Choosing the right fans for your PC build is more important than you might think. With the ongoing quest for quieter, cooler, ...

From there he wrote a RUBY program to monitor the CPU temperature and generate a PWM signal on the serial port, throttling the fan speed as needed.

Of course, the standard way of controlling CPU fans these days is with PWM, so he built a circuit which essentially converts the PWM signal from the motherboard into a phantom thermistor.