best way to design fan blade counter

If I wanted to build a fan blade counter obviously to count the number of times the blade has spun around, which would be the best way to design it?
Would a photodiode be fast enough (and strong enough) with a photo detector on the other side of the blades?
I've never designed anything like this and wasn't sure where I'd even start.
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us:

Attach a "chopper wheel" on the other end of the shaft. It is an opaque colored wheel with one or more small slits in it. Then, you can buy a sender/receiver diode pair that is already spaced apart, made for the purpose. Then, all you need is the counter circuit.
Putting such a thing in front of the blades would require that one be out if the path of the fan's breeze. Also, ambient light source reflections might give rise to errant counts.
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Peter wrote:

There are fans (as in diddy ickle ones on computer graphics cards) and FANS (as in whole-aircraft wind tunnels).
Everything that follows depends on the nauture of the fan and will work with some but not for others.
Generally, I would be looking to glue one or two very small (relative to the fan) magnets onto the fan and positioning a suitable magnetic pickup where it would detect the passage of the magnet. Then multiplying by the number of blades and dividing by the number of magnets. Two magnets, one opposite the other, if you need to balance the rotor.
Optical sensing through the fan is possible but puts one sensor into the path of the outgoing airflow - where it can pick up dust, etc. Better is often to stick bits of reflective tape on the blade and use reflection rather than transmission - with the bits on the intake side of the fan. For a trasnmissive or reflective sensor - "strong enough" is mostly a case of "bright enough". Use a laser diode with the sort of wavelength as the sensor and you can sense the rotation from 30 feet or more if you want. Good idea if the fan blade is on your prototype, notroglycerine-powered, racer...
But, as so often the case, it does so depend on how big it (the fan) is.. Size really does matter, I'm afraid - not just what you are doing with it.
-- HTH
Sue
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Peter, there are all kinds of sensors one could use (like optical Xmission/reflection, magnetic as has been suggested here). You could also 'listen'. A suitable microphone or sound sensor if it was close enough could hear the individual blaldes go by -- think of sonar in subs that can identify the number of blades on a screw and how fast it's turning. A pressure sensor could feel the air pulses from individual blades.
If one had to choose a single one-size-fits-most approach, using a laser aimed at where the blades pass and looking at the modulations in reflected light would probably be the most likely to succeed. It would be easy to configure such a thing as a hand held pistol that could be aimed, and the optical sensors are really good: think of bar code readers.
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