There are several advantages for using a micro controller as a fan control device. First, the micro controller has a built in analog to digital converter and a PWM generator eliminating the need to build this functionality using separate components. The component count can be significantly reduced. Secondly, once the hardware has been designed, the software can be updated to change the temperature to fan speed curve or add extra features. A kick start is easy to add with no additional circuitry.
The basic design consists of a thermistor network, a micro controller with an analog to digital converter and a PWM generator, and an 8 led display. Because 4-wire PWM fans require only a 12 volt source and a PWM signal to control speed, the fan controller does not need a drive section. In other words, the same controller can be used effectively for a 0.2 amp 70mm fan or a 2.0 amp 120mm fan. A 12 volt source can be wired directly from the power supply. The thermistor network and micro controller require 5 volts.
Printed circuit board design detail for PIC16F684 fan controller.
I used Eagle software for the circuit board design and layout.
Printed circuit board (PCB) with surface mount components, PIC16F684 socket, and 4 pin molex connector.
The PIC16F684 socket is mounted above board level components and circuit traces.
Completed PIC16F684 fan controller with 70mm fan. The red wires are thermistor leads.
In operation, the led display shows a digital value of the voltage input to the A/D converter by the thermistor network. The printed circuit board is 1 x 2 inches.
References:
My new 4-wire fans
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