I have a cooler master FA07015L12LPB and the data sheet(?) I found didn't specify the voltage output of the TACH pin.
I tried to measure the TACH pin by using a multimeter but I got funny results.
With 12V supply to the fan, the multimeter reads 120mV.
With 8V, it reads about 110mV.
With 5V(surprise the fan still spins), it fluctuates.
I don't think I am measureing anything meaningful here. How do I know the volatge of the Tach pin?
You need to use an oscilloscope, because a multimeter will not accurately read fast pulses.
If you look at the Intel 4 wire fan specification, you will see that the tachometer output is open collector or open drain.
This is useful, because it would allow you to use the tachometer output with whatever supply voltage you give it.
You could connect the output directly to an Arduino input and use pinMode( ) to select INPUT_PULLUP.
(Note, that I don't know whether your fan follows the Intel Specifications.)
