What is a photoresistor?
A photoresistor is also called a light-dependent resistor (LDR) and is a passive electronic component. Photocell and photoconductive cells are other names for photoresistors, this component is crucial in circuits involving resistors, rheostats, potentiometers, thermistors, and color-coding resistors. The resistance of the photoresistor decreases for the luminosity (light) received on the sensitive surface of the component.

A photoresistor produces the photoconductivity. Hence, the phenomenon of photoconductivity is defined as the resistance of the photoresistor decreasing concerning the increase in incident light intensity.

Behavior of a photoresistor in different lighting conditions
A photoresistor can be utilized in light-sensitive detector circuits, light-activated switching circuits, and dark-activated switching circuits as a resistance semiconductor.
In the dark, the resistance of the photoresistor will be high (mega ohms), and in the light, the resistance of the photoresistor will be low (a few hundred ohms).
How do photoresistors respond to light?
When the incident light on the photoresistor goes beyond a certain frequency, photons will be absorbed by the semiconductor, giving bound electrons enough energy to jump to the conduction band. So, the resulting free electron-hole pairs will be responsible for conducting the electricity, thereby lowering the resistance. The resistance and sensitivity of the photoresistor can vary among dissimilar devices.
Characteristics of a photoresistor
Types of photoresistors and working mechanisms
Depending on the type of materials used, photoresistor can be divided into two types.
Intrinsic
Extrinsic
Intrinsic photoresistors are non-doped materials such as silicon and germanium. Whenever the photons fall on the device, they move the electrons from the valence band to the conduction band. This results in more free electrons in materials that are available for carrying current, and hence resistance is less.
Extrinsic photoresistors are made of dopants. Dopants are defined as materials that are made with impurities. A new energy band is developed above the existing valence band by the dopants, populated by electrons. Due to the smaller energy gap, these electrons need less energy to make the transition to the conduction band.
Both types of devices will exhibit a reduction in resistance when they are illuminated. The greater the light intensity, the greater the resistance drop.
Wavelength dependency
The sensitivity of the photoresistor differs with the wavelength of light. If the wavelength is beyond a certain range, it will not affect the device's resistance. Extrinsic LDRs are usually designed for lights with longer wavelengths with a tendency toward the infrared range. When working in the IR, to avoid heat buildup, care must be taken because measurement will be affected by changing the resistance of the device due to thermal effects.
Sensitivity
Photodiode or phototransistor are more sensitive to light than photoresistor. Photodiode and phototransistor are semiconductor devices; photoresistor is a passive electronic component that doesn't have a PN junction. The photoresistivity of a photoresistor is varied depending on the ambient temperature, and hence, it is not suitable for applications that demand precise measurement of or sensitivity to light photons.
Latency
Between changes in illumination and changes in resistance, there would be a time delay. This is called the resistance recovery rate. When the light is applied after total darkness, it takes about 10 ms for the resistance to drop, but up to 1 s for the resistance to rise to its starting value after the light is removed completely. Due to this reason, LDR cannot be used in applications where rapid light fluctuation needs to be recorded.
Construction of a photoresistor
A photoresistor is constructed by mounting a thin, zigzag-shaped photosensitive device on the insulating material. Cadmium Sulfide (CdS), Cadmium Selenide (CdSe), and Lead Sulfide are the materials used in photoresistors and are sensitive to light. Ceramic is the insulating material used in photoresistors. The metal films are connected with terminal leads. The complete structure of the photoresistor is placed either in a plastic case or resin case to avoid direct sunlight exposure. In the absence of light, very high resistance will be there, and it is about mega ohms. When the light is incident on the photoresistor, there will be a decrease in resistance and an increase in conductivity as well.
Working principle of a photoresistor
When a photoresistor is not exposed to light, the photoconductive material will not have any free electrons or a few free electrons. When a photoresistor is exposed to light, the conduction band breaks, and a greater number of free electrons and holes will be formed. Free electrons and holes will be formed. The free electrons and holes gain energy, and they jump from the valence band to the conduction band. As a result, the current is generated. When the incident light increases on the photoresistor, the resistivity of the photoresistor decreases.
Comparing Rheostats and Photoresistors
While rheostats control current by manual adjustment, photoresistors respond to light intensity.
Rheostats suit applications requiring dynamic control, while photoresistors excel in light-dependent scenarios.
Both components contribute to the flexibility and functionality of electronic circuits.
Roles of LDRs, Potentiometers, Thermistors, and Color-Coding Resistors in Electronics
Light-Dependent Resistors (LDRs),