Arduino speed control for a four-wire desktop computer fan. Good for magnetic stirrers.
Four-wire computer fans have speed controlled by PWM applied to one of the wires. The wire is normally pulled up to 5-12 V, and to reduce the fan speed, we should pull it down. Pulling it down all the time would tell the fan to turn at its lowest speed. Leaving it high lets the fan run at max speed. If we use PWM to pull it down some of the time, the duty cycle will determine the fan's speed. Note that you must use a transistor to pull the connection down, as the voltage sometimes goes much higher than an Anduino port can handle.
The fan may not be able to handle very high frequency PWM, so Arduino is advantageous over a faster dedicated PWM controller.
My circuit has the optional feature of using a momentary push button to change whether the fan is on. Connect one end of the button to Arduino and the other end to ground, so the button press pulls the pin down. If you don't need this, you can hook the fan up to a 12 V power supply directly.
I am using a 10 k potentiometer (as a voltage divider) to control the speed.
Measure voltage from the pot's middle pin. If you want to use some other logic
to determine the desired speed, rewrite the getChosenSpeed function.
My circuit uses two 2222A transistors, but you could use almost any transistor.
Since Arduino is popular with beginners, there are a lot of people that don't have a 555 timer. If you have it and know how to use it, use the 555! You may need to limit the frequency so it doesn't get so high the fan gets confused.
Magnetic stirrers are very easy to make with a computer fan, but people typically control their speed by limiting the voltage with a variable resistor. This works badly because they may not have enough torque to get started until they're at nearly full power. (And good luck trying to stir a thick liquid.) Using the built in speed control doesn't have that problem. However, operation is still limited by the lowest speed. My fan's minimum speed is probably still too fast to stir something very thick like honey.
I learned about the speed control in this video: https://www.youtube.com/watch?v=gKHww3qJbs8