoutput is 1. This is inverted by theNOT gate and thus deactivates therelay turning the motor off.

2 Reversing action of food mixer every 5 s● This type of circuit is sometimes used

in food mixers. Activating the relaydoes not turn the motor on or off, butreverses the direction of the currentand thus the direction in which themotor rotates.

3 Flashing a lamp● The output from a binary counter can

be used to turn a lamp on and off agiven number of times. When theswitch is turned on in this circuit thelamp flashes 6 times.

4 Automatic light buoy● Flashing lamps have many possible

applications including providingwarnings such as buoys which markthe position of an underwater hazard.The circuit can be modified using anLDR so that the light only flashes whenit is dark which preserves the battery.

5 Batch counter● Batches of articles can be counted

either by depressing switches manuallyor having an arrangement in which aswitch is triggered each time an articlepasses a sensor.

batterybinary codebistableclocked logic

light-dependentresistor (LDR)

Key words

1HZLight is on forone second Adaptation so that

light only comeson at night

(pull-upresistornot shown)

+ +

+ +

+ +

5 Batch counter

S Q

1 Switching machine off for 5 s in every 20 s

Reset Hold

Clock

ABCD

2 Reversing action of food mixer every 5 s

ABCD M

M

Reset

Reset

ABCD

Clock

Reset

ABCD

Reset

Clock

AB

C

D

AB

C

DReset

M

a

d

g

e

f

3

Clock

f 1 Hz clock inputg to clock input

Q

S

R

Q

Clock

Q

S

R

Q

Clock

Q

S

R

Q

Clock

a 1 Hz clock inputb driverc relay

h number of articles in batchi number of batchesl articles pass the LDR to

operate clock(pull-up resistor not shown)

k conveyer belt motor

3 Flashing a lamp

d 1 Hz clock input

e 1 Hz clock input

4 Automatic light buoy

Q

S

R

Q

cb

h

j

k

i

*04 Electronics (160-187).qxd 2/2/07 12:24 pm Page 178