Fan Control Circuit

Controlling two 12V fans is what this little project is about.

An ATtiny45 monitors an NTC and starts a fan once a certain temperature is reached. PWM varies the fan speed according to the temperature. When the top speed is reached, a second fan is started and its speed is changed according to the temperature, just like the first.

There are two potentiometers, one to set the minimum temperature threshold for the first fan, the other one to set the temperature range. That is, how much hotter than the threshold temperature it must get, before both fans are run at top speed. In other words, the fan1 is started at the temperature set by pot1 and fan2 is started at pot1+pot2.

Perfboard

Some details:

• Fans are always started at full speed for a brief period, as some fans can keep running at a voltage that is not enough to jolt them into movement.
• There is a delay on the spin down. It's not quite an hysteresis, but, in practice, can effectively avoid quick and repeated on/off/on transitions.
• The first fan to start is alternated between the two, so that wear is evenly spread between the two fans. If you need to disable this feature, for instance if fans are not the same type/size, there is a knob called SWAP_FANS that you can define to zero.
• A moving average is applied to temperature readings, with the effect that fan speed variations are smoother (see CMA_WINDOW) .
• At boot time (that is, when the circuit is powered up) a brief test is performed on the fans to make sure that everything is in working order:
  1. the first fan is briefly turned on at top speed and then off
  2. the same is done for the second fan
  3. after a pause to let the fans settle, both are turned on simultaneously and then off again
  If the self-test keeps repeating or looks like it doesn't run at all, you may have a power supply problem; the fans draw too much current and cause a brownout of the IC.
• A reset button and a LED for each fan are there to help you debugging your circuit.

It is possible to use the circuit with just one fan, but if you don't intend to ever use the second header, you may want to get rid of one MOSFET (and related components), and you'll get a more compact board.

At the beginning of the source file there are some constants you may want to fiddle with. Especially:

• FAN[12]_MIN_DC, the minimum duty cycle, which is different for every fan
• SPINUP, the larger the fan, the longer the spin-up phase required
• MIN_TEMP, that depends on the use you'll make of this device

As you may have guessed from the picture above, my prototype employs a 7805 voltage regulator in TO-220 packaging, which is oversized for the job, and two MOSFETs that could easily drive industrial motors. Those were the parts I had at hand. You are free to use more reasonably sized components and waste less room on your board.

The source code is on GitHub. You'll also need avr-gcc, avrdude and, of course, an ISP programmer. If C programming is not your cup of tea, you may try to upload a pre-compiled image I had laying around. Just make upload and good luck.