Adding a Tachometer to a 2-wire Fan

It's quite simple to add a tachometer output to a 2-wire cooling fan so you can connect it to a fan header on your motherboard and monitor rpms using Motherboard Monitor or similar programs. I added a tachometer to a Pentium I CPU fan. First take your fan apart (they are all different, you'll have to figure out this part yourself) enough to get access to the circuit board. It will look something like this:


Look for the tiny copper wires from the circuit board to the coils. There are usually 2 coils connected together (labelled Common above), and a separate input to each one (labelled Coil 1 and Coil 2). Solder a wire to either one of the coil inputs - the blue wire above (which I later changed to a thinner red wire because the blue one was too thick to allow the fan to be mounted on it's heatsink properly).

Next build the circuit below. You need a 2.7K ohm 1/4W resistor, 4.7K ohm 1/4W resistor, 2N2222 NPN transistor, and a 5.1v Zener diode - the Zener is optional, but will protect your motherboard's sensor input if the fan ever melts down or the transistor fails.

Fan Tachometer Circuit

Install your tachometer circuit on the fan leads however you like, mine looked like this before I covered it in a blob of hot-melt glue to protect it and prevent it from shorting out onto anything.

Fan with Tachometer


Test your circuit before you connect it to the motherboard - if you've made a mistake in the wiring, it will blow your motherboard sensor instantly (or worse). You need a multimeter, the 4.7K ohm resistor left over from the parts list above, and a test jig wired as shown below.

Test Jig

The tachometer output should read around 1.5 - 2v on the DC scale, and 3 - 4v on the AC scale - if it's more than 5v on either scale, something is seriously wrong and needs to be fixed before connecting the fan to the motherboard!

I also built a version out of surface mount components I scrounged off the controller board of an old hard drive. The parts are tiny and hard to solder, but the end result is neater as you can see below.

Fan with Tachometer - SMD Version

Last Updated January 7th, 2004 by P2B