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contact less Infra Red digital tachometer
The contact with the rotating shaft is avoided with an optical sensing mechanism that uses an infrared (IR) light emitting diode and a photo detecting diode. The IR LED transmits an infrared light towards the rotating disc and the photo detecting diode receives the reflected light beam. This special arrangement of sensors is placed at about an inch away and facing towards the rotating disc. If the surface of the disc is rough and dark, the reflected IR light will be negligible. A tiny piece of white paper glued to the rotating disc is just enough to reflect the incident IR light when it passes in front of the sensor, which happens once per rotation (shown below).
When the IR Tx pin is pulled high, the BC547 transistor (NPN) conducts and infrared light is transmitted. This is controlled through RA3 pin of PIC18F255o, and is turned on for 1 sec during which the number of reflected pulses received by the photo detecting diode are counted. The pulses appear at the collector of the BC557 transistor (PNP) goes to RA4/T0CKI pin of PIC18F2550. It is the external input pin for Timer0 module which counts the external pulses arriving at this pin. Under normal condition, the resistance of photo detecting diode is very high and therefore, the BC557 transistor is almost cut-off. The output at its collector is pulled to ground. When the photo detecting diode receives the reflected IR light, it’s resistance drops and BC557 conducts, and the collector output goes high. Thus, this simple circuit converts the reflected IR light from the white mark on the rotating disc into a pulse.
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