Photo interrupter, photogate is a sensor is composed of an infrared emitter and infrared detector placed on face-to-face. It applies the principle that light is interrupted when an object passes through the sensor. Used for many applications including optical limit switches, general object detection, etc. Basically, a photo-interrupter consists of two parts: transmitter and receiver. The transmitter (IR LED) emits light and then the light goes to the receiver. The transmitter (IR LED) emits light and then the light goes to the receiver. If that light beam between the transmitter and receiver is interrupted by an obstacle, the receiver will detect no incident light even for a moment and the output level will change.