So you have a nice little motor and maybe you want to turn it into a robotic project but, alas, it doesn’t have an encoder built into it cause it’s a cheapy thing you found at a swapmeet. Prof. Mason has a nice and simple solution that you can make for $5 and retrofit yourself: a magnet and two hall effect sensors. I bet if you used a Schmitt trigger’d comparator on the outputs you would have a pretty effective encoder that can go straight to your robot brain. I imagine this design is less finicky than optointerrupts + disc. Don’t forget to plug the signals into the microcontroller interrupt pins because they’ll be flying by rather fast!

Is there such a thing as a magnetic set-screw shaft collar? That might avoid the uneven weight problems of gluing a big magnet on.