> I did try the internal temperature sensor, but it didn't measure the ambient temperature. I have read that if you set the ATtiny85 to sleep mode for some time

> I did try the internal temperature sensor, but it didn't measure the ambient temperature. I have read that if you set the ATtiny85 to sleep mode for some time, it will converge to ambient temperature, but it would take too long to validate, and I didn't want to go down another rabbit hole. It's kind of better than ambient, it would reach the temperature of the GPU if thermally coupled. The sleep-mode thing would just be a case of lots of micro sleeps to discount the temperature of the chip, could have even under-clocked too. > USB Bootloader is interesting, but I already had a few ISP loaders. You can make an ISP loader with Arduino as well. The benefit of USB is being able to get debug data from the fan controller as it runs, and be able to re-flash it whilst in the PC by attached to an internal USB bus. This way you could also log out what the GPU reports, vs what you measure, vs what the ATtiny85 measures.