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
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.
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,
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
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
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
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.