While cleaning out the dust bunnys I was reminded I had added a 2-wire
fan designed to cool a P1 CPU to my video card, powered via a molex
connector, but I still had a spare fan header and tach input on the
motherboard. Hmmm...

Turns out only a simple circuit is required to add a tach output to a
2-wire fan. I found suitable components on a board from an old HDD, but
then I screwed up, wired a transistor backwards, and blew the
motherboard sensor :-( I was mad at myself, both for reversing the
transistor and for not measuring the tach output before I plugged it
into the board, and decided to fix it on the spot. Another bad decision
- the sensor chip is 7mm square and has 48pins, 12 on each side, and is
jammed up against a PCI slot. It was not worth the effort of changing
the chip just to fix a fan sensor, but I did it anyway.

I digress... the correctly wired circuit works great and shows my P1 fan
spinning at a healthy 4600rpm - and I added a zener across the output to
protect the motherboard in case it ever fails! If you make the circuit
using surface mount parts from old hard drives, it's no bigger than 2
match heads - just a bump on the power wires covered in heat-shrink tubing.

If there's any interest, I could post more details or do a web page.

P2B