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Light dependent resistors, LDRs or photoresistors are often used in circuits where it is necessary to detect the presence or the level of light.
They can be described by a variety of names from light dependent resistor, LDR, photoresistor, or even photo cell, photocell or photoconductor.
Although other devices such as photodiodes or photo-transistor can also be used, LDRs or photoresistors are a particularly convenient electronics component to use. They provide large change in resistance for changes in light level.
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.
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. With such a wide variation in resistance, LDRs are easy to use and there are many LDR circuits available. 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.
How an LDR works
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 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.
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.
Types of photoresistor
Light dependent resistors, LDRs or photoresistors fall into one of two types or categories:
- Intrinsic photoresistors: Intrinsic photoresistors 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.
- 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 incident light.
The LDR symbol used in circuits is based around the resistor circuit symbol, but shows the light, in the form of arrows shining on it. 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.
The light dependent resistor / photoresistor circuit symbols are shown for both the newer style resistor symbol, i.e. a rectagular box and the older zig-zag line resistor circuit symbols.
Light dependent resistor specifications
There are several specifications that are important for light dependent resistors, LDRs / photoresistors.
These photoresistor specifications include:
|Key LDR / Photoresistor Specifications
|Max power dissipation||This is the maximum power the device is able to dissipate within a given temperature range. Derating may be applicable above a certain temperature.|
|Maximum operating voltage||Particularly as the device is semiconductor based, the maximum operating voltage must be observed. This is typically specified at 0 lux, i.e. darkness.|
|Peak wavelength||This photoresistor specification details the wavelength of maximum sensitivity. Curves may be provided for the overall response in some instances. The wavelength is specified in nm|
|Resistance when illuminated||The resistance under illumination is a key specification is a key parameter for any photoresistor. Often a minimum and maximum resistance is given under certain light conditions, often 10 lux. A minimum and maximum vale may be given because of the spreads that are likely to be encountered. A 'fully on' condition may also be given under extreme lighting, e.g. 100lux.|
|Dark resistance||Dark resistance values will be given for the photoresistor. These may be specified after a given time because it takes a while for the resistance to fall as the charge carrier recombine - photoresistors are noted for their slow response times.|
A typical light dependent resistor, LDR / photoresistor specification may be:
|Example Photoresistor Specifications|
|Max power dissipation||200mW|
|Max voltage @ 0 lux||200V|
|Min. resistance @ 10lux||1.8kΩ|
|Max. resistance @ 10lux||4.5kΩ|
|Typ. resistance @ 100lux||0.7kΩ|
|Dark resistance after 1 sec||0.03MΩ|
|Dark resistance after 5 sec||0.25MΩ|
LDRs are very useful components that can be used for a variety of light sensing applications. As the LDR resistance varies over such a wide range, they are particularly useful, and there are many LDR circuits available beyond any shown here. In order to utilise these components, it is necessary to know something of how an LDR works, which has been explained above.
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