What is the LDR Sensor?

What is the LDR Sensor?


Do you want to Light up your knowledge with LDRs! Get an in-depth understanding of what LDRs are, how they work, different types available, benefits, limitations, and various applications. Discover how these tiny light-sensitive sensors are revolutionizing industries and enhancing daily life. Stay ahead of the curve and find out how you can put LDRs to work for you!


A Light Dependent Resistor (LDR) is a type of passive electronic sensor used to detect light.

It’s made up of two conductors separated by an insulator which becomes more conducting when exposed to high levels of light intensity, forming a variable resistor in the circuit.

This allows it to measure the amount and brightness or darkness within its environment and provide information accordingly.

What is LDR

LDR light sensors are typically used as part of automated lighting control systems for energy conservation purposes such as dimming lights based on natural daylight that can be detected outside without human intervention.

Furthermore, they have found application of LDR sensor in motion detectors like burglar alarm security systems because they can tell if something passes through their beam line quickly enough from dark-to-light conditions created with most forms of ambient lighting sources present today such as LED's etc., making them ideal sensing tools both commercially & domestic level settings alike.

What is LDR?

An LDR, or Light Dependent Resistor is a type of optoelectronic sensor that's used for detecting light in the environment. 

LDR working by sensing changes in illumination and converting them into electrical signals which can be read off from its two terminals.

LDR Sensor

This makes it an essential tool in many automation systems because it helps detect objects without any physical contact with them.

In addition to this, they have low power requirements making them ideal for energy-saving applications like solar cells and garden lighting systems where their high sensitivity allows precise illumination control even under tightly controlled conditions.

Their small size further increases flexibility as well giving these devices versatility across multiple different projects while still being able to fit inside tight spaces too!

Most home appliances, outdoor lighting, and streetlights are normally operated and maintained manually.

This is not only dangerous, but it also wastes energy due to staff negligence or unforeseeable events when turning this electrical equipment ON and OFF. Therefore, by using a light sensor, we can automatically shut off the loads based on the intensity of the daylight.

By sensing the radiant energy present in a relatively small range of frequencies often referred to as "light," which runs in frequency from the "Infra-red" to "Visible" up to "Ultraviolet" light spectrum, a light sensor produces an output signal that indicates the intensity of light.

The LDR light sensor is a passive component that produces an electrical signal from this "light energy," whether it is in the visible or infrared portions of the spectrum.

Because they transform light energy (photons) into electricity, light sensors are more generally referred to as "Photoelectric Devices" or "Photo Sensors" (electrons).

LDR Sensor Pin Diagram

It can be divided into two primary categories: photovoltaics and photo-emissions and photo-resistors and photo-conductors, which modify their electrical properties.

LDR light sensor is also referred to as a photoresistor, photocell, or photoconductor. It is a specific kind of resistor, and the amount of light that strikes its surface affects how much resistance it exhibits.

A light-dependent resistor or LDR is an example of an electrical component that responds to light. When light beams strike it, the resistance changes right away. An LDR's resistance levels can vary by several orders of magnitude. As the light level rises, the resistance value will decrease.

LDR resistance values range from many megaohms in complete darkness to only a few hundred ohms in strong light.

As a result, these resistors are widely used in a variety of applications due to this variation in resistance. The wavelength of the incident light affects the LDR sensitivity as well.

Light Dependent Resistor (LDR) technology is widely used in many industrial, educational and commercial applications. Its highly sensitive contactless operation allows it to replace mechanical switches for a wide range of needs.

The LDR sensor working as light sensors or solar radiation detectors, controlling street lights on highways, automatically changing the direction of traffic signals at intersections, even photoelectric equipment such as smoke detector alarms all rely upon this small but efficient device which monitors changes from darkness into ambient light with impressive accuracy.

LDR working extensively to monitor temperatures inside refrigerators & freezers while keeping an eye out for heat/cool fluctuations that could cause food spoilage - ensuring optimal performance every single day!

In addition inventor hobbyists are making great uses of LDR sensor by integrating them directly into their projects where they provide motion detection functionality and much more – giving everyone the ability to add innovation onto already existing ideas!

Also, read our blog Uses of LDR detailing every related LDR Sensor from the definition, how LDR works, how LDR works in street lights, to the uses of LDR sensors in a variety of projects.

Working Principle of LDR:

Working Principle of LDR


The photoconductivity theory underlies the operation of this resistor. It is nothing more than the fact that when light strikes its surface, the material's conductivity decreases and the electrons in the device's valence band are stimulated to the conduction band.

These incident light photons must have energy larger than the semiconductor material's band gap. Consequently, the electrons quickly move from the valence band to the conduction band.

Read our blog explaining how LDR works, detailing what is an LDR, how LDR Works in Street Lights, what are the types of LDR, how to make an Light Dependent Resistor Sensor Module and applications of an Light Dependent Resistor Sensor.

Variations in resistance to change in light intensity

The light-dependent resistor reacts to light very quickly. When the light is greater, which indicates that the resistance value for the LDR will be drastically reduced as the light intensity rises.

Variations in resistance to change in light intensity

When an LDR light sensor is exposed to light, the resistance reduces; this is known as dark resistance. Conversely, when the resistor is exposed to darkness, the resistance decreases.

Any device that absorbs light will have significantly less resistance. The light intensity will increase, and the current flow will begin to increase if a stable voltage is applied. Therefore, the characteristics between resistance and illumination for a particular LDR are shown in the diagram below.

Since LDRs are not linear devices, the wavelength of the light that strikes them causes a change in their sensitivity. Because it depends on the material employed, some types of photocells are not at all sensitive to a particular range of wavelengths.

When light enters a photocell, the resistance changes within 8 milliseconds from 8 to 12 and takes a few extra seconds to return to its initial value after the light has been turned off. Therefore, this is referred to as a resistance recovery rate. This attribute applies to audio compressors.

Light Dependent Resistor Circuit:

The bridge rectifier circuit or a battery is used to provide the LDR circuit with the necessary DC voltage. This circuit converts the AC source to a DC source. To reduce the voltage from 230 volts to 12 volts, a step-down transformer is used in the bridge rectifier circuit.

DC voltage is converted by connecting diodes in a bridge-like manner. The voltage regulator is utilised to transform the 12v DC to 6v DC, and the circuit is subsequently powered by this DC voltage. The bridge rectifier and load must both have a constant 230v AC supply for the light sensor circuit to operate constantly.

Circuit diagram of the LDR

When the resistance is low around 100 ohms. As can be seen in the light sensor circuit above, the power source passes through the LDR light sensor and ground through the variable resistor and resistor.

This is because the resistance provided by the light-dependent resistor is lower than the resistance of the remaining components of the sensor circuit during the day or when light is falling on the LDR.

We are aware of the current principle, which states that current always flows in the direction of least resistance.

Types of LDR sensors:

There are two kinds of Light-dependent Resistor
1. Intrinsic Photo Resistors
2. Extrinsic Photo Resistors

1. Intrinsic Photo Resistors:

These resistors are pure semiconductors made of germanium or silicon. The number of charge carriers increases because of the electrons being stimulated from the valence band to the conduction band when light strikes the LDR.

2. Extrinsic Photo Resistors

The impurities that are added to these devices result in the creation of new energy bands above the valence band. There are electrons inside these bands. As a result, the band gap is reduced, and less energy is needed to move them. Long wavelengths are the principal application for these resistors.

Another kind of classification is done based on linearity,

  • Because of their linear performance and method of operation, liner-type LDRs are sometimes referred to as photoresistors instead of photodiodes.
  • The polarity via which they unite does not affect the behavior's of non-linear LDRs, even though they are frequently utilized.

How to Test LDR's Functionality:

Step 1: Set the multimeter to the Ohms mode. In this mode, resistance can be measured.

Step 2: Attach the two LDR portions to the multimeter's positive (usually red) and negative (usually black) terminal wires. Verify to ensure that the connections are secure.

Step 3: Use a torch or any other type of light source for lighting the LDR's surface. This will replicate the LDR's lighting condition.

Step 4: Verify the multimeter's reading. Note the value of resistance that the multimeter shows.

Step 5: Put the LDR in a dark area or cover it with your hand at this point. This will simulate situations in which the LDR is not in contact with light.

Step 6: Once the LDR is covered or in darkness, recheck the multimeter reading again. When the multimeter displays the resistance value, note it down.

Step 7: Analyze and compare the readings for the two circumstances. In the first situation, where the object is exposed to light, you will typically find that the resistance value is lower. In the second situation when it is covered in the dark, the resistance value is greater.

Step 8: When an LDR is covered or dark, its resistance is usually high, in the megaohm range. When exposed to light, its resistance decreases, sometimes reaching as low as 100Ω, depending on the strength of the light.

LDR advantages and disadvantages


  • Sensitivity is High
  • Simple & Small devices
  • Easily used
  • Uses of LDR sensor in variety projects.
  • Inexpensive
  • There is no union potential.
  • The light-dark resistance ratio is high.
  • Its connection is simple


  • The spectral response is limited.
  • The best materials have limited temperature stability due to the hysteresis effect.
  • Its chemical reaction in stable materials.
  • LDR Sensor is only used in situations when the light signal fluctuates dramatically.
  • It is not a particularly responsive tool.
  • As soon as the operating temperature changes, it gives the wrong results.



LDR sensors find diverse applications in daily life, serving as key components in various systems.

The uses of LDR sensors can be from controlling streetlights and enhancing security systems to optimizing solar panel efficiency and aiding photographers in capturing the perfect shot. Uses of LDR sensor in daily life includes.

  • The photoresistor is typically used to measure light intensity and presence.
  • LDR sensors used in lights that turn on and off automatically based on light
  • Smoke Detector Alarm, Automatic Lighting Clock
  • LDR sensors used in design of optical circuits
  • Proximity switch for photos
  • Security measures utilising lasers
  • In Solar street lighting LDR sensors can be used
  • Light metres for cameras
  • LDR sensors cna be used in Radio clocks
  • Can be used in dynamic compressors; some compressors adjust the signal gain by connecting LDR and LED to the signal source.



In this blog post, we have learned that Light Dependent Resistors (LDRs) play a vital role in modern technology, providing an effective way to detect light levels, control various electrical systems, and various uses of LDR sensors.

With a deep understanding of their LDR sensor working principle, types, advantages, disadvantages, applications, and dynamic range, one can take full advantage of this amazing technology and optimize the LDR light sensor range for specific needs.

Whether you're an engineer looking to improve your designs or an enthusiast seeking to learn more about LDRs, this blog has got you covered. Don't miss out on the opportunity to experience the power of LDR technology.


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Frequently Asked Questions

1. What is LDR sensor and how it works?

A Light Dependent Resistor, aka an LDR, is an enigmatic device that is deployed to gauge the magnitude of luminosity. It employs a mystical substance, frequently cadmium sulfide, that exhibits peculiar changes in its resistance in response to fluctuations in the illumination it's subjected to. 

As the illumination intensifies, the resistance of the LDR decreases in a paradoxical manner, and when the light recedes, the resistance rises accordingly. This bizarre phenomenon of resistance alteration can be monitored and analyzed to decipher the luminosity levels. The LDR is widely utilized in a plethora of applications that require the measurement of light, such as automatic lighting control systems, cameras, streetlights and more.

2. Where is LDR sensor used?

The LDR (Light Dependent Resistor) sensors, prevalent in various settings where light level detection holds paramount importance, perplexes with its versatility and application. These enigmatic sensors, commonly encountered in automatic lighting control systems, street lights, camera light meters, burglar alarms, and environmental control systems, burst forth with their ability to detect fluctuations in light intensity and dynamically adjust their electrical resistance in response. 

This metamorphosis in electrical resistance then serves as the controlling mechanism for other electronic devices. LDR sensors are also utilized in automation systems, orchestrating lighting and heating systems based on occupancy and ambient light levels with unfathomable precision.

3. How does LDR detect light?

A Light Dependent Resistor, or an LDR, is a bewildering component that flummoxes conventional understanding of resistance. It's a resistor that's like a chameleon, adapting its resistance level according to the light it's exposed to. This resistor comprises of a semiconductor material that transforms into a conductor when illuminated, thus reducing its resistance. 

This drastic shift in resistance causes an upheaval in the current flow, which can be monitored and utilized to detect the existence or lack thereof of light. LDRs are widely utilized in lighting control systems, camera flash units, and light-sensitive alarms, providing a straightforward and economical solution for light sensing across various applications.

4. How to connect Raspberry Pi to LDR sensor?

Connecting a raspberry pi 4 2gb to an LDR sensor is relatively straightforward. To begin, you must first install the necessary software packages onto your raspberry pi 4 2gb; such as Raspbian and GPIO Zero libraries for Python 3. Once these have been installed, then it's time to physically connect the LDR sensor with its corresponding pins on your raspberry pi 4 2gb board. 

By matching up pin numbers or labels correctly (examples may include GND/Ground & VCC/Voltage) – one can easily establish connection between components through connecting wires & breadboarding of each module present in circuit design projects respectively if required by application itself during experimentation phase(s). 

Finally, once this has all been complete - testing out code arrangements via script programming knowledge that best suits user’s needs should be attempted within monitoring environment ; so conditions like light intensity can be adjusted if certain levels are exceeded when running program before final deployment into home automation system related tasks or IoT Network technology development operations at large scale production level situations where having accurate data output values are paramount towards project progress further down line from network security standpoint!

5. Why resistor is used with LDR?

A voltage divider circuit is developed by combining a resistor and an LDR. This allows the LDR's changing resistance in response to light intensity to be converted into an equivalent voltage change for measurement or control in electronic circuits.

6. Is A LDR passive or active?

The LDR is a passive component. A passive component doesn't require an external power source to work. An LDR's resistance varies only in response to the amount of light that hits it. Rather than actively creating or strengthening a signal, it reacts to changes in its surroundings. As a result, an LDR is classified as a passive component in electronic circuits since it operates only on the light it receives and requires no additional power.

7. Which material is used for making LDR?

LDRs are typically made of semiconductor materials such as cadmium sulfide. When this semiconductor material is exposed to light, its electrical resistance changes from several thousand Ohms in the dark to only a few hundred Ohms when lighted.

8. How is LDR used in street lights?

LDRs control solar streetlights by detecting light levels. When daylight decreases, the LDR activates the lights, and when natural light increases, it turns them out, saving energy. The LDR automatically controls the operation of the light by utilizing transistors, resistors, and other components. Modern solar street lights also have motion sensors and filtering features to increase energy economy, making them a popular and environmentally responsible outdoor lighting solution.

9. How can the resistance of an LDR be measured?

The resistance of an LDR can be measured using a multimeter set to resistance (ohms) mode. Connect the multimeter leads to the LDR terminals. The multimeter will show the resistance value, which varies depending on how much light hits the LDR. High resistance suggests decreased light levels, whereas low resistance suggests higher light intensity.

10. What unit is an LDRs resistance measured in?

The resistance of an LDR is measured in ohms (Ω).

11. What is the minimum resistance of a LDR?

The resistance of an LDR fluctuates with the quantity of light it is exposed to. LDRs can have extremely high resistance in darkness, reaching up to 1.02 megaohm (1.02 MΩ) or greater. However, when exposed to light, the LDR sensor's resistance decreases significantly.

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