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Photoresistor(LDR) : Working Principle, Types and Applications

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.

Photoresistor or LDR
Photoresistor or LDR

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.

Symbol of LDR
Symbol of LDR

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.


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.


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), also known as photoresistors, share common ground with potentiometers, thermistors, and color-coding resistors in the realm of electronics. Here's how they are related:


  • Relationship: Both LDRs and potentiometers are types of resistors.

  • Connection: Potentiometers are variable resistors that allow manual adjustment of resistance. In contrast, LDRs exhibit variable resistance based on light intensity. Both contribute to the overall resistance in a circuit.


  • Relationship: LDRs and thermistors are both types of resistors with variable characteristics.

  • Connection: While LDRs respond to changes in light, thermistors respond to changes in temperature. Both introduce variability in resistance, making them crucial in circuits that require sensitivity to specific environmental factors.

Color-Coding Resistors

  • Relationship: LDRs share the resistor category with color-coding resistors.

  • Connection: Color-coding resistors use a standardized system of colored bands to indicate their resistance values. While LDRs don't use color coding, they align with color-coded resistors in their broader category as resistive components in electronic circuits.

LDRs, potentiometers, thermistors, and color-coding resistors are all types of resistors, each with unique characteristics. LDRs stand out by responding to light, potentiometers by offering manual resistance adjustment, thermistors by responding to temperature, and color-coding resistors by providing a standardized identification system for resistance values. Despite their specific functions, they collectively contribute to the diversity and versatility of resistive components in electronic circuits.

Advantages of photoresistor

  • LDR price is less.

  • Different sizes and shapes are available on the market.

  • In the practical LDR, there are different sizes available on the market, and the most popular size is the 100-mm phase diameter.

  • LDR operation requires less power and less voltage.

  • LASER security systems

  • Relative fluid density measurement

  • Dark sensors in street lights

Limitations of a photoresistor

  • Slow to respond with respect to changes in light intensity

  • It can be influenced by fluctuations in temperature.

  • Consistent performance is difficult to achieve because resistance can vary between individual components.

Applications of photoresistor

  • Light-sensitive detector circuits

  • Light-Activated Switching Circuits

  • Dark-activated switching circuits

  • To measure the light intensity

  • To determine whether the light is present or absent

  • Automatic lighting clock

  • Smoke detector alarm

  • Solar street lighting

  • Optical circuit designs

  • Utilized in lights that turn on/off automatically based on light

  • Automatic security light

  • Light intensity meters

  • LDR sensor modules

  • Infrared astronomy

  • Television receivers for automatic contrast control and brightness control

  • Optical coding

  • Photosensitive relay

  • Proximity switch

  • Light-activated control circuits


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