Dreamcast Dreamcast: Faking the FANs PWM-Signal

[rayik24]

This is a cut and paste of a guide by Nold at BitBuilt found here. I found this while stumbling around one day.

Faking the FANs PWM-Signal
Why use Arduinos or 555's... damn... just use some solder!

[See attached "Image 1 - fake_pwm_hack.png "]

Some background: The DC uses IC601 (a multivibrator IC) as some kind of ADC (Analog-Digital-Converter), to convert the PWM-Signal from the fan (up to 2.5v), to 3.3v (= logical "true"). So when there's no fan, it'll put out 0v (= logical "false"). So all we need to do is pull-up the output signal of the IC & done! How nice Sega prepared this mod for us & gave us some free solder-pads.. (It might be safer to use a resistor here, but it should even work without the IC, anyway; also there is a capacitor, protecting a possibly connected fan.)

 

[Nopileus]

Alternatively if you are still using a 5v fan with tacho signal (like the noctua) you can connect a pullup resistor (i used 10k) between tacho and 5v so the system will detect fan RPM correctly.
It's really easy to add to the underside of the controller PCB, directly to the fan connector.