In the PN junction of some semiconductor materials, the superfluous energy will be released in the form of light when the injected minority carriers are combined with the majority carriers, thus directly converting electric energy into light energy. PN junction plus reverse voltage, a few carriers are difficult to inject, so it does not emit light. This kind of diode using injection electroluminescence principle is called light-emitting diode, commonly known as LED. When it is in the positive working state (that is, the two ends of the positive voltage), the current from the LED anode to the cathode, the semiconductor crystal will emit different colors of light from ultraviolet to infrared, the strength of the light is related to the current. At first, LED was used as an indicator light source for instruments and meters.