Circuit

Photoresistor – Light Dependent Resistor (LDR)

Photoresistor-or-Light-Dependent-Resistor-(LDR)

A photoresistor or light dependent resistor (LDR) is an electronics component that is used to detect light and control the operation of electronic circuit whose depend upon the light variation. It has popular with other names from light dependent resistor, LDR, photoresistor, or even photo cell, photocell or photoconductor.

It is easy of manufacture, low cost and eases of use in verity of applications in photographic light meter and detects light levels etc.

What is a Photoresistor or Light Dependent Resistor (LDR)?

Photoresistor is also known as Light Dependent Resistor. It us a electronics component whose resistivity is a function of the incident electromagnetic radiation. Hence it is a light sensitive device. So, it is called photoconductive cells, photoconductor or simply photocells. It is constructed by a semiconductor material with high resistance.

“Photoresistor is a variable resistor when light incident on it the resistance decreases. It is inversely proportional with the intensity of light”

Photoresistor symbol

The symbol of Photoresistor is based around the resistor circuit but it shows the light in the form of arrow. The figure shows the symbols of light dependent resistor or photoresistor.

photoresistor-or-light-dependent-resistor

Technical detail of photresistor

Dimensions of photresistor

  • Length- 4.46mm/0.18in
  • Width- 5mm/0.20in
  • Height- 2.09mm/0.08in
  • Weight- 0.25g/0.01oz

Types of photoresistor or Light Dependent Resistors -LDR

Photoresistor or LDR are made of semiconductor material and it is a passive device. The photoresistor are categorized in two types depending on the materials used to construct them. These two types are given below:

  1. Intrinsic photoresistors (Undoped semiconductor)- This type of photoresistor is made of pure semiconductor material and these are silicon and germanium. When photon energy fall on it the electron in valance band get excited and move towards to conduction band
  2. Extrinsic photoresistors– In Extrinsic semiconductor material are doped impurity and is called dopants. These dopants create new energy level which is above the valance band. Due to this the energy gap reduces. The extrinsic photresistor are useful for long wavelength.

Basic structure and construction of Photoresistor

The photo resistor is a light dependent resistor and it is made by semiconductor materials. These materials are cadmium sulphide having negligible free electrons when no light fall on it. In absence of light, photo resistor offers high resistance in mega ohms. However in presence of light the photo resistor has low resistance in few hundred ohms.

The basic construction and symbol is given below:

basic-construction-and-symbol

The zigzag patterns are made of cadmium sulphide. This zigzag pattern are use to get the expected power rating and resistance. The hole structure are covered in plastic so as to have direct exposure to the incident radiation.

Working principle of a Photoresistor

So how does a photoresistor (LDR) work? The working of photoresistor is based on the concept of photoconductivity. The photoconductivity is the optical phenomena. When material of conductivity is increased when the absorbed of light by material.

When light fall or photon fall on it, the electrons of semiconductor in valance band are excited to the conduction band. The energy of these photons is greater than the band gap of the semiconductor material to jump the electron from valance band to conduction band.

Hence when the sufficient light energy strikes on it, more and more electron in valance band get excited to moves toward to conduction band in a large number of charge carriers. Due to this more and more current starts flowing through it when circuit is close and we say resistance of the circuit is decreases.

Thus, as the light intensity increases, the number of free electron increases due to this the photoconductivity of material increases and photoresistor is decreases of the material. This is called working principle of light dependent resistance-LDR.

working-principle-of-photoresistor

The approximately relationship between resistance and illumination is:

R=E A 

Here, R is resistance in ohm
E is illumination
A and α are constant
The value of constant α is decided by manufacturing procedure and Cadmium sulphide.

Characteristics of Photoresistor (LDR)

Photoresistor is a light dependent resistor. When light energy fall on it the resistance decreases and no fall of light resistance increases. When we kept in dark the resistance is high in mega ohms. It also called dark resistance. When constant voltage applied on is and light intensity increases the current starts increasing. The resistance vs illumination curve for photoresistor is shown in below:

resistance-vs-illumination-curve-for-photoresistor

Photodiode and photoresistor are non-liner passive device. Its sensitivity depends on the wavelength of light. Some photocell or photoresistor are made for certain range of wavelengths. Based on the material are uses on it. It has different spectral response curves.

When light is incident on photocell. It usually takes 8 to 12 ms for change in resistance. While it take one or more second for resistance to rise back again to its initial value after removal of light and this phenomenon is known a resistance recovery rate. And it is use in audio compressors. The LDR is less sensitive than photodiode. The photodiode and photoresistor are not same, a photodiode is a P-N junction diode that converts light energy to electrical energy while photoresistor is a passive component and it does not convert light energy to electrical energy.

Projects Based on Photoresistors

Many electronics project are based on photodiode and photresistance. The applications of photresistor are use in medical, astronomical and embedded field. Some project base on photresistor is given below:

  1. Photoresistor based, photometer and its application in forensic analysis of dyes.
  2. Integration of biocompatible organic resistive memory
  3. photoresistor for wearable image sensing application.
  4. Photogate timing with a smartphone.
  5. Design and implementation of simple acousto optic dual control circuit.
  6. System for light source location detection.
  7. Overheat protection device.
  8. Device for detecting electromagnetic radiation.
  9. Automatic dual-axis solar-powered lawnmower for agricultural application.
  10. The light-induced luminous keyboard is designed using photoresistors.
  11. Street light system for smart cities using photoresistors.
  12. Tracking of MRI interventional devices with computer-controlled detunable markers.
  13. These are used in Light-activated blinds.
  14. Photoresistors are also used for automatic contrast and brightness control in televisions and smartphones.
  15. For designing of proximity controlled switch photoresistors are used.

Advantages of Photoresistor

  • The internal photoelectric effect is zero
  • LDR provides good voltage and power handling capability.
  • It is a low-cost good reliability device and is readily available.

Disadvantage of Photoresistor

  • The variation of resistance is slow when rapid light action.

Applications of Photoresistors

Photoresistors have low cost and simple structure. It is often used as light sensors. Some application of photoresistor is given below

  • It is use in camera light meter for Detect absences or presences of light.
  • Used in street lighting for automatic ON and OFF
  • Alarm clocks
  • Burglar alarm circuits
  • Light intensity meters
  • It is use  SCADA industry to perform functions such as counting the number of packages on a moving conveyor belt.

Photoresistor specifications

PARAMETER EXAMPLE FIGURES
Max power dissipation 200mW
Max voltage @ 0 lux 200V
Peak wavelength 600nm
Min. resistance @ 10lux 1.8kΩ
Max. resistance @ 10lux 4.5kΩ
Typical resistance @ 100lux 0.7kΩ
Dark resistance after 1 sec 0.03MΩ
Dark resistance after 5 sec 0.25MΩ

 

The photodiode and photoresistor are not same, a photodiode is a P-N junction diode that converts light energy to electrical energy while photoresistor is a passive component and it does not convert light energy to electrical energy.

Frequently ask question about photoresistor

What is Photoresistor material?

The photo-resistor are made by silicon and germanium. It also use some other type of semiconductor materials for construction of photo-resistor such as cadmium sulfide or cadmium selenide.

What is a photoresistor sensor?

photo-resistor-sensor

Photoresistor Sensor (LDR)
it is also known as light dependent resistors (LDR), are light sensitive devices are often used to indicate the presence or absence of light, LDRs have a sensitivity and non-liner device and depending upon the wavelength of the light applied

How does a photoresistor sensor work?

photo-resistor-sensor

Hence when the sufficient light energy strikes on it, more and more electron in valance band get excited to moves toward to conduction band in a large number of charge carriers. Due to this more and more current starts flowing through it when circuit is close and we say resistance of the circuit is decreases.

What is the difference between photodiode and Photoresistor?

The photodiode and photo resistor are not same, a photodiode is a P-N junction diode that converts light energy to electrical energy while photo resistor is a passive component and it does not convert light energy to electrical energy.

What are the two types of Photoresistor?

Photo resistor or LDR are made of semiconductor material and it is a passive device. The photo-resistor are categorized in two types depending on the materials used to construct them. Is

Why is Photoresistor used?

Photo-resistor is a light dependent resistor it is mostly use to detect the absence and presence of light and measuring the intensity of light.

Also read:- photodiode, diode,

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