Tachometer using arduino




Digital tachometer using arduino plus speed control.

Tachometer is a device used for measuring the number of revolutions of an object in a given interval of time. Usually it is expressed in revolutions per minute or RPM. Earlier tachometers purely mechanical where the revolution is transferred to the tachometer through mechanical coupling (cable or shaft) , the rpm is determined using a gear mechanism and it is displayed on a dial. With the advent of modern electronics, the tachometers have changed a lot. This article is about a contactless digital tachometer using arduino. The speed of the motor can be also controlled using the same circuit. The RPM and all the other informations are displayed on a 16×2 LCD screen. The circuit diagram of the digital tachometer using arduino is shown below.

Tachometer using arduino




RPM Sensor.

An IR photo transistor and IR LED forms the sensor. IR photo transistor is a type of photo transistor which responds to infra-red waves only. The use of IR phototransistor avoids other light interferences from the environment. The photo transistor and IR diode are aligned side by side. Resistor R2 limits the current through the IR diode. A reflective strip is glued on the rotating object (shaft, disc or fan) in line with the sensor. I used a 9V/100mA cooling fan. The clearence between the sensor and reflective strip has to be less than 1cm. When the reflective strip passes in front of the sensor, IR waves are reflected back to the photo transistor. The photo transistor conducts more at this moment and as a result the voltage across R3(68K resistor) shoots up at this moment. The result will be a waveform like what shown below at the emitter of the photo transistor. RPM can be determined by counting the number of upward shoots in a given interval of time.

 

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