This is an easy tutorial showing how you can control the brightness of an LED or adjust the speed of a DC fan using Arduino Board, a small BC547 transistor and a potentiometer. In my example the whole ...

A (10K) potentiometer attached to each axis provides proportional feedback of the up/down and left/right positions. The joystick is spring-loaded, so that it always returns to its centered position ...

Which led him to designing his own prototype Series Elastic Actuators using an Arduino, potentiometers, some elastics, and a geared DC motor.