Automatic PC fan control
Use a Atmel ATTINY26
as a serial port based fan controller for a PC. Tracy’s machine is very noisy but has built in sensors for the CPU and case temperature. Use a PC app to do closed loop control on the temperature. Run it hot-ish as the cooling effect increases with the temperature differential so the hotter it runs the slower the fan needs to run.
Use a FET for the fan drive. The reset case on the micro will be full noise. Could also use the on-board ADC and a resistor divider to sense the back EMF when the PWM is off to get an idea of fan speed.
It should be very cheap – the micro is $6.20 and there is _very_ little support circuitry. Instead of using the CPUs sensor, you could add 1-wire or similar temperature gauges directly to the Atmel but at significant additional cost.