The video is a primer on encoder design, walking you through the basics of sensing rotation with phototransistors, and how a pair of detectors is needed to determine the direction of rotation.

Rotary encoders are electromechanical devices used for sensing in myriad applications — on motors paired with drives and automated machinery for everything from consumer electronics ...

Continuing his tradition of making bits of wire and scraps of wood work wonders, [HomoFaciens] is back with a unique and clever design for an electromechanical encoder. There are lots of ways to bu… ...