You are trying to measure about 800Hz (=24krpm 2blades/rotation) with a fill
factor of about 1/15 (~7%) and a contrast of maybe 6dB. So the actual frequency
response should be computed for ~10-12kHz (the pulse width for each passing
blade should be about 100usec wide and these pulses should be cleaned and
applied to your counter. This is not so easy even with a normal photodetector
imho. Most photodetectors have problems with high frequency due to internal
capacitance combined with low photocurrent.

Peter



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