What is LDR and how it works with circuit to try on?

 

LDR (Light-dependent resistor)

A photo-resistor (or light-dependent resistor, LDR, or photo-conductive cell) is a light-controlled variable resistor. The resistance of a photo-resistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. [1]

In the dark, their resistance is very high, sometimes up to 1MΩ, but when the LDR the sensor is exposed to light, the resistance drops dramatically, even down to a few ohms, depending on the light intensity. LDRs have a sensitivity that varies with the wavelength of the light applied and are nonlinear devices.

For example it can be used to turn on a light when the LDR is in darkness or to turn o­ a light when the LDR is in light. It can also work the other way around so when the LDR is in the light it turns on the circuit and when it’s in darkness the resistance increases and disrupts the circuit.

How it Works?

The way an LDR works is that they are made of many semi-conductive materials with high resistance. The reason they have a high resistance is that are very few electrons that are free and able to move because they are held in a crystal lattice and are unable to move. When light falls on the semiconductive material it absorbs the light photons and the energy is transferred to the electrons, which allow them to break free from the crystal lattice and conduct electricity and lower the resistance of the LDR.

The common CdS (cadmium sulfide) LDRs have peak spectral response at about 550 nm which is the green region of the visible spectrum.

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. [2]

Types of photoresistor

Light-dependent resistors, LDRs or photoresistors fall into one of two types or categories:

 

• Intrinsic photoresistors:   Intrinsic photo resistors use un-doped semiconductor materials including silicon or germanium. Photons fall on the LDR excite electrons moving them from the valence band to the conduction band. As a result, these electrons are free to conduct electricity. The more light that falls on the device, the more electrons are liberated, and the greater the level of conductivity, and this results in a lower level of resistance.

                                

In intrinsic photoresistors, the resistance decreases slightly with the increase in light energy. Hence, intrinsic photoresistors are less sensitive to light. Therefore, they are not reliable for practical applications.

 

• Extrinsic photoresistors:   Extrinsic photoresistors are manufactured from semiconductor of materials doped with impurities. These impurities or dopants create a new energy band above the existing valence band.

As a result, electrons need less energy to transfer to the conduction band because of the smaller energy gap.                                    

 

 

Regardless of the type of light-dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of the incident light.

In extrinsic photoresistors, we already have a large number of charge carriers. Hence, providing a small amount of light energy generates even more number of charge carriers. Thus, the electric current increases rapidly.  Extrinsic photoresistors are reliable for practical applications.

LDR frequency dependence

 

The sensitivity of photoresists is shown to vary with the wavelength of the light that is impacting the sensitive area of the device. The effect is very marked and it is found that if the wavelength is outside a given range then there is no noticeable effect.

Devices made from different materials respond differently to light of different wavelengths and lots can be developed for the sensitivity and the light wavelength.

It is also found that extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared. However, when working with infrared, care must be taken to avoid heat build-up caused but he elating effect of the radiation

TRY the following the circuit and see what happens?

 Also, try changing NPN transistor to PNP transistors and see the difference.

References:

[1] (“Photo-resistor”, In Wikipedia, Para 1, Retrieved on 30 April 2019‎)

https://en.wikipedia.org/wiki/Photoresistor

 

[Figure: 1] IndianMart. 2011. LDR (Light Dependent Resistor). [ONLINE] Available at: https://www.indiamart.com/proddetail/ldr-light-dependent-resistor-8320577633.html. [Accessed 6 May 2019].

 

[Figure: 1.1] Explore & Learn. 2011. Photocells CdS Cells. [ONLINE] Available at: https://learn.adafruit.com/photocells/overview. [Accessed 6 May 2019].

 

[2] Resistor Guide. 2019. Photoresistor. [ONLINE] Available at: http://www.resistorguide.com/photoresistor/. [Accessed 6 May 2019].

 

[Figure: 1.4] LEDnique. N.A. Light-dependent resistor (LDR). [ONLINE] Available at: http://lednique.com/opto-isolators-2/light-dependent-resistor-ldr/. [Accessed 6 May 2019].

 

[3] electronicsnotes. 2016. Light Dependent Resistor LDR: Photoresistor. ONLINE Available at: https://www.electronicsnotes.com/articles/electronic_components/resistors/light-dependent-resistor-ldr.php. [Accessed 6 May 2019].

 

[Figure: 1.2 & 1.3] physics-and-radio-electronics.com. N.A. Photoresistor. [ONLINE] Available at: https://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/passive-components/resistors/photoresistor.html. [Accessed 6 May 2019].