29-01-2013, 02:53 PM
DIGITAL SPEEDOMETER
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This instrument displays the speed
of the vehicle in kmph. An
opaque disc is mounted on the
spindle attached to the front wheel of
the vehicle. The disc has ten equidistant
holes on its periphery. On one side of
the disc an infrared LED is fixed and on
the opposite side of the disc, in line with
the IR LED, a phototransistor is
mounted. IC LM324 is wired as a
comparator.
When a hole appears between the
IR LED and phototransistor, the
phototransistor conducts. Hence the voltage
at collector of the phototransistor
and inverting input of LM324 go ‘low’,
and thus output of LM324 becomes logic
‘high’. So rotation of the speedometer
cable results in a pulse (square wave) at
the output of LM324. The frequency of
this waveform is proportional to the
speed.
Let ‘N’ be the number of pulses in
time ‘t’ seconds and numerically equal
to the number of kilometres per hour
(kmph). For a vehicle such as LML
Vespa, with a wheel circumference of
1.38 metres, and number of pulses equal
to 10 per revolution.