equivariant

DIY tachometer

If you have a photoresistor and an oscilloscope, making a simple tachometer is straightforward.

  1. Build this circuit:

    9V battery with a 4kΩ resistor and photoresistor in series. Oscilloscope probe connected between the two resistors and ground connected to the negative terminal of the battery.

  2. Make a high-contrast mark on the thing you're measuring the speed of:

    PC fan with a circular piece of paper taped to the center section. Two 45° notches are cut out of the circle opposite each other.

  3. Power it on and hold the photoresistor a couple millimeters away:

    Fan running with photoresistor held in front of the spinning paper pattern.

  4. Check the oscilloscope:

    Oscilloscope screen with two patterns: a 0-5V/44Hz square wave and a ~1.5-2.5V/44Hz approximately sinusoidal wave.

The smooth signal is from the photoresistor and the square wave is from the fan's internal tachometer. As you can see, they have the same frequency. In my case, the mark passes the photoresistor twice per revolution, so the speed is:

44.0 Hz/(2 passesrevolution)⋅60 smin=1320 RPM.44.0\,\mathrm{Hz} {\huge /} \left(2\,\frac{\mathrm{passes}}{\mathrm{revolution}}\right) \cdot 60\,\frac{\mathrm{s}}{\mathrm{min}} = 1320\,\text{RPM}.