PWM Fan Controller schematic

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.
PWM Fan Controller schematic - schematic

The system is quite simple, a temperature sensor will be connected to the input of a microcontroller. There will be two outputs, one for operating the fan and one for operating a status LED. During construction a serial output will also be used for debugging purposes. The LM35 temperature sensor package being used is the TO-92, it is small enough for this project and is leaded so that we can easily connect it to some wires. It is also nice since we can extend it by simply plugging it into a 0.1 inch female wire harness. This is a Celsius device and since the output is 10mV per degree it is very simple to calculate the temperature. It is even nice and simple to directly view the temperature on a multimeter as can be seen in the video. The LM35 outputs 10mV per degree Celsius, this can be read very easily on the meter but unfortunately is isn’t quite that simple to read it within the microcontroller. The output of the LM35 is connected to an ADC (analog to digital converter) so that we can get a usable reading from it, but first we need to do a small calculation. 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...

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