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 lights, and in areas where we need to have light sensitivity. It is also called a Light Sensor.

LDR are usually available in 5mm, 8mm, 12mm and 25mm dimensions.

How are LDRs Made?

The LDRs made with photosensitive semiconductor materials like Cadmium Sulphides (CdS), lead sulfide, lead selenide, indium antimonide or cadmium selenide and they are placed in Zig-Zag shape as you can see in the pic below, and two metal contacts are placed on both ends of the Zig-Zag shape these metal contacts helps in creating a connection with the LDRs.

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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

Working and 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 that is in the valence band get excited and go to the conduction band and thus increasing the conductivity as per the increased 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.

A Simple LDR Component

Difference Between Photocell and LDR

Photodiodes give quick response and are used where needed to detect quick response 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 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

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 having an adequate amount of energy falls on this then electrons gain that energy and got excited and a few of them go to the conduction band.

Extrinsic Photoresistor

This type of photoresistor uses the 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 LDRs are Tested

  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 an 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 the 2nd case. In the dark, LDRs resistance are high as several megaohms, while in the light, it can get reduced to 100Ω also.


  • 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 few seconds to return to their normal resistance once the light source is removed.
  • The sensitivity of an LDR 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 using LDRs in your project.



  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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