Fan Revolution Counter

[ibwev]

I am trying to find the most affordable and reliable method of counting the revolutions of a 2.5" diameter fan (see pic) using a micro-controller. Currently, I am using a light sensor (GP2A200LCS0F) which cost over $5 per sensor. Is there a cheaper, yet dependable, component for this?

 

[Ice-Tea]

Depends a bit on what type of fan that is?

For some types, you can use the commutation current profile to generate a pulse for each commutation. Divided by the amount of commutations/windings you get RPM.

 

[ibwev]

Depends a bit on what type of fan that is?

It is a 2.5" diameter plastic fan connected to a bearing. I am using it to measure air current across the blades. It is not powered by a motor.

... commutation current profile to generate a pulse for each commutation.

Does this method only work on fans powered by a motor?

 

[FvM]

Does this method only work on fans powered by a motor?

I presume it's a BLDC motor with built-in electronics. The BLDC can be used as a synchronous generator, but you need to remove the electronics and connect a winding directly. I don't expect that you get a usable signal at the open supply pins, but may be worth a try. Removing the electronics will however reduce an electrical breaking effect by the BLDC generator load.

 

[ibwev]

I presume it's a BLDC motor with built-in electronics.

There is not a motor connected to the fan. The fan only revolves when air is forced across the blades. I am currently using a light sensor to count the blades. Is there a better method?

 

[ard]

This is a very nominal requirement. You should be able to use an IR LED and a photo diode, they both come very cheap.

Just reverse bias the photo diode (Anode to ground, cathode thru' 2.2k to Vcc), and the junction of the cathode and the resistor would be able to drive your CPU directly.

Regards,

Anand