digital speedometer circuit schematic

Posted on Feb 4, 2014

This circuit serves to show 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 about 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 LM

digital speedometer circuit schematic
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324 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.

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