A photoresistor, often referred to as a light dependent resistor, is a resistor that reacts to increased exposure to light by decreasing its resistance in a circuit. They are used in a variety of devices that require sensitivity to light to operate, such as glow-in-the-dark watches and streetlamps that turn on when the sun goes down. Photoresistors are part of a larger group of sensors known as photodetectors, which are devices that react to light.
Resistors are present in almost all types of electric circuits. They function to block the flow of electricity through a circuit so that it stays within a safe range. In the case of a photoresistor, they also serve as a switch, regulating the flow of electricity based on the amount of light that they are exposed to.
Photoresistors are essentially semiconductors, meaning they conduct electricity by means of the flow of electrons. They typically have two prongs connected to a photosensitive plate. When light shining on the plate reaches a sufficiently high level frequency, this stimulates the electrons in the device and gives them enough energy to break free from their bonds. These freed electrons allow electricity to flow through the photoresistor.
The uses for a photoresistor are widespread. They are commonly seen in smaller devices in the form of the cadmium sulfide (CdS) cell. The CdS cell, a term that is largely considered synonymous with the term photoresistor, is found in many forms of clocks and watches, light meters in cameras, and street lamps.
A CdS cell is made by putting cadmium sulfate that has adulterants added, a process known as doping, on a ceramic base. The doping material is closer to where the band for conducting electricity is, so the electrons don’t have to move as far to be unbound and conduct electricity. This means it doesn’t need as high a frequency of light to change the resistance in this type of photoresistor. The main advantage of the CdS cell then is that it is more sensitive to the spectrum of light that humans use, which is one reason why it’s used in items like camera light meters and street lamps.
The other type of commonly seen photoresistor is made of silicon. Silicon photoresistors are not as good a semiconductor as CdS cells because they need higher frequencies of light to conduct electricity due to the fact that the bands to move the electrons are not as easily accessible. A silicon photoresistor is more commonly seen in devices that are sensitive to infrared or red light, such as infrared detectors.
