Anyone who uses a computer for long periods will appreciate the benefits of a silent PC. Quite a few websites now sell computer accessories specifically designed to make your desktop run more quietly. The CPU fan is often the main culprit in a noisy PC; in many cases it can be replaced by a large passive heat sink to dissipate the heat more efficiently. The heat sink fins are arranged to make optimum use of air blown through the case by the power supply fan. The specification of Intel’s ATX type PC form factor even suggests that the cooling air should be used in this way but to be successful on modern machines it is necessary to pay careful attention to a number of factors. Firstly it is important to use a processor which has the lowest possible power consumption (especially in idle mode), the lower cost 45 nm technology chips are a good place to start here. Secondly it is important to pay attention to the air flow in the case to ensure that it is ducted efficiently from the PSU through the passive CPU heat sink. The main drawback with this setup is that fan speed is only controlled by the temperature of the PSU, not the processor.
The solution of course is to install a new fan speed controller and fit a temperature sensor to the CPU heat sink. The controller senses the air temperature in the PSU as well as the processor heat sink and adjusts the fan speed according to the warmest reading. This approach ensures that everything remains cool. With this in mind the author built this versatile fan speed controller using little more than a small microcontroller, a few transistors and two NTC thermistors. The main circuit element IC1 is an 8-pin 8-bit ATtiny13 microcontroller from Atmel. This controller has more than enough 10-bit resolution analog inputs for the job. The circuit diagram is not so complicated: Two thermistors are connected between NTC1 and NTC2 of K3 and ground. Together with R1 and R2 they form two voltage divider networks.
The voltages produced at NTC1 and NTC2 are proportional to the measured temperatures. These are sampled by the analog inputs ADC2 and ADC3 of the microcontroller. The controller will select one of ten fan speed settings depending on the measured values of temperature. The higher of the two temperature readings will always be used. The output from pin 6 is a pulse modulated waveform to control fan speed. The output Darlington configuration of T1/T2 drives the fan from the PWM waveform integrated by R6/C2. This low pass network filters out the 15 Hz fundamental of the PWM output signal to reduce any PWM noise generated in the fan windings.