Photoresistors are special resistors made of semiconductor materials such as cadmium sulfide or cadmium selenide, which work based on the internal photoelectric effect. The stronger the light, the lower the resistance value, with the increase of the light intensity, the resistance value decreases rapidly, bright resistance value can be as small as 1KΩ below. Photoresistor is very sensitive to light, it is in the absence of light, high resistance, dark resistance generally up to 1.5MΩ. With the development of science and technology, the special performance of photoresistor will be widely used.

Photoresistor, also known as photoconductivity detector, is a kind of resistor whose resistance value changes with the intensity of incident light, which is made by using the photoconductivity effect of semiconductor. Incident light intensity, the resistance decreases, incident light weak, the resistance increases. And then there's the other one where the incident light is weak and the resistance goes down, and the incident light is strong and the resistance goes up.

Photoresistors are generally used for light measurement, light control, and photoelectric conversion (converting changes in light into changes in electricity). Commonly used photoresistor cadmium sulfide photoresistor, which is made of semiconductor material. The sensitivity of the photoresistor to light (that is, the spectral property) is very close to the response of the human eye to visible light (0.4~0.76) μm, as long as the light that the human eye can perceive, will cause its resistance value to change. Design of light control circuit, all use incandescent bulb (small electric bead) light or natural light as the control source, so that the design is greatly simplified.