Light-dependent resistors also known as LDRs, or photoresistors are electronic components used to detect light and change the operation of a circuit dependent upon the light levels. Light-dependent resistors are often used in electronic circuit design, where the presence or level of light needs to be detected. This resistor has a variety of names from a light-dependent resistor, LDR, photoresistor, photocell, photocell, or photoconductor. Other electronic components can serve their purposes such as photodiodes, or photo-transistor, but LDRs or photo-resistors are ideal and convenient in many electronic circuit designs.
In this article, you’ll get to know the definition, applications, diagram, symbol, types, structure, construction, working, specification, characteristics, frequency dependence, and latency of light-dependent resistors LDRs. You’ll also get to know the advantages and disadvantages of these LDRs
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What are light-dependent resistors LDRs?
A light-dependent resistor LDR is an electronic component that is sensitive to light, when it experiences light or light fall upon it, the resistance changes instantly. The 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. In another word, LDR or photoresistors are responsive to light, once light rays drop on them, their resistance instantly changes. Having in mind the resistance value drops when the light level increases.
In the darkness, the resistance values of LDR are several megaohms while in bright light it reduces or drops to hundred ohms. Because of the change in resistance, light-dependent resistors can be used in different applications. The sensitivity of these types of resistors also changes due to incident light’s wavelength. The light-sensitive properties of LDRs are due to the semiconductor materials they are made of. Although many materials can be used, one of the most popular materials is cadmium sulfide, Cds. Semiconductor materials are used in this component because they are purely passive devices, they do not possess a PN junction, and this separates them from other photodetectors such as photodiodes and phototransistors.
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Applications of LDR
The applications of light-dependent resistors are common due to their simplicity and low cost. They are used in applications where there is a need to sense the presence and absence of light and they are used as light sensors. LDRs are mainly seen in alarm clocks, street lights, light intensity meters, burglar alarm circuits, light beam alarms, reflective smoke alarms, outdoor clocks, and camera light meters. However, photoresistors are often used in electronic circuit designs where it is important to detect the presence of a level of light.
LDR / photoresistor symbol used in electronic circuits depends on the resistor symbol. The light rays are illustrated in the arrow form just as that of phototransistor and photodiode circuit symbols. The figure below shows the symbol LDRs:
Types of light-dependent resistors LDRs
The various types of LDRs can be classified based on linearity or photosensitive materials. Materials often used in their design include cadmium sulfide, thallium sulfide, cadmium selenide & lead sulfide. LDRs generated with cadmium sulfide chemicals are very responsive to all kinds of light radiation that is noticeable within the spectrum of human beings. LDRs made of lead sulfate chemicals are particularly responsive to IR radiation. However, the most frequent classification of LDR is linear and nonlinear.
- Linear LDR: These types of LDRs are called photodiodes but it is used as photoresistors in some applications due to the linear performance during operation.
- Nonlinear LDR: the nonlinear LDRs are mostly used but their behavior does not depend on the polarity through which it unites.
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Diagram of LDR:
Photoresistor structure and construction
The structure of photoresistors is light-sensitive having a horizontal body that is exposed to light. In most 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 metallization on the surface of the active area letting the light through. The two metalized areas act as the two contacts for the resistor, this area is designed to be relatively large because the resistance of the contact to the active area needs to be minimized.
The construction of LDR or photoresistor is achieved with a light-sensitive material that is placed on an insulating substrate like ceramic. The material is placed in a zigzag shape so that the required power rating and resistance can be obtained. The area of zigzag demarcates the metal-placed areas into two regions. The ohmic contacts are made either on the sides of the area and the resistances of the contacts must be less as possible. This will ensure the resistance mainly varies due to the light effect only. The use of lead and cadmium materials is avoided as they are hazardous to the environment.
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The working principle of light-dependent resistors is very easy and can be easily understood. They work on the principle of photoconductivity, which is nothing but an optical phenomenon. That is, when the light is absorbed by the material then the conductivity of the material enhances. When the light falls on the LDR, then the electrons in the valence band of the material are severe to the conduction band. However, the photons in the incident light mush have superior energy to the bandgap of the material to make the electrons jump from one band to another band (valence to conduction).
When light having is ample energy, more electrons are excited to the conduction band which grades in a large number of charge carriers. If the effect of this process and the flow of the current starts flowing more, the resistance of the device decreases.
Watch the video below to learn more on the working of light-dependent resistors LDRs:
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Just as earlier mentioned, light-dependent resistors are very responsive to light. When the light is stronger, the resistance is lower. This means when the light intensity increases then the value of resistance for the LDR will decrease drastically to below 1k. when light falls on LDR, the resistance will be decreased, and when the resistor is placed in the dark, the resistance will be increased which is known as dark resistance. The resistance will reduce radically if any device absorbs light. If a stable voltage is given to it, the light intensity will be increased and the flow of current starts increasing.
Specifications of a light-dependent resistor
The table below shows the specifications of photoresistor and their examples:
|KEY LDR / PHOTORESISTOR SPECIFICATIONS|
|Max power dissipation||This is the maximum power the device can dissipate within a given temperature range. Derating may be applied 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 are given under certain light conditions, often 10 lux. A minimum and maximum value 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.|
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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|
Frequency dependence of LDR
The sensitivity of photoresistors is shown to change with the wavelength of the light that is reflecting the sensitive area of the device. The effect is marked and if the wavelength is outside a given range no effect will occur. Devices are made from different materials which make them respond differently to light of wavelengths. This means that different electronics components can be used for different applications. extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared.
Latency of LDR
The latency of LDR is the time taken to respond to the components for any changes. This feature is essential while designing a circuit. It takes a visible amount of time from any changes within a light level before the LDR attains its last value for the new level of light. Therefore, a light-dependent resistor is not a better option when there is a reasonably quick changing of light values. Once the light changes occur over a certain period then they are more than sufficient.
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The recovery rate of resistance is the rate at which point the resistance changes. Normally, the LDR responds in a few tens of millions of secs once the light is given after complete darkness, but once the light is removed, then it can take up to a second. In most components’ datasheets, LDR specifications are normally quoted at the dark resistance after a specified time like in seconds. The values are frequently quoted as one value for 1 sec and another one for 5 sec. The resistor’s latency is determined by these values.
Advantages and disadvantages of Light-dependent resistors LDRs
Below are the benefits of LDR in their various applications:
- The device is simple and small
- Sensitivity is high
- Can be easily used
- Widely available
- There is no union potential
- The light-dark resistance ratio is high
- Its connection is simple
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Despite the good advantages of LDR, some limitations still occur. Below are the disadvantages of this LDR in their various applications:
- Hysteresis effect
- Spectral response is narrow
- Temperature stability is ow for the best materials
- The incorrect result is provided once working temperature changes
- The device is not much responsive
- LDR is limited where the light signal changes rapidly.
A light-dependent resistor is also known as LDR, photoresistor, photocell, photocell, or photoconductor. They are often used in electronic circuit design, where the presence or level of light needs to be detected. Note that, LDR is an electronic component that is sensitive to light, when it experiences light or light fall upon it, the resistance changes instantly. That is all for this article, where the definition, applications, diagram, symbol, types, structure, construction, working, specification, characteristics, frequency dependence, latency, advantages, and disadvantages of light-dependent resistors LDRs are being discussed.
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