Photodiode has a PN junction like ordinary diode, but the difference is that there is a transparent window on the shell of photodiode to receive light irradiation and realize photoelectric conversion. In the circuit diagram, the character symbol is generally VD. Photodiode works under the action of reverse voltage. When there is no light, the reverse current is extremely weak, which is called dark current. When there is light, the reverse current rapidly increases to tens of microamperes, which is called photocurrent. The greater the intensity of light, the greater the reverse current. The change of light causes the change of photodiode current, which can convert optical signals into electrical signals and become photoelectric sensing devices.