Light dependent resistor LDR or photoresistor changes its resistance when light fall on it, thus it detects the presence of light.
In this article, we will learn about Light dependent resistors (LDR or photoresistors), their symbol, construction, working, types, latency, technical specifications, and applications of LDR.
What is a Light Dependent Resistor (LDR)?
LDR is a light-dependent resistor. It is also called Light controlled variable resistor, photoresistor, and photo-conducting cell. As the name suggests, its resistance changes when the light falls on it. So, it is an electronic component, that is sensitive to light.
It is made of semiconductor material, and its resistance decreases when the intensity of light hitting on it increases. Ultimately, its resistance is more when the intensity of light is less.
Symbol of Light Dependent Resistors (LDR)
The circuit symbols of the light-dependent resistor (LDR) are shown in the following figure.
Construction of Light Dependent Resistors (LDR)
LDR is made from a semiconductor material having a property called photoconductivity. The most common semiconductor material used for making the LDR is Cadmium Sulfide and Cadmium Selenide.
- The core of LDR is made of semiconductor material like Cadmium Sulfide.
- The semiconductor material is directly linked with two metal electrodes to allow the current through the LDR.
- LDR is placed in the protective casing to prevent it from external factors such as dust, moisture, and physical damage.
Working of Light Dependent Resistors (LDR)
It works on the principle of photoconductivity. When light strikes an LDR, the photons’ energy is absorbed by the semiconductor material. This absorbed energy by the semiconductor material causes it to release the charge carriers which increases the conductivity. Actually, bound electrons in the valence band get the energy and they jump into the conduction band where due to the formation of free electrons, the conduction of electricity starts.
As a result, its resistance decreases when the intensity of light hitting on it increases. Ultimately, its resistance increases when the intensity of light is less because the formation of free electrons is less.
Types of Light Dependent Resistors (LDR)
There are two types of Light-dependent resistors as shown in the following:
- Intrinsic LDR
- Extrinsic LDR
(1). Intrinsic LDR:
This type of photoconductive cell or LDR is made of pure semiconductor material like Silicon or Germanium. There is no doping with the impurities. When light falls on it, electrons get sufficient energy and come into the conduction band and when the number of electrons increases in high intensity of light, the resistance will be less.
In such a material, the number of charge carriers is solely determined by the thermal excitation and electrical conductivity is relatively low.
(2). Extrinsic LDR:
This type of photoconductive cell or LDR is made of semiconductor material like Silicon or Germanium and doped material. There is doping with the impurities. When light falls on it, electrons need less energy and come into the conduction band and when the number of electrons increases in the high intensity of light, the resistance will be less. In extrinsic LDR, the addition of impurities causes a reduction in the gap between the valance and conduction band, and thus, electrons need less energy. As a result, electrons enter into the conduction band even in the low intensity of light.
Extrinsic LDR is more sensitive to light, because, here, electrons enter into the conduction band even in the presence of low-energy photons.
Latency of LDR
The term Latency refers to the delay between input signal and output response. LDRs do not have significant latency. It is also called Response time.
The response time or latency of LDR mainly depends upon the physical properties of semiconductor material and the design of the LDR circuit. LDRs are relatively slow in response time in comparison to other light-sensitive devices.
The response time or latency of LDR can be in the range of milliseconds to ten milliseconds, which is often sufficient for many applications.
The exact latency of LDR can be varied according to the model of LDR, integrated circuit, and environmental conditions.
Light-dependent resistors also have a lag between light exposure and resistance decrease, which is typically around 10 milliseconds. When switching from illuminated to dark situations, the lag period is even longer, frequently lasting up to one second.
LDR Frequency Dependence
The sensitivity of LDR depends on the wavelength of the light. The responsive region of LDR shifts with a change in the incident light’s wavelength. LDRs that are made with different materials( Intrinsic/ extrinsic) have different responses to the wavelengths of light. Therefore, the various components can be used for a variety of applications.
The extrinsic-type LDRs are likely to be more responsive to light wavelength & they can be used for infrared. However, we need to take care of the heat dissipation of the device.
Technical Specifications of LDR
An LDR’s (Light-Dependent Resistor) technical parameters may vary based on the type and manufacturer. Common technical specifications of LDR can be:
- Resistance
Dark Resistance – Dark resistance values are specified after a given time because the resistance decreases after some time on the recombining of charge carriers. For example-Dark resistance after 1 sec is 0.04MΩ/Dark resistance after 5 sec is 0.30 MΩ
Light Resistance– The minimum and maximum resistance values are specified under certain light conditions. The minimum and maximum light intensity may be between 10 lux to 100 lux. - Illuminance range
- Spectral resonance- The sensitivity of an LDR changes with a change in the wavelength of light. Thus, the spectral response of LDR shows the response of the LDR for a particular range of wavelengths.
- Peak wavelength – Specified in nm
- Response time- The response time shows how quickly an LDR changes its resistance under changed light conditions. when LDR is exposed to light, it decreases its resistance. The time taken to decrease its resistance is called rise time. When light is removed, the LDR increases its resistance, and the time taken to increase its resistance is called decay time. Typically, the rise time is less than the decay time. The rise and decay time in milliseconds.
- Power rating- The maximum power the LDR can dissipate within a specified temperature range. Operating above a specified temperature cause LDR power derating. For example- power rating=200mW
- Recovery Rate: When LDR is exposed to light, it changes its state from dark to light condition. On removing light the LDR goes into its original state. The rate at which an LDR returns to its original resistance state is called the recovery rate. The factors like temperature, humidity, and aging affect the recovery rate of LDR.
- Operating temperature range
- Tolerance
- Material and construction
- Operating Voltage – Specified at 0 lux(darkness)
- Sensitivity: The ratio of change in resistance to change in light intensity is called the LDR sensitivity, expressed in percentages or decibels (dB). The higher sensitivity shows that LDR can detect the smaller change in light intensity.
Because different LDRs are designed for different purposes, understanding their technical requirements is critical when selecting the proper component for your project.
Characteristics of LDR
The resistance of LDR changes with a change in the intensity of light. The resistance of LDR decreases with an increase in light intensity, and the resistance decreases sharply to below 1000 lux. The resistance vs light intensity curve of the LDR is given below.
The resistance of LDR decreases when the light on LDR is decreased and when the LDR is kept in the dark its resistance increases and it is called the dark resistance.
Applications of LDR
Because of their light sensitivity, light-dependent resistors (LDRs) have a wide range of applications in a variety of industries as listed below.
- Automatic street light control system
- Day-night switches for outdoor lighting
- Security system
- Solar-powered lighting system
- Light-sensitive Garden watering system
- Photographic light meters
- Light-sensitive doorbell activation
- Light-sensitive pet feeders
- Optical circuit design using the LDR
- Clock radios with LDR
- Simple smoke detector alarm by using the LDR
- Light-sensitive exterior vehicle lighting
- Used in infrared astronomy
- Automatic contrast and brightness control in television receivers
These applications demonstrate LDRs’ versatility and practicality in responding to changes in light intensity and being integrated into a variety of devices and systems for automatic and responsive functionality.
Conclusion
A light-dependent resistor is an LDR. It is also known as a light-controlled variable resistor, a photoresistor, or a photo-conducting cell. When exposed to light, its resistance changes, as the name implies. As a result, it is a light-sensitive electrical component.
It is made of semiconductor material, and its resistance reduces with increasing light intensity. When the intensity of light is reduced, its resistance increases. It operates on the photoconductivity principle.
When light strikes an LDR, the photons’ energy is absorbed by the semiconductor material. The absorbed energy by the semiconductor material allows the charge carriers to be released, increasing conductivity. There are two types of LDR that were discussed in detail. In the end, we studied the latency of LDR, technical specifications and applications of LDR.
