It utilizes optical elements to change the light beam in the media and makes use of the light beam to reflect the object, instantly sending the light beam back after long distance. The photoelectric sensor is composed of the transmitter, the receiver and the testing circuit. The emitter emits a beam at the target. The light beam emitted generally comes from the light emitting diode (LED) and the laser diode. It continuously emits light beam or change the width of the pulse. The radiation intensity of the pulse-modulated beam runs indirectly towards the target  after many times of selection during the emission. The receiver is composed of a photodiode or a phototriode. In front of the receiver, it is equipped with optical elements such as lenses and apertures. Behind them is a testing circuit that filters out valid signals and uses them.

A photocoupler is a kind of electric-optical-electric converter which transmits electrical signals by light. It consists of the light source and the light receiver. The light source and the light receiver are assembled in the same closed shell and are isolated by transparent insulator. The pin of the light source is the input end, and the pin of the light receiver is the output end. Light source we usually see is the light-emitting diode, and the receiver is the photodiode, or phototriode and so on. There are many types of photocouplers. What we usually see are photodiode type, phototransistor type, photoresistor type, optically controlled thyristor type, photo darlington type, and integrated circuit type, etc. The working principle is adding electrical signals to the input end of the photocoupler to make the light source luminous.The intensity of the light depends on the magnitude of the excitation current. After the light is irradiated onto the enclosed receiver, the photocurrent will come into being for the photoelectric effect, which is extracted by the output end of the receiver. In this way, the electric-optical-electric conversion is realized.

The modulated pulse generated by the oscillator circuit is emitted by the light emitting tube GL after the reflection circuit. When the tested object enters the scope of the light receiver, the reflected light pulse enters phototriode DU. Photoelectric sensors demodulate an optical pulse into an electric pulse signal in a receiving circuit. Then after amplification through the amplifier and simultaneously shaping, it eliminates the interference by digital integral or RC integral, and finally triggering the driver to output the control signal of the photoelectric sensor with or without retardation. Usually, photoelectric sensors have fine return difference, hence, even the tested object wobbles in a small area, it won't affect the output state of the driver, thus maintaining it in a stable working area. At the same time, the self-diagnosis system can also display the light-receiving state and the stable working area, monitoring the work of the photoelectric sensor at any time.