What Is a Light Dependent Resistor?

Photoresistor or light-dependent resistor (the latter is abbreviated as LDR) or photoconductor. Commonly used materials are cadmium sulfide, and also selenium, aluminum sulfide, lead sulfide, and bismuth sulfide. These manufacturing materials have the characteristic that the resistance value decreases rapidly under the irradiation of light of a specific wavelength. This is because the carriers generated by the light all participate in the conduction and make a drift movement under the action of an external electric field. The electrons run to the positive pole of the power supply, and the holes run to the negative pole of the power supply, so that the resistance of the photoresistor decreases rapidly.

Usually, the photoresistor is made into a sheet structure to absorb more

According to the spectral characteristics of the photosensitive resistor, it can be divided into three types of photosensitive

First, according to semiconductor materials: intrinsic photoresistors, doped photoresistors. The latter has stable performance and good characteristics, so it is mostly used.
2. According to the spectral characteristics of the photoresistor, it can be divided into three types of photoresistors:
1. Ultraviolet photoresistors: more sensitive to ultraviolet light, including cadmium sulfide, cadmium selenide photoresistors, etc., for detecting ultraviolet light.
2.Infrared photosensitive

(1) Use a black piece of paper to cover the transparent window of the photoresistor. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinite. The larger the value, the better the performance of the photoresistor; if the value is small or close to zero, the photoresistor is damaged and cannot be used. (2) A light source is aimed at the light transmission window of the photoresistor. At this time, the pointer of the multimeter should swing to the right to a large extent, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If this value is very large or even infinite, it means that the photoresistor has an internal open circuit damaged and cannot be used. (3) Align the light-transmitting window of the photoresistor with the incident light, and use a small piece of black paper to shake the upper part of the light-shielding window of the photoresistor to intermittently receive light. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper If the pointer of the multimeter always stops at a certain position and does not oscillate with the shaking of the paper, the photoresistor is damaged. ^[1]

The lead sulfide photoresistor has a high sensitivity in a wide spectral range, and its peak is in the infrared region; the peaks of cadmium sulfide and cadmium selenide are in the visible light region. Therefore, when selecting a photoresistor, the material of the photoresistor and the type of light source should be considered in order to obtain a satisfactory result. ^[1]