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Topic: Measure wind velocity (anemometer) with pc fan (Read 9789 times) previous topic - next topic


Hi guys!
I'm new to arduino but not to open source and forums ;)

I would like to build a weather station with my arduino. I already have a temperature and a light sensor.
I would like to build an anemometer, and my idea is to start using the hall-effect sensor of the unexpensive pc fans (we all have an old pc fan at home).

I can correctly measure pc rotation speed according to this http://arduino.cc/playground/Main/ReadingRPM but this is not exactly what I need. In fact I don't want to power the fan: the fan should rotate with the wind. I would like to measure only the signal coming from the hall sensor...
Mantaining the same circuit depicted here: http://arduino.cc/playground/Main/ReadingRPM without giving power to the fan I get no signal/counts from the hall sensor.

Any hint?


To get a standard PC fan rotating in the wind you must wait for a storm. You have to build a device which rotates with less wind power. The motor of the fan put too much resistance to the rotor and without the motor you don't have the magnet for the reed sensor.


I'm using the fan just as a support... I will build bigger fan blades later.
Anyway this has nothing to do with the problem itself.
If you have any hint on how to measure turns with a pc fan without powering it, then you are welcome.


it's mototr is brushed? if it is, it will act as a dinamo, so you just have to use an analog pin. and then estimate the energy producted/rpm
if it is brushless, you can use one of the cable as hall sensor for the rotor passage, so you have a real rmp count. (number of passage/number of magnet)
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You have to open it and disconnect the motor from the Vcc line (but just the motor not the sensor). Not an easy task though without destroying it at least for the models I know.

You can see how this is wired in the section 3-wire PC Fans on this page:


If you have a 4-wire fan you simple don't connect the control line.

If this works depends on the construction of the motor. If the hall sensor senses a constant magnet you'll be probably successful. If it's constructed like the one disassembled on the above page, you have to wait for the fan to generate enough power to have a current flow. In this case you have close the motor circuit with a resistor to burn the energy (and let a current flow!).

As the produced energy will always put resistance on the rotor we will be back on my first post. A motor without power but driven mechanically is a generator.


Those informations are exactly what I was looking for! Thanks!
I have a brushless 3-wire fan, and I suspect I don't have a permanent magnet inside the fan... so it's not straightforward ;)
Maybe it's easier and cheaper to buy an hall sensor like this http://www.sparkfun.com/products/9312 and build directly an anemometer (or find a 4-wire fan).
Thanks for all the infos, I will report the solution I will implement

ps: I already used the fan as lesto suggested, but I preferred to use hall information. Anyway this can be a good way for a test mode ;)

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