How about mounting a small (eg 1" diameter) DC motor in the
centre of the fan blade as a generator. The motor's rotor is held
still by the frame that the fan sits in and the case (which now
becomes the "rotor") of the motor is fixed to the circuit, with power
taken out of the motor's wires. I've seen this done to make an
anemometer. Voltage out measures speed. The only proviso is
that you need a "smooth" motor which is very easy to turn. Something
like that in a cassette or CD player for example, but maybe you have
a fan motor that's grunty enough to turn any small motor

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