Wednesday, May 22, 2024

What Is Light Dependent Resistor (LDR)?

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LDR (Light Dependent Resistor) as the name states is a special type of resistor that works on the photoconductivity principle means that resistance changes according to the intensity of light. Its resistance decreases with an increase in the intensity of light.

It is often used as a light sensor, light meter, Automatic street light, and in areas where we need to have light sensitivity. LDR is also known as a Light Sensor. LDR are usually available in 5mm, 8mm, 12mm, and 25mm dimensions.

How are LDRs Made?

The Light-dependent resistors made with photosensitive semiconductor materials like Cadmium Sulphides (CdS), lead sulfide, lead selenide, indium antimonide, or cadmium selenide and they are placed in a Zig-Zag shape as you can see in the pic below.

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Two metal contacts are placed on both ends of the Zig-Zag shape these metal contacts help in creating a connection with the LDRs.

Now, a transparent coating is applied on the top so that the zig-zag-shaped photosensitive material gets protected and as the coating is transparent the LDR will be able to capture light from the outer environment for its working.

LDR Symbol
LDR Symbol

LDR Working Principle

It works on the principle of photoconductivity whenever the light falls on its photoconductive material, it absorbs its energy and the electrons of that photoconductive material in the valence band get excited and go to the conduction band and thus increasing the conductivity as per the increase in light intensity.

Also, the energy in incident light should be greater than the bandgap gap energy so that the electrons from the valence band got excited and go to the conduction band.

The LDR has the highest resistance in dark around 1012 Ohm and this resistance decreases with the increase in Light.

Light Intensity V/S Resistance

As per the property of LDRs, the amount of light entering the LDR the inversely proportional to the resistance of the sensor, and the graph is hyperbolic in nature.

Light Dependent Resistor

Difference Between Photodiode and LDR

Photodiodes give quick responses and are used where needed to detect quick responses on and off like in optical communication, and optoisolators. The photodiodes are semiconductor devices and work on PN junctions.

The photodiode works on the principle of converting the light energy into electric energy while the LDR is resistance and its resistance decreases with the increase in light intensity. They are generally used in automatic security lights.

Whereas the LDR, Photocell, a photoelectric, photovoltaic effect, or photoconductivity is used to generate a current or a voltage when exposed to light or other electromagnetic radiation. They are generally used in burglar alarms.

Types of LDR or Photoresistors

1. Intrinsic Photoresistor

This type of photoresistor is made with pure semiconductors without any doping. This kind of photoresistor uses pure semiconductors like silicon and germanium. when the incident light with an adequate amount of energy falls on this, electrons gain that energy and get excited, and a few of them go to the conduction band.

2. Extrinsic Photoresistor

This type of photoresistor uses a doped semiconductor; this means some impurities are mixed with the semiconductor such as phosphorus to make this photoresistor.

Extrinsic light-dependent resistors are generally designed for longer wavelengths of light, with a tendency towards infrared (IR).

How to Test LDRs?

  1. Take a multimeter and set it up in Ohms mode.
  2. Now connect the positive terminal and negative terminal wires to the two sections of the LDR
  3. Place a glowing torch light or any medium of light onto the surface of the LDR and check the reading.
  4. Now place a hand over the LDR or place the LDR in the dark and check the multimeter reading.
  5. You can see that in 1st case the value of Ω would be lower than in the 2nd case. In the dark, LDR’s resistance is high as several megaohms, while in the light, it can get reduced to 100Ω also.

LDR Applications

  • The photoresistor is generally used in detecting the presence and intensity of light
  • Used in automatic lights that switch on and off according to light
  • Simple Smoke Detector Alarm, Clock with automatic light
  • Optical circuit design
  • Photo proximity switch
  • Laser-based security systems
  • Solar Street Lamps
  • Camera light meters
  • Clock radios
  • Can be used in Dynamic Compressors, some compressors use LDR and LED connected to the signal source to create changes in signal gain.


  • LDRs require a few milliseconds or more to respond fully to the changes in light intensity, i.e. they require a few seconds to return to their normal resistance once the light source is removed.
  • The sensitivity of a Light-dependent resistor varies with the light wavelength. If the wavelength is outside a certain range, it will not affect the resistance at all.
  • Light-dependent resistors have lower sensitivity than photodiodes and phototransistors.

Given here are the Top 10 LDR-based Electronics Projects to try out.

Light Dependent Resistor – FAQs

Q: What are the applications of LDRs?

LDRs are commonly used in light-sensitive circuits such as streetlights, photographic light meters, solar cells, and automatic lighting systems. They are also used in some electronic musical instruments, where they can be used to control the pitch or volume of the sound.

Q: How can the resistance of an LDR be measured?

The resistance of an LDR can be measured using a digital multimeter or an analog ohmmeter. However, it is important to note that the resistance of an LDR is not a fixed value and can vary depending on the light intensity.

Q: What is the dark resistance of an LDR?

The dark resistance of an LDR is the resistance of the LDR when there is no light falling on it. This value can range from several hundred ohms to several megaohms, depending on the type and size of the LDR.

Q: Can LDRs be damaged by exposure to bright light?

Yes, LDRs can be damaged by exposure to very bright light, such as direct sunlight or a laser beam. This can cause a permanent decrease in the resistance of the LDR or even damage the semiconductor material. It is important to handle LDRs carefully and avoid exposing them to excessive light.

Q: Can LDRs be used with digital circuits?

Yes, LDRs can be used with digital circuits by connecting them to an analog-to-digital converter (ADC) or a comparator circuit. The output of the ADC or comparator can then be used to control a digital output, such as an LED or a relay.

Q: What is the spectral sensitivity of an LDR?

The spectral sensitivity of an LDR refers to the range of wavelengths of light that the LDR is sensitive to. Different types of LDRs have different spectral sensitivities, with some being more sensitive to visible light and others being more sensitive to infrared light.

Q: Can LDRs be used in high-temperature environments?

LDRs are typically sensitive to temperature changes, and their resistance can vary significantly with temperature. Therefore, it is important to choose an LDR that is rated for the temperature range of the application.

Q: Can Light Dependent Resistors be used in wet environments?

LDRs are typically not waterproof and can be damaged by exposure to moisture or water. Therefore, they should be used in dry environments or protected with a waterproof enclosure.

Q: Can a Light-dependent resistor be used for measuring light intensity?

Yes, LDRs can be used to measure light intensity by converting the resistance of the LDR to a voltage using a simple voltage divider circuit. The output voltage can then be measured using a voltmeter or connected to an analog-to-digital converter (ADC) for digital measurement.

Q: Can LDRs be used for color sensing?

Light-dependent resistors are not typically used for color sensing because they are only sensitive to the intensity of light and not the color. Color sensors typically use more complex devices such as photodiodes or phototransistors that are sensitive to specific wavelengths of light.

Q: Can LDRs be used for detecting motion?

LDRs are not typically used for detecting motion because they only respond to changes in light intensity and not a movement. Motion sensors typically use other technologies such as passive infrared (PIR) or ultrasonic sensors.


  1. I have a circuit using LDRs to monitor lighting in a room with florescent fixtures throughout. When directly under a fixture, and the LDR is covered, a separate LED is lit. Got that, no prob.

    BUT when the same LDR circuit is placed in a part of the room not directly under the florescent fixture, the LDR doesn’t “see” enough light (as if covered) and thus the LED is constantly lit – that is a problem!

    So, how can I vary the LDR’s sensitivity to react to the uneven amount of florescent illumination (illuminance) as in a corner of the room, etc)?

    Should I be using photodiodes instead of LDRs?

    Application: I’m a model railroader. I have RR crossing signals located where roads cross or intersect RR tracks (just as in prototypical situations). I’ve used OR IC chips so that multiple LDRs can detect when they are covered by freight cars or engines as they roll over them. When that happens, light is blocked and the RR crossing signals begins alternating flashings until the cars are past the LDRs when light is, once again, detected and the flashing stops as default.


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