Light is a type of electromagnetic radiation. The electromagnetic spectrum is divided into several bands, and the term “light” usually refers to the visible portion of the spectrum. However, in physics, other forms of radiation such as gamma rays, X-rays, microwaves, and radio waves, are also categorized as light.

The visible light spectrum includes wavelengths between 400 and 700 nanometers, positioned between the infrared and ultraviolet ranges. Light carries energy in the form of photons. When photons interact with other particles, they transfer energy through collision.

This fundamental property of light has led to the development of many practical devices, such as photodiodes, photoresistors, and solar panels.

What Is a Photoresistor?

Light has a dual nature, meaning it behaves both as a wave and as a particle. When light strikes a semiconductor material, the photons (light particles) are absorbed by electrons, causing them to become energized and jump to higher energy levels.

A photoresistor, also known as a light-dependent resistor (LDR), is a type of resistor whose resistance changes based on the amount of light hitting its surface. As the intensity of the incoming light increases, the resistance of the photoresistor decreases.

Photoresistors operate based on the principle of photoconductivity. Compared to photodiodes and phototransistors, they are less sensitive to light. Additionally, the resistance of a photoresistor can be affected by changes in the surrounding temperature.

How a Photoresistor Works

Unlike photodiodes, photoresistors do not contain a P-N junction and are considered passive components. They are typically made from high-resistance semiconductor materials.

When light strikes the surface of a photoresistor, the photons are absorbed by the semiconductor. This energy excites the electrons, and if they receive enough energy, they break free from their atomic bonds and move into the conduction band. As more electrons become free to conduct, the material’s resistance drops. As a result, the conductivity of the photoresistor increases as the light intensity increases.

The resistance range and sensitivity of a photoresistor vary depending on the type of semiconductor material used. In complete darkness, a photoresistor can have a very high resistance—often in the megaohm range. However, when exposed to light, its resistance can drop significantly, sometimes down to just a few hundred ohms.

Types of Photoresistors

Photoresistors are classified into two main types—intrinsic and extrinsic—based on the characteristics of the semiconductor material used in their construction. These two types respond differently to various wavelengths of light.

Intrinsic photoresistors are made from pure (intrinsic) semiconductor materials. These materials rely solely on their natural charge carriers. In their default state, there are no free electrons in the conduction band. Instead, they contain holes in the valence band that contribute to electrical conductivity when exposed to light.

To excite the electrons in an intrinsic semiconductor, enough energy must be supplied to move them from the valence band to the conduction band—effectively crossing the full bandgap. This means higher-energy photons are needed to activate the device. As a result, intrinsic photoresistors are designed to detect higher-frequency (shorter-wavelength) light.

In contrast, extrinsic semiconductors are created by adding impurities (a process called doping) to intrinsic semiconductors. These impurities introduce additional free electrons or holes that lie closer to the conduction band. Because of this, less energy is needed to move these carriers into the conduction band. Extrinsic photoresistors are typically used to detect lower-frequency (longer-wavelength) light.

The more intense the light, the greater the drop in resistance within the photoresistor. However, the sensitivity of a photoresistor depends on the wavelength of the incoming light. If the wavelength isn’t sufficient to excite the electrons, the device won’t respond. Extrinsic photoresistors are capable of detecting infrared light, while intrinsic types are suited for sensing higher-frequency light waves.

Symbol of a Photoresistor

Photoresistors, often labeled as LDRs (Light Dependent Resistors), are used to detect the presence or absence of light. They are commonly made from materials such as cadmium sulfide (CdS), lead sulfide (PbS), or lead selenide (PbSe). These devices are sensitive not only to light but also to temperature variations. As a result, even if the light intensity remains constant, changes in temperature can still cause fluctuations in their resistance.

Photoresistor-Symbol

Applications of Photoresistors

The resistance of a photoresistor changes in a nonlinear way with respect to light intensity. While they are less sensitive compared to photodiodes or phototransistors, photoresistors are still widely used in various light-sensing applications. Common uses include:

• Light detection and ambient light sensing
• Measuring light intensity in different environments
• Used in night lights and photographic light meters
• Their response delay (latency) is useful in audio compression and outdoor light detection
• Found in everyday devices like alarm clocks, garden or outdoor clocks, and solar-powered street lamps
• Utilized in specialized fields such as infrared astronomy and infrared spectroscopy, particularly for detecting the mid-infrared region of the spectrum

Projects Based on Photoresistors

Photoresistors are widely used by hobbyists and researchers alike, thanks to their simplicity and versatility. Numerous electronic projects and research studies have explored their use across various fields, including medical devices, embedded systems, and astronomy. Below are some notable projects and applications that incorporate photoresistors:

• A student-designed photometer using a photoresistor for forensic dye analysis
• Integration of biocompatible organic resistive memory with photoresistors for wearable image sensing
• Photogate timing experiments using a smartphone
• Development of a simple acousto-optic dual control circuit
• Systems for detecting the location of light sources
• A mobile robot activated by sound and guided by external light
• An open-source monitoring system for thermodynamic analysis in buildings
• Overheat protection circuits
• Devices for detecting electromagnetic radiation
• An automatic dual-axis solar-powered lawnmower for agricultural use
• A water turbidity sensing system using LEDs and photoresistors for in-situ environmental monitoring
• A light-sensitive illuminated keyboard
• A novel electronic lock using Morse code and IoT technology
• A smart street lighting system designed for urban infrastructure
• MRI tracking systems using computer-controlled detunable markers
• Light-activated window blinds
• Automatic brightness and contrast control in TVs and smartphones
• Proximity-controlled switches using photoresistor input

Due to environmental regulations in Europe, the use of cadmium-based photoresistors (like CdS and CdSe) is restricted. However, photoresistors remain popular in many applications because they are easy to implement and can be seamlessly interfaced with microcontrollers.

These components are readily available in the market as integrated circuit (IC) sensors, including types like ambient light sensors, light-to-digital converters, and LDRs (light-dependent resistors). Some commonly used examples include the OPT3002 light sensor, a digital ambient light sensor from Texas Instruments, and basic LDR-based passive light sensors. Detailed electrical specifications, features, and performance data for the OPT3002 can be found in the datasheet provided by Texas Instruments.

Can Photoresistors Be Used as an Alternative to Photodiodes?

While photoresistors and photodiodes both respond to light, they are not always interchangeable. The key differences lie in their response time, sensitivity, and linearity:

• Photoresistors are slower to respond to changes in light and have higher latency. They are suitable for general-purpose light sensing where speed and precision aren't critical, such as in streetlights or light-activated switches.
• Photodiodes, on the other hand, offer much faster response times and greater sensitivity to light changes. They are ideal for high-speed or low-light applications, such as optical communication, precise measurement systems, or camera light meters.
• Photoresistors are analog and non-linear, while photodiodes offer more linear and controllable responses, especially in reverse-biased mode or when used with amplifiers.

In summary, photoresistors can be used as a low-cost alternative to photodiodes in simple applications. However, for high-speed, accurate, or low-light detection, photodiodes are the preferred choice.

Frequently Ask Questions

What Are Photoresistors Used For?

Photoresistors are mainly used as light sensors. They detect whether light is present or absent and can measure the intensity of light.

How Sensitive Is a Photoresistor?

A common type of photoresistor is made from cadmium sulfide (CdS), a semiconductor material. The sensitivity of these photoresistors closely matches the human eye’s response to visible light, roughly within the wavelength range of 0.4 to 0.76 micrometers.

Do Photoresistors Increase or Decrease Resistance with Light?

Most photoresistors decrease their resistance as the light intensity increases. To measure this resistance change using an analog-to-digital converter (ADC), the resistance is usually converted into a voltage signal. The simplest method for this is a voltage divider circuit.

What Is the Difference Between a Photocell and a Photoresistor?

The terms photocell and photoresistor are often used interchangeably. Both refer to devices that detect light and convert it into an electrical signal. They are commonly found in applications like light meters, automatic streetlights, solar panels, and more.

What Devices Use Photoresistors?

Photoresistors come in various types. Affordable cadmium sulfide (CdS) photoresistors are commonly used in consumer electronics such as camera light meters, clock radios, alarm systems (to detect light beams), nightlights, outdoor clocks, solar street lamps, and solar road studs.

Do Solar Panels Use Photoresistors?

Photoresistors can be part of solar tracking systems. In these systems, photoresistors detect the sun’s position and send data to a controller, which adjusts stepper motors to reposition the solar panel for optimal sunlight exposure.

How to Connect a Photoresistor?

Hardware connection example:

• Connect one terminal of the photoresistor to 5 volts (5V).
• Connect the other terminal to an analog input pin (e.g., Analog Pin 0).
• Attach a 10kΩ resistor between the analog input pin and ground (GND).

This setup creates a voltage divider, where the photoresistor and the fixed resistor form the two resistors, allowing the analog pin to read a voltage that varies with light intensity.