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The Essential Guide to LDR Circuit (Light Dependent Resistor) & Its Working

Sometimes, home appliances and lights need to operate and controlled manually on various occasions and events. However, human negligence and unwanted situations may lead to power wastage of appliances. Therefore, to control the situation, engineers provide a better alternative to control.

LDR, which refers to Light Dependent Resistor, helps in controlling loads of the circuit based on light intensity. LDR also serves as a photoresistor device that comprises semiconductor material of high resistance.

This article gives you a detailed insight into LDR, its functioning, working principles, types, as well as advantages and disadvantages.

What Is an LDR Circuit (Light Dependent Resistor)?

LDR refers to an essential electrical component of a circuit that responds to light. It changes the resistance as the rays of light fall on it. The value of resistance of the LD resistor changes many times. However, the value of resistance drops immediately as the level of light increases.

The LDR resistance value may vary in darkness and bright light. It gives several Mohms value in darkness, yet it decreases to a few hundred ohms in bright ray light. This change in resistance makes LDR be used in various types of applications and devices. Moreover, the sensitivity of LDR also alters through the wavelength of incident light.

Semiconductor materials have light-sensitive properties. Therefore, designers use semiconductor material in LDRs because of this feature. Cadmium sulfide is a widely used material for the composition of LDRs. At the same time, some European countries restrict the use of this material along with Cadmium Selenide because of environmental concerns. Moreover, they can use Indium antimonide and Lead sulfide.

Resistors use semiconductor material because they do not contain any PN-junction and act as passive devices. Therefore, simple resistors do not fall into the category of LDRs like photodiodes and phototransistors.

LDR Circuit Symbol

LDR Circuit schematic
LDR Circuit schematic

The LDR symbol basically represents the symbol of the resistor in electronic PCBs. However, it may describe the rays of light in the form of arrows. This way, the LDR symbol follows the same principle as the photodiode and phototransistors symbol, in which arrows illustrate the drop of light over the components.

Construction Of An LDR Circuit

The fabrication of LDR contains light-sensitive or semiconductor material which is placed over the ceramic for insulation. Designers put the material in a zigzag form. This gives desired resistance and power rating. The zigzag area is distinct from the area of metal-placed in two separate spaces.

On both sides of the surface, it creates ohmic contacts. Here, designers keep the resistance low as much as they can to ensure that resistance only alters over the light. Moreover, they avoid using cadmium and lead semiconductor materials as they affect the environment. 

Working Principle Of Light Dependant Resistor

The LDR working principle follows photoconductivity which represents an optical function. The material conductance enhances as the light falls on it.

Whereas, when it falls over the LDR, the electrons present in the material valence band start tending to the conducting band. But the incident ray light has photons that carry superior energy to the material band gap. This enables the electrons to jump from the valence band to the conduction band.

Moreover, when light contains ample energy, it excites more electrons to a conduction band. Whereas, when current flow and process effect start increasing, it automatically decreases the device resistance.

Light Dependant Resistor Circuit

The LDR circuit consists of LDR, diode, resistors, and Darlington pair. In this voltage supply connects with the load. A battery or a bridge rectifier is used to provide Direct Current voltage to the circuit of LDR. Moreover, the bridge rectifier utilizes a step-down transformer for voltage step-down from 230 to 12 volts. Diodes are connected in a bridge that alters the Alternating Current-voltage into Direct Current.

It also uses a voltage regulator, which helps in changing the DC-DC, 12-6 volts. This current then goes to the whole circuit. An AC supply of 230 volts to both the load and bridge rectifier. Moreover, ensure to keep them constant for light-sensor type circuits. 

This sensor shows low resistance in the daytime, around 100 ohms. Hence, the power flows across the LDR and ground going through resistors and variable resistors. This happens because of LDR. During day time when light drops over the LDR, the resistance decreases as compared to other parts of the light sensor circuit. As we know, the current always goes through the path of less resistance. 

Therefore, they really do not provide enough power supply. Thus turning off the light in daylight. Similarly, the LDR resistance increases at night time by about 20 megohms. Because of high resistance, current flow decreases and gets zero. Here, current flows through the path of low resistance, which increases the Darlington pair base voltage up to 1.4 volts. The Darlington pair then provides enough power supply to the relay, which turns on the light at night time.       

Frequency Dependence of LDR Circuit


The LDR sensitivity depends on the wavelength of the light as it affects the responsive area of a device. However, this effect can be easily found and noticeable when the light wavelength goes outside the determined range. After this range, one cannot see the effect. Different types of material in the device react and respond differently to the light wavelength. 

Moreover, extrinsic resistors type respond more quickly to the wavelength of light as well as IR. But make sure to take precautionary measures for dissipating heat released from IR while working on it.

Latency of LDR Circuit

LDR is highly associated with an important factor, latency. Latency depicts the time required for responding to any changes in the components. Therefore, these features make it important to consider while designing a PCB. Latency shows the time at which a person can easily see a note. This time refers to changes that LDR attains from one light level to another light level. Therefore, LDR does not fit well for devices where light changes its value quickly. If the occurrence of light changes remains for quite a time, then it pars the sufficient level.

The resistance recovery rate determines the rate of change in resistance. Generally, LDR responds to the light in a few mili seconds after thorough darkness. After that, the time limit rises to a second. However, in the datasheet of components, the specifications of LDR are written as the resistance to darkness after a certain time, such as in seconds. The most used quoted values are 1 and 5 seconds. These values clearly indicate the latency of a resistor. 

Types Of LDR Circuit

LDR has various types based on the material’s photosensitive and linearity. LDR uses different types of materials. These include lead sulfide, cadmium selenide, thallium sulfide, and cadmium sulfide. 

LDR that comprises cadmium sulfide proves responsive to almost all sorts of light radiations and rays a human can see within their spectrum. However, LDR that comprises lead sulfate can only respond to Infrared radiation.

Based on the types of materials and their photosensitive ability, LDR divides into two types: Linear LDRs and Non-Linear LDRs.

Linear LDR also refers to photodiodes. Due to its linear operation and performance, it serves as a photoresistor in several applications. Non-Linear Light Dependent Resistors are found more commonly in devices. However, polarity does not determine its function and operation, which unites it.




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