Photoresistor – Working, Types And Applications

Light is a form of electromagnetic radiation. The electromagnetic spectrum is divided into many bands from which Light usually refers to the Visible Spectrum. But in physics gamma rays, X-rays, microwaves and radio waves are also considered as Light. The visible light spectrum has wavelengths in the range of 400-700 nanometers, lying between infrared ray spectrum and ultraviolet spectrum. Light carries energy in the form of photons. When these photons come in contact with other particles, energy gets transferred due to the collision. By making use of this principle of light, many useful products such as Photodiodes, Photoresistors, Solar panels, etc… were invented.

What is A Photoresistor?

Light has Wave-particle duality nature. Which means that light has both particle-like and wave-like nature. When light falls on semiconductor material, photons present in light are absorbed by electrons and they get excited to higher energy bands.

A photoresistor is a type of light-dependent resistor that varies its resistance values based on the light incident on it. These photoresistors tend to decrease their resistance values with an increase in the intensity of the incident light.

Photoresistors exhibit photoconductivity. These are less photo-sensitive devices compared to photodiodes and phototransistors. Photoresistivity of a photoresistor varies with change in ambient temperature.

Working Principle

The photoresistor doesn’t have a P-N junction like photodiodes. It is a passive component. These are made up of high resistance semiconductor materials.

When light is incident on the photoresistor, photons get absorbed by the semiconductor material. The energy from the photon gets absorbed by the electrons. When these electrons acquire sufficient energy to break the bond, they jump into the conduction band. Due to this, the resistance of the photoresistor decreases. With the decrease in resistance, conductivity increases.

Depending upon the type of semiconductor material used for photoresistor, their resistance range and sensitivity differs. In the absence of light, the photoresistor can have resistance values in megaohms. And during the presence of light, its resistance can decrease to a few hundred ohms.

Types of Photoresistors

Depending on the properties of semiconductor material used for designing a Photoresistor, these are classified into two types – Extrinsic and Intrinsic photoresistors. These semiconductors react differently under different wavelength conditions.

Intrinsic photoresistors are designed using Intrinsic semiconductor material. These intrinsic semiconductors have their own charge carriers. No free electrons are present in their conduction band. They contain holes in the valence band.

So, to excite electrons present in an intrinsic semiconductor, from the valence band to conduction band,  sufficient energy should be provided so that they can cross the entire bandgap. Hence we require higher energy photons to trigger the device. Hence, Intrinsic photoresistors are designed for higher frequency light detection.

On the other hand, Extrinsic semiconductors are formed by doping intrinsic semiconductors with impurities. These impurities provide free electrons or holes for conduction. These free conductors lie in the energy band closer to the conduction band. Thus, a little amount of energy can trigger them to jump into the conduction band. Extrinsic photoresistors are used for detecting the longer wavelength and lower frequency light.

Higher the light intensity, larger the resistance drop of the photoresistor. The sensitivity of photoresistors varies with the wavelength of the light applied. When there is no sufficient wavelength, enough trigger the device, the device doesn’t react to the light. Extrinsic photoresistors can react to infrared waves. Intrinsic photoresistors can detect higher frequency light waves.

Symbol of Photoresistor

Photoresistors are used to indicate the presence or absence of light. It is also written as LDR. These are usually made up of Cds, Pbs, Pbse, etc… These devices are sensitive to temperature changes. So, even when light intensity is kept constant, change in resistance can be seen in the photoresistors.

Applications of Photoresistor

The resistance of photoresistor is a nonlinear function of light intensity. Photoresistors are not as sensitive to light as photodiodes or phototransistors. Some of the applications of photoresistors are as follows-

• These are used as light sensors.
• These are used to measure the intensity of light.
• Night light and photography light meters use photoresistors.
• Their latency property is used in audio compressors and outside sensing.
• Photoresistors can also be found in Alarm clocks, outdoor clocks, solar street lamps, etc…
• Infrared astronomy and Infrared Spectroscopy also use photoresistors for measuring mid-infrared spectral region.

Projects Based on Photoresistors

Photoresistors have been a handy device for many hobbyists. Many new research papers and electronic projects based on photoresistors are available. Photoresistors have found new applications in medical, embedded and astronomical fields. Some of the project designed using photoresistor are as follows-

• Photoresistor based, student-built photometer and its application in forensic analysis of dyes.
• Integration of biocompatible organic resistive memory and photoresistor for wearable image sensing application.
• Photogate timing with a smartphone.
• Design and implementation of simple acousto optic dual control circuit.
• System for light source location detection.
• The mobile robot turned on by sound and directionally controlled by an external light source.
• Design of an open-source monitoring system for thermodynamic analysis of buildings and systems.
• Overheat protection device.
• Device for detecting electromagnetic radiation.
• Automatic dual-axis solar-powered lawnmower for agricultural application.
• Sensing mechanism for water turbidity using LED for an in-situ monitoring system.
• The light-induced luminous keyboard is designed using photoresistors.
• Novel electronic lock using morse code based on the internet of things.
• Street light system for smart cities using photoresistors.
• Tracking of MRI interventional devices with computer-controlled detunable markers.
• These are used in Light-activated blinds.
• Photoresistors are also used for automatic contrast and brightness control in televisions and smartphones.
• For designing of proximity controlled switch photoresistors are used.

Due to the ban on cadmium in Europe, use of Cds and Cdse photoresistors is restricted. Photoresistors can be easily implemented and interfaced with microcontrollers.

These devices are available in the market as IC sensors. They are available as ambient light sensors, Light to digital sensors, LDR, etc… Some of the popularly used products are OPT3002 light sensor, LDR passive light sensor, etc… The electrical characteristics, specifications, etc.. of OPT3002 can be found in the datasheet provided by texas instruments. Can we use photoresistors as an alternative for photodiodes? What makes the difference?