Structure and principle of photoresistor

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A photoresistor, also known as a light-dependent resistor (LDR), is an electronic component that detects optical signals by utilizing the semiconductor photoconductive effect. It can be made from various forms of materials, such as single-crystal sheets, polycrystalline sheets, sintered polycrystalline films, vacuum-evaporated films, chemically deposited films, or sputtered films. These devices are typically constructed using semiconductor materials and are widely used in applications where light detection and conversion into electrical signals are required. The structure and wiring diagram of a photoresistor are illustrated in Figure 9-2. When exposed to light, the internal photoelectric effect causes the material to conduct more electricity, resulting in a decrease in resistance. This change in resistance allows for a variation in current through a load resistor (RL) and the voltage across it. The stronger the illumination, the greater the current and the lower the resistance. Conversely, when the light source is removed, the photoelectric effect ceases, and the resistance returns to its original level. If a photoresistor is connected within a closed circuit, the current flowing through the loop can be controlled by adjusting the intensity of the light. This makes it possible to convert optical signals into electrical signals, which is essential for many sensing and control systems. Common photo-sensitive semiconductor materials include silicon, germanium, cadmium sulfide, lead sulfide, indium antimonide, and cadmium selenide. Pure semiconductors without luminescent properties can be doped with small amounts of impurities to enhance their photoelectric response. These materials often consist of metal sulfides, selenides, and tellurides, such as cadmium sulfide, lead sulfide, antimony sulfide, cadmium selenide, lead selenide, and lead telluride. The performance of a photoresistor depends on several key characteristics, including dark current, photocurrent, volt-ampere characteristics, illumination response, spectral sensitivity, frequency response, temperature dependence, sensitivity, time constant, and optimal operating voltage. Understanding these factors helps in selecting the most suitable photoresistor for specific applications.