Next, the encoder that we will use is quite common and can be found at the following link: http://www.ebay.com/itm/Wheel-Encoder-Kit-For-Robot-Car-/221245494078?hash=item338342533e. The encoder comes with a disk and a chip, which does the job with the light beam explained earlier.

Next, you can see the part of the encoder that actually spins are the wheel spin and let the light beam reach its destination through the holes. Expensive robots have more than 2000 holes but, as said, they are really expensive:

Next you can see the electronic part of the encoder. This black plastic contains a sensor that activates a beam from the one edge to the other. If there is a hole between the beam, it reaches its destination and we call it as HIGH or 1 and if something is between we call it as LOW or 0. As the mobile robot moves, we have many 0 and 1 and this is a pulse: