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This is a circuit I designed to reduce the noise made by all the fans in my case. I thought fellow OCAU readers may be interested in it.

Now, don't get me wrong - I'm all for my case sounding like a jet aircraft during a LAN-fest to keep my overclocked processor and graphics card cool. When I'm just browsing the Internet or word processing though, all those fans are damn loud!

What I wanted was something that would make the fans quiet when I'm not stressing the system, and will crank them up when things get hot. I could have installed some switches, but that didn't seem the most elegant solution - I'd have to switch the fans on/off. No, an automated solution was needed.

Since I had a few 10k NTC thermistors lying around (the type you connect to most motherboards) I figured a thermally regulated solution was in order. The circuit uses an NTC thermistor mounted in the case to sense the air temperature in the case. When the temperature is below about 20°C, the circuit idles the fans at about 4V DC. This turns them over very slowly but still quickly enough to push a little air around. The fans are nearly inaudible at this speed. As the temperature in the case increases, the circuit increases the voltage supplied to the fans (which then spin faster), until it supplies the maximum voltage at about 35°C.

Some notes about this circuit:

• because of the presence of the transistor, the fans never get the full 12V - the maximum will be about 10.5V. This may reduce the appeal to some people.
• the thermistor can be put anywhere in the case. If you attach it to your CPU heatsink, the circuit will regulate the speed of the case fans according to the temperature of the heatsink. It may also have other uses.
• I take the 12V and 0V rails from the power connectors in the case
• the 5k potentiometer at the positive input to the op-amp is used to adjust the threshold temperature at which the thermistor circuit starts to power the fans
• the 5k potentiometer at the base of the transistor is used to adjust the voltage supplied to the fans when the temperature is low. To calibrate it, remove the thermistor from the circuit and gradually increase the 5k pot until the fans start to turn. This ensures that the fans always receive enough power to turn them, even if the thermistor circuit is providing an output lower than that.

You can add more fans to the circuit - just add them in parallel to those already shown. A lot of fans will means a few Amps will be running through the transistor, so people will need to choose a transistor capable of dealing with the expected current, and possibly add a small heatsink if it gets hot. I'm using a BC548 NPN transistor, which is a very common unit. It's only small, and with the 2 fans I have connected at the moment, it doesn't get warm.