I made a PWM Fan Controller to cool my external hard drive that got quite warm when it was running for an extended period of time. There are 4 videos documenting the build below. Have a look at the project page for the schematic, code and additional information.
The heart of the PWM Fan Controller is a PIC 12F675 microcontroller. This microcontroller is reading the analog output of a LM35 temperature sensor using a ADC (analog to digital converter) . The resulting digital value is converted to a temperature and a fan is powered proportionally to how hot the sensor is. The sensor is mounted against the hard drive chassis so it is measuring the actual drive temperature and not just the air temperature inside the housing.
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Nicely done, haven’t you thought of lowering the temperature of shutting off these fans a little bit lower than the temperature where they are switched on? avoiding fans from rapidly turning on/off when the temperature happens to be in that switching point?