> hi
> i have one of those 3 wire fans on my cpu. My question is how do i measure
> the rpm of my fan from it. not through software like lm_sensors but in real life
> ie so it dosent even have to be in a pc.
>
> Is there some kind of eq i can use so i can measure the voltage across the 3rd
> wire and get a rpm value.
Black and Red are the 12V power for the fan.
Yellow is an open collector output that changes state 4 times per
revolution. (on-off-on-off per rev).
To use it you connect it to 5v or 12v through a resistor (say 1K) this
will then give you a rough square wave at a frequency twice the fan
rpm.
The voltage you connect the resister to should match the input to the
counting cct you have chosen - 5v for digital inputs.
You could feed this to an unused data pin on your PC (ie printer
port) and count the pulses. Or, if you want an RPM gauge, you
could try one of the electronic tacho IC's around such as the
LM2917 (Dick Smith).
Cheer
-Rod
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