So [Faransky] uses a X9C104P 100KOhm 8-bit digital potentiometer as a voltage divider on the chip’s output.

It’s a simple enough set-up in which a voltage controlled current source feeds the pot while an Arduino with a motor controller turns it through a stepper motor, and takes a voltage reading from ...

We set the potentiometer pin as 0 (A0 on the Arduino Uno board), the CONTROL variable is the voltage that is used to cotrol the led brightness of the speed of the fan.