Here's how to use the 555 PWM Output STEMMA to trick your PC into thinking there's a fan running. This is useful when you've replaced the air cooling fans on your CPU or other parts of the system with liquid cooling radiators and pumps, such as a water cooled or oil cooled rig.
Some motherboard and BIOS combinations may not allow you to disable the fan alarms and may not allow you to boot past the POST test without a fan (or fan dummy) in place.
Since the fan headers on the motherboard contain a signal pin that watches for 5V square wave pulses, we can plug the 555 PWM Output STEMMA in and tell a big fat lie to the motherboard!
Normal System Fan Operation
Here we have a small (and noisy!) system fan connected to the motherboard SYS FAN header.
As it spins, a small magnet on the fan hub spins and triggers pulses from a Hall effect sensor. It's reporting Current SYSTEM FAN Speed 1190 RPM, which makes the BIOS happy.
Fan Unplugged
I've unplugged the system fan!
The BIOS causes the system speaker to blare a horrible, annoying beep and reports Current SYSTEM FAN Speed 0 RPM
555 PWM Output STEMMA
Here comes the 555 PWM Output STEMMA to the rescue!
I've plugged in a STEMMA JST 3-pin cable with DuPont female connectors:
- Red to 5V
- Black to GND
- White to tachometer signal
Consult your motherboard's manual to be certain of pinout.
555 Range
The range selector switch on the 555 PWM Output STEMMA should be flipped left, to the lower range.
The LED indicators will light up for power (green LED on top) and signal (red LED on bottom, which pulses at the output frequency).
SUPER SONIC
You can use a small flathead screwdriver to turn the trimmer pot to the right to increase the pulse speed. Now the BIOS thinks the fan is roaring along at over 8500 RPM!
This won't impact the computer at all, although some BIOS setups may dislike speeds outside of a normal range, usual around 1000 RPM.
Just Right
There we are, 1025 RPM ought to keep everyone happy and now you can get on with your liquid cooling dreams! All thanks to a venerable 555 timer and a little bit of fibbing.
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