Photoresistors work on the basis of the internal photoelectric effect. The electrode leads are installed on both ends of the semiconductor photosensitive material, and the photosensitive resistance is formed by packaging it in a shell with a transparent window. In order to increase the sensitivity, the two electrodes are often made into a comb shape. The materials used to make photoresistors are mainly metal sulfides, selenides, tellurides and other semiconductors. Usually, coating, spraying, sintering and other methods are used to make a very thin photosensitive resistance body and comb ohm electrode on an insulating substrate, and then connect the lead and package it in a sealed shell with a transparent mirror, so as not to affect its sensitivity by moisture. When the incident light disappears, the electron-hole pair generated by photon excitation will recombine, and the resistance value of the photoresistor will return to the original value. The metal electrode at both ends of the photosensitive resistance with a voltage, which will have a current through, by a certain wavelength of light irradiation, the current will increase with the increase of light intensity, so as to achieve photoelectric conversion. Photosensitive resistor has no polarity, is purely a resistance device, can be used to add dc voltage, also add AC voltage. The conductivity of a semiconductor depends on the number of carriers in its conduction band.