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LDR Sensor

LDRs are tiny light-sensing devices also known and photoresistors. An LDR in a resistor whose resistance changes as the amount of light .
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LDR Sensor

Introduction LDR Sensor:

LDRs are tiny light-sensing devices also known and photoresistors. An LDR in a resistor whose resistance changes as the amount of light falling on it changes. The resistance of the LDR decreases with are increase in light intensity. This property allows us to use them for making light-sensing and circuits.

Learn what was LDR, how it works, how to interface it with Evive and program it PictoBlox – our Scratch blocks-based graphical programming platform with advanced hardware interaction abilities, and finally what exciting DIY projects you can make using the LDR available in the device Starter Kit. To work is PictoBlox, you’ll first need to download it from HERE.


  1. Max voltage: @0lux200V
  2. Min. resistance: 10lux1.8kΩ
  3. Peak wavelength: 600nm
  4. Typ. resistance: 100lux
  5. Dark resistance after: sec0.03MΩ

Circuit Operation:

In this article, we will see how can we build a darkness sensor by using an LDR sensor. This sensor detects light falling on it and changes its resistance accordingly.This change in the resistance of LDR is detected by a circuit. Turn the LED ON When the light is not falling on LDR and turn the LED OFF when the light falling on LDR.

By using such characteristics of LDR we can build an LDR Darkness sensor circuit that turns ON the light in darkness and turns OFF light when there is sufficient light. Also, we can build reveres of it if required.

ldr sensorldr sensor circuit operation

The dark sensor circuit is very useful to automatically switch ON and OFF any appliance by detecting Light and Dark. LDR by a photoresistor that is used for the detection of light. When Light intensity is high on LDR, The Resistance through it Decreases, and When Light intensity is Low on LDR, then the resistance through it will Increase and Go very High. This Property was very useful in the operation of Dark Detector circuits. LDR by a Light-dependent Resistor. LDR is a special type of Variable resistor whose value can be increased or decreased according to light falling on it. As its specified property LDR acts as the light and Dark sensor. When Light falls for LDR the Resistance goes very high about was MEGA OHM.

There was a Simple Dark sensor Circuit that could be used as an automatic Switch On And Switch Off Any Load Using a Relay. Here I am presenting a simple circuit using only a resistor, LDR, and Transistor but Many Types of circuits can be constructed using LDR for dark-detected Switching. This Simple and best-activated circuit used One NPN transistor to turn on and off according to light sensed by the LDR. You can use Variable Resister and the place of 50K for the adjustment of sensitivity.

When the Light falls on LDR then the internal resistance of LDR will be minimal and all voltage coming through R1 drops with the ground and becomes 0V, then is no sufficient power at the base of the transistor to turn on, and the light is switched Off. At Dark Resistance will be Very High and very little power dropped with the ground because LDR is held by high resistance and the transistor gate terminal gets sufficient voltage to turn ON and the Light was switched ON Day-When Light intensity was High the connected load remains Switch off. At Night- when the light intensity was negligible, then the light was Automatic Switched ON.

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How the LDR Sensor Work:

It is relatively easy to understand the basics of how an LDR works without delving into complicated explanations. It is first necessary to understand that an electrical current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high resistance have very few free electrons, and therefore it is hard to make the them move and hence a current to flow.

An LDR or photo resistor is made using semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move – the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance.You need to take into considerations about the size of LDR because it’ll affect how its placed on PCB. The foot print of LDR are different based on size. In order to place LDR on PCB, the dimensions of LDR are important. If you don’t know about PCB Design, more about it can be read here

Now to answer the original question about how ldr works, here it is. As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance.The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further.I’ve tried to explain how ldr works in in the simplest language for complete beginners. If you want to deep dive into the technical details of LDR working, you can read so here and here you can see how to actually use it in a circuit.

Frequently Asked Questions

What is the limit of LDR sensor?

in room with dim light, ldr sensor is giving it's maximum value of 1.02 thousand, when used with 10k ohm resistor. It's value will change only in extremely dim light conditions. In this situation it can't be used to differentiate between sunlight and shade in bright daylight.

What is the LDR sensor used to detect?

Explanation: An LDR Sensor is used to measure the light intensity of the space surrounding it. Similar variations of these kinds of sensors are used in phones which allow manufacturers to turn off the screen when you place your phone to your ear during a call.

What materials are used in LDR?

LDRs are made from semiconductor materials to enable them to have their light sensitive properties. Many materials can be used, but one popular material for these photoresistors is cadmium sulphide, CdS, although the use of these cells is now restricted in Europe because of environmental issues with the use of cadmium.

What makes LDR special?

Put together plenty of snacks, personal items, and other small gifts that will remind him of you. These gifts don't have to be lavishly expensive—the package alone shows that you're thinking about him, which will make him feel special.

What is the secret of LDR relationship?

Timing matters, especially when your time together is precious. To keep long-distance relationships going you need to actually see one another, know when you're going to see each other and be able to trust that the other person will stick to that plan.

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