What Is Light Dependent Resistors (LDR)?


LDR (Light Dependent Resistor) as the name states it 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 light sensor, light meter, Automatic street lights and in area where we need to have light sensitivity.

How are LDRs Made?

The LDRs made with photosensitive materials like Cadmium Sulphides (CdS) and are pleased on Zig-Zag Shape as you can see in the pic below.and two metal contacts are placed on both end of the Zig-Zag shape this metal contacts helps in creating 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 get light from the environment for its working.

LDR Symbol
LDR Symbol

Working and Principle

It works on the principle of photoconductivity whenever the light on its photoconductive material falls 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 increases 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 conduction band.

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

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, 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 a resistance and its resistance decreases with the increase in light intensity.

Types of LDR

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 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 to make this photoresistor.


  • The photoresistor are generally used in detecting the presence and intensity of light
  • Used in automatic lights that switch on and of according to light
  • Smoke alarms, Clock with automatic light
  • Optical circuit design
  • Photo proximity switch



  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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