What is a Photoresistor? Structure and principle of operation?

A photoresistor, or LDR (Light Dependent Resistor), is an electronic component that changes its resistance value according to the intensity of incoming light. With this ability, photoresistors are widely applied in many fields such as medical, security systems or light sensors. In this article, join RX Tradex to learn about what is a photoresistor? The structure and principle of operation of photoresistors.

1. What is a photoresistor?

A photoresistor, also known as a photoresistor or LDR (Light Dependent Resistor), is a type of electronic component capable of changing its resistance value based on the intensity of the light hit.

2. The principle of operation of photoresistors

The principle of operation of photoresistors is based on the internal photoelectric effect. The internal photoelectric effect is the phenomenon of releasing free electrons from atoms or ion atoms when they absorb photons of sufficiently large energy. The energy of the photon must be greater than or equal to the binding energy of the electron to the atom or ion atom.

When there is light shining on the photoresistor:

• Photons in light will collide with electrons in the semiconductor’s conduction region.
• If the energy of the photon is large enough, the electron will be released from the atom and become a free electron.
• An increased number of free electrons leads to increased electrical conductivity, and the resistance of the photoresistor decreases.

The degree of decrease in resistance of the photoresistor depends on the intensity and wavelength of the incoming light.

When there is no light hitting the photoresistor:

• Free electrons will recombine with atoms or ion atoms.
• A decrease in the number of free electrons leads to a decrease in electrical conductivity, and the resistance of the photoresistor increases.

The resistance of the photoresistor can vary from a few MΩ in the absence of light to several kΩ  when light is hit.

3. Resistive unit

Ohm (symbol: Ω) is a unit of resistance in the International System of Measurements (SI), named after German physicist Georg Simon Ohm. 

One ohm is equivalent to one volt per ampere (1 Ω = 1 V/A). Resistors can have a variety of values, including milliohm (1 mΩ = 10-3 Ω), kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω), which flexibly measure and represent different resistance levels in circuits and electronic applications.

4. What is the application of photoresistors?

Thanks to its ability to change the resistance value when light hits, photoresistors are widely applied in many different fields such as:

4.1. Light sensor

• Automatic light on and off: Photoresistors used in light sensors automatically turn on lights when it’s dark and turn lights off when it’s light.
• Adjust screen brightness: In smartphones, tablets, and laptops, photoresistors are used to adjust screen brightness automatically according to ambient light conditions.
• Automatic door sensors: Used in automatic door sensors to open the door when someone approaches and close the door when people pass.
• Camera: In-camera use to measure the amount of light and adjust exposure settings.

4.2. Optical counter

• Counting the number of objects: Photoresistors are used in optical counters to count the number of objects passing through a production line. For example, in the food industry, photoresistors are used to count the number of cans of soft drinks passing through the production line.
• Traffic control: Used in traffic control systems to count the number of vehicles passing through an intersection.

4.3. Optical relay

Control of electrical equipment: Photoresistors are used in optical relays to control other electrical equipment. For example, optical relays can be used to turn water pumps or blowers on and off when there is light hitting the photoresistor.

4.4. Security system

Intrusion detection: Application to security systems to detect intrusions. When a thief passes, they obscure the light shining on the photoresistor, triggering an alarm.

4.5. Application in the medical field

Photoresistors are used in medical devices to measure biological parameters such as heart rate and blood pressure.

5. Is the guide for testing photoresistors still working?

To check if the photoresistor is still working, we need to perform the following steps:

• Switch VOM to resistance measurement mode: Set the multimeter (VOM) to resistance measurement mode (ohm scale).
• Connect the VOM to the photoresistor: Place the two probes of the VOM on the legs of the photoresistor.
• Test in the dark: Cover the resistor with your hand or any object to create a dark environment, observing the number of ohms displayed on the VOM. When masked, the ohm number of the photoresistor will increase.
• Test under light: Shine light on the surface of the resistor, as intensely as possible. Observe the number of ohms on the VOM. In the presence of bright light, the number of ohms measured decreases.

Test result:

• If the number of ohms changes dramatically when switching from a dark environment to a light environment and vice versa, the photoresistor still works fine.
• If there is no change or very little change, it is possible that the resistor has failed or is not working properly.

6. Summary

With the above information, hopefully you can better understand what photoresistors are, and their operating principles and structure to apply in your production process. If you want to learn more about efficient manufacturing solutions in the field of electronics industry, you can register to participate in NEPCON Vietnam 2024 organized by RX Tradex next August.