You can see the PWM Fan Controller project here.
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.
Step 1: PWM Fan Controller - Parts
Step 2: PWM Fan Controller - LM35 Sensor Overview
Step 3: PWM Fan Controller - Reading Data With the Microcontroller
The analog input has a range of 0 to 5 volts and the digital resolution is 10 bits. This means that the 0 to 5 volt range will be represented by a number from 0 to 1023 in the microcontroller. If we divide 5 volts by 1024 we get the number of volts per digital increment. In this case it is 0.004883 volts per increment. So if the voltage was 1 volt on the microcontroller pin we would expect to have a reading of around 204 as the digital number (1/0.004883).
To convert the digital value to a degrees Celsius number we need to do a bit of math. If we take the digital result and multiply it by 49 then divide it by 100 we will get a result that is very close to the correct result. The calculation looks a bit weird since we are working with integer math. With integer math the information after the decimal is lost. For example 25/3 would be 8 (not 8.333).
Here is a full example. If we have a temperature of 15 degrees Celsius the voltage out of the LM35 would be around 10mV * 15 = 0.15V we know that each 0.004883 volts is one count for the digital side of the ADC therefore 0.15V / 0.004883 V = 30.73 this would provide a value of simple 30. Finally if we use the formula 30 * 49 / 100 = 14.7 but it would be stored as 14.