Optical encoder kit
I designed this kit based on the encoder kit sold at Solarbotics. Importing this kit into India was way too expensive at that time. So I had to come up with my own version to go along with the robots that we were selling at my former workplace. Everything had to start from scratch, including the wheels.
The design uses two opto-reflector sensors (mostly used in copy machines to detect the presence of paper) to detect the strips on the wheel. The signals are then filtered through schmitt triggers to give out a clean quadrature signal.
The design went through a few iterations before everything worked nicely and was ready to ship: (pardon me for the poor image quality)
I made a few fancy wheel designs but in the end the simplest of them was chosen to reduce machining and molding costs.
You can buy these wheels here.