Switching diode is a kind of semiconductor diode, which is specially designed and manufactured in order to play the role of "on" and "off" in the circuit. When the semiconductor diode is turned on, it is equivalent to the switch being closed (the circuit is connected), and when it is turned off, it is equivalent to the switch being opened (the circuit is cut off), so the diode can be used as a switch. The commonly used model is 1N4148. Because the semiconductor diode has the characteristic of unidirectional conduction, the PN junction is conductive under positive bias, and the resistance in the conductive state is very small, about tens to hundreds of ohms; Under the reverse bias, it is in the off state, and its resistance is very large. Generally, silicon diodes are above 10 μ Ω, and germanium tubes are also tens of kiloohms to hundreds of kiloohms. Using this characteristic, the diode will control the current on or off in the circuit and become an ideal electronic switch.