I have a small cabinet in which I keep my video gaming consoles, as well as a Mac Mini. It tends to get quite hot. I needed to install a fan in it, but since the cabinet is located in my bedroom I needed the fan to be very quiet most of the time. I decided to have it controlled via a temperature sensor so the fan really only spins up if the cabinet goes above 80F.
What we did
I used a USB-powered Thermaltake Mobile II fan which has a potentiometer control. I removed the potentiometer and used the Arduino to emulate it.
Using an Arduino PWM pin to send variable voltage to the analog fan, this created a whining sound as the Arduino rapidly switches voltage on and off. I built a simple low pass filter to act as DAC and the whining was resolved.
I used a DH11 sensor which was a part of an OSEPP MHUM-01 board. It took a bit to figure out the pin out of the MHUM-01, but once that part started working as well, the project was basically done.
Some shrink wrap packaging later and I was ready to frag (or be fragged).