So I have one of those thermoelectric fans for our wood stove and have wondered how fast it goes (I already knew it really doesn't move much air).
I used a OPB606A Reflective Object Sensor to detect a fan blade moving past a given point. Here is a link to the data sheet.
It consists of an infrared LED and a phototransistor mounted together in an opaque housing. When it "sees" something reflective, the transistor turns on and drives the corresponding pin near ground. I had to use a Schmitt trigger (74LS14) to get good rise times (when the transistor turns off) because I had a fairly long RC time constant. Long wires with the signal adjacent to ground and a 20K pullup.
I implemented an circular buffer of 15 samples of 4 seconds each. Every four seconds, I compute the total of the samples, round it, and divide by two (two-bladed fan). This gives me the total of revolutions for the preceding 60 seconds (RPM).
I display the current RPM and maximum RPM on an 8-digit MAX7219 stick.
Re-inventing the wheel is not a waste of time if, when you are done, you understand why it is round.
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