It is not necessary an optical apparatus for achieve that. There are fan models available with 3 wires: two for motor supply, but another one acting as an internal tachometer ( output open collector ). The purpose of such feature is the ability to evaluate the "health" of the fan, that due to being exposed to dust, often reduce spin rate, or even stops.
Probably you will need to take a research on a manner to cut off supply of internal motor, leaving energized only the circuit responsible to measure rotation.
Another issue necessary to check is if the wind is strong enough to move the propeller of the fan.
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