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Table of contents
Characteristics of LDRs
Types of LDRs
Working of LDRs
Projects Based on LDRs
What is LDR?
Where are photoresistors used?
What is LDR made of?
Explain Photoconductivity.
Last Updated: Mar 27, 2024

Light Dependent Resistors (LDRs)

Author Harsh
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Many electronic devices are manually operated which leads to energy wastage when used with carelessness, users might forget to turn on or off these electronic devices. To overcome this problem we can use Light-Dependent resistors to turn off the loads automatically based on the strength of daylight.

In this blog, we will learn about light-dependent resistors, how they work, and we’ll also brief you about the projects that can be created using LDRs.


A photoresistor, photocell, or photoconductor is a light-dependent resistor (LDR). It's a form of resistor whose resistance changes depending on how much light hits its surface. The resistance changes when light shines on the resistor. These resistors are typically utilized in many circuits that need the detection of light. The uses and resistance of these resistors are diverse. When the LDR is in the dark, it can be used to turn on a light, and when it is in the light, it may be used to turn off a light. A typical light-dependent resistor has a resistance of 1MOhm in the dark and a resistance of a few KOhm in the light.

LDR (Source: WatElectronics)

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Characteristics of LDRs

LDRs are light-dependent devices that have a lower resistance when exposed to light and a higher resistance when exposed to darkness. When a light-dependent resistor is maintained in the dark, it has a very high resistance. Dark resistance is the name for this type of resistance. It can be as high as 1012 ohm and allowing the device to absorb light reduces its resistance dramatically. When a steady voltage is provided, and the light intensity is raised, the current increases. The resistance vs. illumination curve for a certain LDR is shown in the diagram below.

Resistance V/S Illumination in LDRs

When light strikes a photocell, it takes around 8 to 12 milliseconds for the resistance to change and one or more seconds for the resistance to return to its original value when the light is removed. Resistance recovery rate is the term for this phenomenon. Audio compressors make use of this characteristic. Also, unlike photodiodes and phototransistors, LDRs are less sensitive.

Types of LDRs

Light dependent resistors are classified based on the materials used

  • Intrinsic Photo Resistors: Pure semiconductor materials such as silicon or germanium are used. When photons of sufficient energy fall on electrons, they are stimulated from the valence band to the conduction band, increasing the number of charge carriers.

  • Extrinsic Photo Resistors: These devices are doped with impurities, which cause additional energy bands to form above the valence band. Electrons are abundant in these bands. As a result, the band gap is reduced, and just a little amount of energy is required to move them. Long wavelengths are the primary application for these resistors.

Working of LDRs

A light dependent resistor works on the principle of photo conductivity.

  • Photo Conductivity: Photoconductivity is an optical and electrical phenomena in which electromagnetic radiation, such as visible light, causes a material to become more electrically conductive.

When photons from light strike a device, electrons in the valence band of the semiconductor material are stimulated to the conduction band. To cause electrons to jump from the valence band to the conduction band, these photons in the incident light must have energy larger than the band gap of the semiconductor material. As a result, when sufficient energy light strikes the device, more electrons are driven to the conduction band, resulting in a large number of charge carriers. As a result of this process, more and more current begins to flow through the device when the circuit is closed, and the gadget's resistance is said to have lowered.

Projects Based on LDRs

  1. Security System Controlled by an Electronic Eye: A light-dependent resistor is used in this project. When light falls on the LDR sensor, the resistance of the sensor lowers, causing an alarm to sound and the user to be alerted. This project is appropriate for providing security systems for lockers and cash boxes found in banks, shopping malls, and jewellery stores.
    This project's circuit is installed inside cash boxes at shopping malls and bank lockers in such a way that when a thief opens the cash box or locker and searches the valuables with a torch light, the circuit is activated. When light shines on the circuit, which incorporates an electronic eye, the ripple counter receives a command. This sets off the alarm and indicates a break-in attempt. When light falls on the sensor, a bulb is utilised to notify the theft.
  2. LDR Based light Intensity Control for Street Lights: In the proposed system, highway lighting is typically provided by HID lamps. Because these bulbs have a high energy usage. LEDs are used in this project to address the disadvantages of HID bulbs. The use of light emitting diodes as a light source is demonstrated in this project. These lights use less energy and have a longer lifespan than HID lamps. The light is detected using a light-dependent resistor. The LDR's resistance decreases dramatically when exposed to sunlight.
    The light intensity is maintained at a high level during peak hours, and when traffic on the roadways tends to drop late at night, the light intensity falls till dawn. Finally, the street lights go out fully in the morning and come back on at 6 p.m.
  3. Lighting Switch from Sunset to Sunrise: The light illuminated on the LDR sensor is controlled by this sunset to sunrise lighting switch.
    The LDR sensor's resistance changes as the intensity of light falling on it changes. This sensor output is wired in bistable mode to an IC 555 timer. The IC 555 timer's o/p is utilised to control the load prompting via a TRIAC. As a result, this circuit automatically turns on the load at dusk and turns it off at sunrise.
  4. LDR PC Desk Lamp: When there is an input power, most of the PC desk lamps on the market light up. These calculations ignore whether or not the light is actually required. The problem is solved with this smart PC desk lamp circuit.
    It detects the amount of light in the room to determine whether it is actually needed and turns on only when necessary. It is meant to function with a computer and only turns on when the computer in the table is turned on. MOC3021 is used.


What is LDR?

It is a light dependent resistor (LDR). It's a form of resistor whose resistance changes depending on how much light hits its surface.

Where are photoresistors used?

Some photoresistors are utilised in consumer products such as light metres in cameras, light sensors in robotic projects, clock radios, and so on.

What is LDR made of?

The Light Dependent Resistor (LDR) is a piece of exposed semiconductor material, such as cadmium sulphide, that alters its electrical resistance from thousands of Ohms in the dark to only a few hundred Ohms when light shines on it, forming hole-electron pairs in the material.

Explain Photoconductivity.

Photoconductivity is an optical and electrical phenomena in which electromagnetic radiation, such as visible light, causes a material to become more electrically conductive.


In this article, we have extensively discussed the Light dependent resistors, their working and what are the different projects that can be made using LDR.

We hope that this blog has helped you enhance your knowledge about Light-Dependent Resistors and if you would like to learn more, check out our articles on our Website

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