So I have yet to implement this myself, but I plan to within the next few weeks. But it simulates the way I expected it to work. The circuit produces a switching signal that changes the speed that your DC fan runs at. The MCP9700A is the temperature sensor...I modeled it with a voltage source because there was no symbol for it in my schematic program. Just connect power and ground to the actual sensor, and connect its output to the plus terminal where the voltage source is in the schematic.
The circuit is by no means accurate, the temperature thresholds can vary by about 5 degrees celsius. The circuit is set up to run the fan at 40% full speed if the temperature is less than 35C (95F), 60% full speed between 35C and 50C (122F), and at full speed if it gets above that. I just sort of chose those values semi-randomly, if anyone has suggestions for more reasonable temperature values I can just change some resistors. Just make sure the MCP9700A is in good thermal contact with the heat-sink on the main processor.
I plan to use this circuit just to keep fan noise to a minimum, but it will also save a decent amount of power if you're running off a battery. Since the fan won't constantly be pulling the maximum current when it doesn't have to. If you want different temperatures or fan speeds, they're just resistor values that need to be changed. I'll leave it up to the reader to determine the necessary values to get the desired results
