An encoder is a simple device that can output a digital signal for each small portion of a movement. To do this, the encoder disk or strip is divided into small sections.Each section is either opaque or transparent(it can also be either reflective or nonreflective). A light source, such as an LED on one side, provides a beam os light to the other side of the encoder disk or strip, where it is seen by a light-sensitive sensor,such as a phototransistor.If the disk’s angular position is such that the light is revealed, the sensor on the opposite side will be turned on and will have a high signal.