Could have been simpler still. You can read the internal temperature of the ATiny85 [1]. You can also get the ATiny85 to speak USB and even be flashed via USB via the micronucleus bootloader [2].

[1] https://andrey.mikhalchuk.com/2011/06/20/reading-attiny85452...

[2] https://github.com/micronucleus/micronucleus

Would be nice if you posted a picture of the schematic in addition to the PCB rendering.

I have been wanting a PWM controller for my Tesla cards in my workstation. Currently I open the case and manually turn up their fan speed when I’m going to use them, and turn it back down afterward. What I would like, but am not very interested in spending the time on, is a microcontroller that controls the fan speed based on messages it gets over serial from a script running on the host that tells it what speed to run based on GPU temperature. When I first set it up, I also placed thermistors that the microcontroller could use as a fallback if it doesn’t receive commands.

Seems like something similar should already exist, but I haven’t been able to find anything that is a close enough fit.

I have a pretty custom GPU cooling setup on a few machines (I run ML workloads locally and I want stuff to be quiet).

Couple of gotchas that I ran across. I found that on Linux, desktop PC fan control support is pretty abysmal. The sensor library that everyone relies on, lm_sensors, is semi-abandoned and didn't recognize sensors on my relatively popular, 7 year old ATX motherboard and GPU. It also requires having Perl installed.

About GPU cooling in particular - modern NVidia cards in particular seem to have a built-in minimum of 30% fan speed when controlling them manually. The connectors are also a different, smaller connector (perhaps a JST PH?).