What is a Light Dependent Resistor?

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- GoPhotonics

Jun 6, 2023

A Light-Dependent Resistor (LDR), (also known as a photoresistor, photocell, light-dependent resistor, or photo-conductive cell), is a light-sensitive resistor or a light-sensitive sensor used in electronic circuit designs for the purpose of detecting the presence or intensity of light. The resistance values of LDRs can vary across multiple orders of magnitude, with resistance decreasing as the light level increases. That is, it exhibits a change in resistance when exposed to light. 

The sensitivity of LDRs or photoresistors also varies depending on the wavelength of the incident light. Unlike other types of resistors commonly used in electronic designs, such as carbon film resistors, metal oxide film resistors, and metal film resistors, LDRs are specifically designed to be light-sensitive and exhibit changes in resistance accordingly.

LDRs are manufactured using semiconductor materials to confer their light-sensitive properties. Various materials can be employed, with cadmium sulfide (CdS) being a popular choice for these photoresistors. Lead sulfide (PbS) and indium antimonide (InSb) are examples of other materials that are used.

Even though LDRs are made of semiconductor materials, they are considered passive devices since they lack a PN junction, which distinguishes them from other photodetectors such as photodiodes and phototransistors.

The low cost, ease of manufacturing, and ease of use of LDRs have led to their widespread application in diverse fields. They have been extensively used in photographic light meters and various applications requiring light level detection.

Symbol of LDR

The symbol used to represent LDRs in electronic circuits is based on the resistor circuit symbol but incorporates arrows to depict the presence of light. This symbol is similar to that of photodiode and phototransistor circuit symbols, where arrows represent incoming light.

The circuit symbols for light-dependent resistors or photoresistors are illustrated using both the newer style of resistor symbol, represented by a rectangular box, and the older zig-zag line resistor circuit symbol.

Structure of LDR

The photoresistor is a light-sensitive resistor that has a flat, horizontal body which is exposed to light. The lightly doped active semiconductor region is deposited onto a semi-insulating substrate and this arrangement ensures the light-sensitivity of the photoresistor.

In many discrete photoresistor devices, an interdigital pattern is employed to enhance the area of the photoresistor that is exposed to light. This pattern is etched into the metallization layer on the surface of the active region, allowing light to pass through. The two metallized areas serve as the contacts for the resistor. It is essential for this area to be relatively large to minimize the resistance at the contact points with the active region.

Semiconductor materials used for constructing photoresistors include CdSe, CdS, CdTe, InSb, InP, PbS, PbSe, Ge, Is, and GaAs. Each material offers distinct properties in terms of wavelength sensitivity and other factors.

Working Principle of LDR

Light-dependent resistors operate based on the principle of photoconductivity. The principle of photoconductivity states that certain materials, known as photoconductors, exhibit changes in their electrical conductivity when exposed to light. In the case of an LDR, which is a photoconductive material, when light falls on it, the energy carried by the light is absorbed by the material. As a result, the electrons in the valence band of the photoconductive material become excited and go to the conduction band. This process leads to an increase in conductivity proportional to the intensity of the incident light.

For this phenomenon to occur, the energy carried by the incident light must be greater than the bandgap energy of the photoconductive material. Only then can the electrons in the valence band be excited to the conduction band.

In its unilluminated state (darkness), an LDR exhibits the highest resistance, typically around 1012 Ohms. However, as light intensity increases, the resistance of the LDR decreases accordingly.

Light Intensity Vs Resistance

An electrical current is characterized by the movement of electrons within a material. Materials with high conductivity possess a large number of free electrons capable of drifting in a specific direction when subjected to a potential difference. For insulators with high resistance have a minimal amount of free electrons, making it difficult for them to move and for a current to flow.

An LDR or photoresistor is constructed using a semiconductor material with high resistance. Its high resistance arises from the lack of free electrons capable of movement. Most electrons are tightly bound within the crystal lattice and unable to migrate, resulting in a high resistance state for the LDR.

When light illuminates the semiconductor material, the photons within the light are absorbed by the lattice structure of the semiconductor. Some of the energy from the photons is transferred to the electrons within the material. This energy transfer provides a sufficient energy to some electrons, enabling them to break free from the crystal lattice and become capable of conducting electricity. Hence, the resistance of the semiconductor decreases, leading to a reduction in the overall resistance of the LDR.

Types of LDR or Photoresistor

There are two main categories of light dependent resistors or photoresistors:

Intrinsic photoresistors: Intrinsic photoresistors use semiconductor materials like silicon or germanium that have not been doped with impurities. When photons strike the LDR, they excite electrons, causing them to excite from the valence band to the conduction band. This transition allows the electrons to move freely and conduct electricity. As more light reaches the device, a greater number of electrons are released, resulting in increased conductivity and a decrease in resistance.

Extrinsic photoresistors: Extrinsic photoresistors are created by adding impurities or dopants into the semiconductor materials. These dopants introduce a new energy band above the existing valence band. Hence, the electrons require less energy to move to the conduction band due to the reduced energy gap. With the presence of light, the electrons can easily move to the conduction band, enhancing conductivity and reducing resistance.

Both intrinsic and extrinsic photoresistors show an increase in conductivity or a decrease in resistance when they are exposed to higher levels of incident light, regardless of their type.

LDR as a Potential Divider

Light dependent resistors find application in various circuits, employing components such as bipolar transistors, field-effect transistors (FETs), operational amplifiers, and more. The fundamental element in most LDR circuits as a potential divider, which can be combined with different circuits to manipulate the voltage according to the desired requirements.

A typical potential divider comprises two resistors connected in series, with one end usually linked to a fixed potential and the other end connected to the ground. This arrangement forms the basis for many LDR circuits, allowing for the voltage to be divided and processed in different ways, depending on the specific application.

The output voltage of the circuit can be calculated using the formula:

In this case, there is no load that affects the voltage, and hence the potential divider circuit is assumed to function as expected. When a high impedance load is present, the circuit operates optimally. But, if the load and R2 (LDR) affect the voltage, their combined resistance should be calculated in parallel to determine the overall resistance of the lower branch of the potential divider. 

As R2 varies, the output voltage of the potential divider also fluctuates. This output voltage can be connected to various components such as transistors, FETs, operational amplifiers, or other appropriate circuits. It can be utilized to amplify the difference or in other circuits, depending on the specific application.

Advantages of LDR

  • High sensitivity
  • Less expensive
  • Small and simple
  • High light-dark resistance ratio
  • No union potential

Disadvantages of LDR

  • Limited spectral response
  • Wrong results obtained as operating temperature changes
  • Not a particularly responsive tool

Applications of Photoresistor

LDRs, or photoresistors, have a simple structure and are known for their affordability and durability. They find applications in various electronic devices and circuit designs due to their versatility. These photoresistors are extensively used in photographic light meters to measure the intensity of light and determine the appropriate exposure settings for cameras.

They play an important role in fire and smoke alarms by detecting changes in ambient light levels. When smoke or fire particles disrupt the light falling on the LDR, it triggers an alarm, alerting occupants to potential hazards.

These photoresistors are used in burglar alarms to sense changes in light levels caused by intruders. When an unauthorized person enters an area and interrupts the light falling on the LDR, it triggers the alarm system.

LDRs are used as light sensors in lighting control systems, particularly for street lamps and outdoor lighting. They help adjust the brightness of the lights based on ambient light conditions, enabling energy-efficient and automated lighting management.

Extrinsic photoresistors, which offer sensitivity to longer wavelengths, are commonly utilized as infrared photodetectors. They are employed in various electronic circuit designs that involve the detection and measurement of infrared radiation.

They can also be utilized for detecting nuclear radiation. When exposed to certain types of radiation, the conductivity of the LDR can change, allowing it to serve as a sensor for radiation detection.