ELECTRONICS PROJECTS Vol. 19140

This instrument displays the speedof the vehicle in kmph. Anopaque disc is mounted on the

spindle attached to the front wheel ofthe vehicle. The disc has ten equidistantholes on its periphery. On one side ofthe disc an infrared LED is fixed and onthe opposite side of the disc, in line with

DIGITAL SPEEDOMETERNARENDRA WADHWANI

the IR LED, a phototransistor ismounted. IC LM324 is wired as acomparator.

When a hole appears between theIR LED and phototransistor, thephototransistor conducts. Hence the volt-age at collector of the phototransistorand inverting input of LM324 go ‘low’,

and thus output of LM324 becomes logic‘high’. So rotation of the speedometercable results in a pulse (square wave) atthe output of LM324. The frequency ofthis waveform is proportional to thespeed.

Let ‘N’ be the number of pulses intime ‘t’ seconds and numerically equalto the number of kilometres per hour(kmph). For a vehicle such as LMLVespa, with a wheel circumference of1.38 metres, and number of pulses equalto 10 per revolution, we get therelationship:

Therefore, time ‘t’ in seconds= 0.4968 second.

As shown in the timing diagram, att=0, output of astable flip-flop IC1(a) i.e.½556 goes low and triggers monostablemultivibrator IC1(b) i.e. ½556. Pulsewidth of monostable IC1(b) = 0.5068 sec.For IC1(a), t(on) = 0.51 sec. and t(off)=0.01 sec. The outputs of IC1(a) andIC1(b), and the signal from thetransducer section are ANDed. Thenumber of pulses counted during thegating period (0.4968 sec.) is the speedN in kmph (kilometres per hour).

At the end of the gating period,output ‘B’ of monostable IC1(b) goes lowand B goes high. The rising edge of B isused to enable the quad ‘D’ flip-flops IC6and IC7.

At this instant, i.e. at t=0.5068 sec.,the number (speed) N will be latchedcorresponding to the ‘D’ flip-flops anddisplayed. At t=0.52 sec., output ofastable flip-flop IC1(a) goes low andremains low for 0.01 sec. Thiswaveform is inverted and applied tothe reset terminals of all counters (ac-tive high).

Thus the counters are reset and

N pulsest

= N kmph

Nx1000 metres per second3600x1.38

=

Nx1000x10 pulses per second3600x1.38

=

ELECTRONICS PROJECTS Vol. 19 141

counting begins afresh at t=0.53 sec. upto the time t=0.52+0.2068 sec. Howeverthe ‘D’ flip-flops are not enabled and the

previous speed is displayed. Thenew speed is displayed at t=0.52 +0.5068 sec. In this way the speedwill be updated every 0.52 sec.

This speedometer can measureup to 99 kmph with a resolution of1 kmph. The range can be increasedup to 999 kmph by adding anotherstage consisting of one each of ICs7490, 74175, 7447 and a 7-segmentdisplay. The voltage supply requiredfor the operation of the circuit isderived from the vehicle powersupply (12V).

The calculations shown above are forLML Vespa and Kinetic Honda. The cal-culations for using this speedometer for

Yamaha, whose circumference of wheel= 1.8353m, can be obtained in a similarfashion. The gating period will simplyvary in direct proportion to the wheeldiameter. It will be 0.6607 sec. forYamaha.

The same speedometer can be usedfor other vehicles by making similar cal-culations. In all the calculations it hasbeen assumed that the speedometercable makes one revolution for everyrevolution of the wheel of the vehicles.Note that on/off periods of the wave-forms have to be precise. High quality

multiturn pots and low temperature co-efficient components should be used inthe timer ICs.