An LDR or a photoresistor has a resistance which changes based on the amount of visible light that falls on it. The light falling on the zigzag lines on the sensor. PDF | Light-dependent resistances (LDR) are cheap light sensors. A less known lightdetector is the electret microphone, whose electret membrane functions as. An LDR (light dependent resistor) is a component that has a (variable) resistance that changes with the Download a pdf version of this page here

Light Dependent Resistor Pdf

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Light Dependent Resistor - LDR. Two cadmium sulphide(cds) photoconductive cells with spectral responses similar to that of the human eye. 5/8/Light Dependent Resistor Brighton Webs Ltd statistics for energy and the environment Home Index. LIGHT DEPENDENT RESISTOR (LDR) - Free download as PDF File .pdf) or read online for free. A photoresistor or light dependent resistor (LDR) is a resistor .

Due to these charge carries the conductivity of LDR increases, increasing its resistivity. Photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.

In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire band gap. Extrinsic devices have impurities, also called dopants, and added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons i.


If a sample of silicon has some of its atoms replaced by phosphorus atoms impurities , there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Photo resistors are basically photocells. Fig 1.

The light sensitive part of the LDR is a wavy track of cadmium sulphide. CdS cells rely on the materials ability to vary its resistance according to the amount of light striking the cell. Fig 2. WORKING There are two basic circuits using light dependent resistors - the first is activated by darkness, the second is activated by light.

The two circuits are very similar and just require an LDR, some standard resistors, a variable resistor aka potentiometer , and any small signal transistor Fig 3.

Light sensitive operated relay type 1 In the circuit diagram above, the relay up whenever the LDR is in darkness. The 10K variable resistor is used to fine-tune the level of darkness required before the Relay up.

The 10K standard resistor can be changed as required to achieve the desired effect, although any replacement must be at least 1K to protect the transistor from being damaged by excessive current. Fig 4. Light sensitive operated relay type 2 By swapping the LDR over with the 10k and 10k variable resistors as shown above , the circuit will be activated instead by light. Whenever sufficient light falls on the LDR manually finetuned using the 10k variable resistor , the relay will up.

In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell. In view of their low cost, ease of manufacture, and ease of use LDRs have been used in a variety of different applications. At one time LDRs were used in photographic light meters, and even now they are still used in a variety of applications where it is necessary to detect light levels. Typical leaded light dependent resistor What is light dependent resistor, LDR or photoresistor A photoresistor or light dependent resistor is a component that is sensitive to light.

When light falls upon it then the resistance changes.

Values of the resistance of the LDR may change over many orders of magnitude the value of the resistance falling as the level of light increases. It is not uncommon for the values of resistance of an LDR or photoresistor to be several megohms in darkness and then to fall to a few hundred ohms in bright light.

The sensitivity of light dependent resistors or photoresistors also varies with the wavelength of the incident light. 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.


Similarly cadmium CdSe is also restricted. Other materials that can be used include lead sulphide, PbS and indium antimonide, InSb. Although a semiconductor material is used for these photoresistors, they are purely passive devices because they do not possess a PN junction, and this separates them from other photodetectors like photodiodes and phototransistors.

In this way it follows the same convention used for photodiode and phototransistor circuit symbols where arrows are used to show the light falling on these components. 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 photoresistor is made any 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.

2 Light Dependent Resistor (LDR) ..

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.

The basic format for a photoresistor is that shown below: Photoresistor structure The active semiconductor region is normally deposited onto a semi-insulating substrate and the active region is normally lightly doped.

In many discrete photoresistor devices, an interdigital pattern is used to increase the area of the photoresistor that is exposed to light. The pattern is cut in the metallisation on the surface of the active area and this lets the light through.

The two metallise areas act as the two contacts for the resistor. This area has to be made relatively large because the resistance of the contact to the active area needs to be minimised.

Photoresistor structure showing interdigital pattern to maximise exposed area.This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance.

In many discrete photoresistor devices, an interdigital pattern is used to increase the area of the photoresistor that is exposed to light. Related titles.

Very in accurate 2. Mrinalini Dixit.

The rate at which the resistance changes is called the resistance recovery rate. Divyang Vijay.

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.