Contactless Tachometer

Posted on Feb 7, 2014

The techometer also called revolution counter is an instrument that measure the speed of a rotating machinery. Generally these measurements are rated in revolutions per minute (R. P. M). In this project we have decided to make our own sensor using a led as light source and a photo diode to pick up the pulse. A arrangement has to be made to make a sm

Contactless Tachometer
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all hole about 5mm near to the outer surface of the wheel. Led or any light source is fixed to the one side of the rotating wheel and photo diode to the other side of the wheel. Light will fall on the photo diode when the hole on the rotating wheel comes in line with the photo diode and light source or led. This way a pulse is picked up every revolution. Resolution can be increased by increasing number of holes in one cycle. Above is the circuit diagram of sensor circuit. In our actual sensor circuit we used a photo diode instead of photo transistor as photo diode was much easily available in market. It is basically a type of proximity sensor which gives a HIGH output when there is no obstacle between LED and photo diode and gives a LOW when there is an obstacle. Now whenever Infrared rays fall on photo transistor or photo diode free electrons and holes are produced. This leads to increase in current and thereby reducing the voltage at pin number 2 of op-amp (operational amplifiers). When no infrared rays are incident on photo transistor or photo diode current reduces thereby increasing the voltage at pin number 2 of op-amp. The expression for voltage at pin 2 is: Vcc * { Rsensor / (Rsensor+R1) } Rsensor is resistance of sensor, which is low when light false on it otherwise it is high. The value of R1 should be carefully chosen to obtain considerable voltage swing in output voltage. We also use a TTL level convertor which...

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