What is Light Dependent Resistor : Circuit & Its Working

The controlling of lights and home appliances is generally operated and maintained manually on several occasions. But the process of appliances controlling may cause wastage of power due to the carelessness of human beings or unusual circumstances. To overcome this problem we can use the light-dependent resistor circuit for controlling the loads based on the intensity of light. An LDR or a photoresistor is a device that is made up of high resistance semiconductor material. This article gives an overview of what is LDR or light-dependent resistor circuit and its working.

What is Light Dependent Resistor?

An electronic component like LDR or light-dependent resistor is responsive to light. Once light rays drop on it, then immediately the resistance will be changed. The resistance values of an LDR may change over several orders of magnitude. The resistance value will be dropped when the light level increases.

The resistance values of LDR in darkness are several megaohms whereas in bright light it will be dropped to hundred ohms. So due to this change in resistance, these resistors are extremely used in different applications. The LDR sensitivity also changes through the incident light’s wavelength.

The designing of LDRs can be done by using semiconductor materials to allow their light-sensitive properties. The famous material used in this resistor is CdS (cadmium sulfide), even though the utilization of this material is currently restricted in European countries due to some environmental issues while using this material. Likewise, CdSe (cadmium selenide) is also restricted and additional materials that can be employed mainly include PbS (lead sulfide), InS ( indium antimonide).

Even though for these resistors, a semiconductor material is used, because they are simply passive devices and they do not have a PN-junction. This detaches them from other LDRs such as phototransistors & photodiodes.

Light Dependent Resistor
Light Dependent Resistor

LDR Symbol

In electronic circuits, the LDR symbol is used that mainly depends on the resistor symbol; however, it illustrates the light rays in the arrows form. In this way, it follows the same principle which is used for phototransistor & photodiode circuit symbols wherever arrows are utilized to demonstrate the light dropping on these types of components. The LDR circuit symbols are shown below.

LDR Symbols
LDR Symbols

Construction of an LDR

The construction of an LDR includes a light-sensitive material that is placed on an insulating substrate like ceramic. The material is placed in a zigzag shape in order to get the required power rating and resistance. The area of zigzag separates the metal-placed areas into two regions.

Lighr Dependent Resistor Construction
Light Dependent Resistor Construction

Where the Ohmic contacts are made either on the sides of the area. The resistances of the contacts must be as less as possible to make sure that the resistance, mainly varies due to the light effect only. The use of lead & cadmium materials is avoided as they are injurious to the environment.

Working Principle of Light Dependent Resistor

The working principle of an LDR is photoconductivity, which is nothing but an optical phenomenon. 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 eager to the conduction band. But, the photons in the incident light must have energy superior to the bandgap of the material to make the electrons jump from one band to another band (valance to conduction).

Hence, when light having ample energy, more electrons are excited to the conduction band which grades in a large number of charge carriers. When the effect of this process and the flow of the current starts flowing more, the resistance of the device decreases.

Light Dependent Resistor Circuit

The circuit of LDR is an electronic circuit built with LDR, relay, Darlington pair, diode, & resistors shown in the below circuit diagram. A voltage supply is given to the load

The required DC voltage of the LDR circuit is supplied from a bridge rectifier circuit or a battery. This circuit changes the AC supply into a DC. The bridge rectifier circuit uses a step-down transformer to step-down the voltage from 230v into 12v. The diodes are connected in the form of a bridge used to alter the AC voltage into DC. The voltage regulator is used to change the 12v DC-6v DC, and then, this DC voltage is supplied to the entire circuit. A 230v AC supply for both the bridge rectifier and the load is to be kept continuously for continuous operation of the light sensor circuit.

LDR Circuit Diagram
LDR Circuit Diagram

In the morning time, this sensor has a low resistance of around 100Ω. Thus, the power supply flows through the LDR & ground through the variable resistor and resistor as shown in the above light sensor circuit. This is due to the resistance offered by the light-dependent resistor in the daytime or when the light falls on the LDR, then it is less compared to the resistance of the remaining part of the sensor circuit. We are alert of the principle of current, that the flow of current always flows in the path of low resistance.

Therefore, the relay coil does not get sufficient supplies to get strengthened. Hence, the light is switched off in the daylight. In the same way, during the night time, the resistance of the LDR increases to a high value (20MΩ). Thus, due to the high resistance of the resistor, the flow of current is low or almost zero. Now, the flow of current through the low-resistance lane such that it increases the base voltage of the Darlington pair to reach more than 1.4v. As the Darlington pair transistor is triggered, the relay coil acquires enough supply to get energized, and hence, the light switches in the nighttime.

LDR Frequency Dependence

The sensitivity of LDR varies based on the light’s wavelength that is affecting the responsive region of the device. This effect is noticeable & found when the wavelength of light is outside of a specified range then a noticeable effect will not be there. Devices that are made with different materials will respond in a different way to the wavelengths of light, which means that the various components can be employed for a variety of applications.

It is also found that extrinsic type resistors tend to be more responsive to lights wavelength & can be utilized for infrared. But once working with IR, we should take care to evade heat build-up caused due to the heating effect of the radiation.

Light Dependent Resistor Latency

Latency is the main aspect associated with an LDR which means the time taken to respond to the components for any changes. So, this feature is particularly significant for 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. So for this reason, the light-dependent resistor is not a better choice wherever there is a reasonably quick changing of light values. Once the light changes occur over a certain time period then they are above sufficient.

The recovery rate of resistance is nothing but the rate at which point the resistance changes. Usually, The LDR responds in a few tens of milli secs once the light is given after complete darkness, however, once the light is removed, then it can take up to a second. In the component’s datasheet, LDR specifications are normally quoted as the dark resistance after a specified time like in seconds. The frequently quoted values are one value is for 1 sec & another one is for 5 sec. These values give an indication of the resistor’s latency.

Light Dependent Resistor Specifications

The LDR specifications mainly include maximum power dissipation, maximum operating voltage, peak wavelength, dark resistance, etc. The values of these specifications mentioned below.

  • Maximum power dissipation is 200mW
  • The maximum voltage at 0 lux is 200V
  • The peak wavelength is 600nm
  • Minimum resistance at 10lux is 1.8kΩ
  • Maximum .resistance at 10lux is 4.5kΩ
  • Typical resistance at100lux is 0.7kΩ
  • Dark resistance after 1 sec is 0.03MΩ
  • Dark resistance after 5 sec is 0.25MΩ

Characteristics of LDR

The light-dependent resistor is very responsive to light. When the light is stronger, then the resistance is lower which means, when the light intensity increases then the value of resistance for the LDR will be decreased drastically to below 1K.

LDR Characteristics
LDR Characteristics

When the light drops on LDR, the resistance will be decreased and when the resistor is placed in the dark then the resistance will be increased which is called dark resistance. If any device absorbs light then its resistance will be reduced radically. If a stable voltage is given to it, the light intensity will be increased & the flow of current starts increasing. So, the following diagram represents the characteristics between resistance & illumination for a specific LDR.

LDRs are not linear devices and their sensitivity changes through the light’s wavelength which drops on them. Some kinds of photocells are not at all sensitive to a specific range of wavelengths because it depends on the used material.

Once light rays fall on a photocell, the resistance will be changed in 8 ms to 12, while it uses few more seconds to rise the resistance back again to its early value once the light is removed. So this is known as a recovery rate of resistance. In audio compressors, this property is applicable.

In addition, these resistors are low responsive to phototransistors & photodiodes. A photo-diode is a PN-junction semiconductor device, used to change the light to electricity, while a photocell is a passive device and it doesn’t include a PN junction but is used to convert light to electricity but it is not.

Types of LDR

The classification of LDR can be done based on linearity or photosensitive materials. The most frequently used materials while designing LDR mainly include cadmium sulfide, thallium sulfide, cadmium selenide & lead sulfide.

  • The LDRs generated with this cadmium sulfide chemical are very responsive to all kinds of light radiation that are noticeable within the spectrum of human beings.
  • The LDRs made with lead sulfate chemicals are particularly responsive to IR radiation.

The most frequent classification of LDR is linear as well as nonlinear

  • Liner type LDRs are called as photodiodes however in some applications areas it is used as photoresistors because of the linear performance they present & their operation.
  • Non-linear LDRs are commonly used but their behavior does not depend on the polarity through which it unites.

LDR Advantages

The advantages of LDR include the following.

  • Sensitivity is High
  • Simple & Small devices
  • Easily used
  • Inexpensive
  • There is no union potential.
  • The light-dark resistance ratio is high.
  • Its connection is simple

LDR Disadvantages

The disadvantages of LDR include the following.

  • Spectral response is narrow
  • Hysteresis effect
  • Temperature stability is low for the best materials
  • In stable materials, it responses very slowly
  • The use of LDR is limited where the light signal changes very quickly
  • It is not so much a responsive device.
  • It provides incorrect result once working temperature alters

Applications of LDR

Light-dependent resistors are simple and low-cost devices. These devices are used where there is a need to sense the presence and absence of light is necessary. These resistors are used as light sensors and the applications of LDR mainly include alarm clocks, street lights, light intensity meters, burglar alarm circuits. For a better understanding of this concept, here we have explained one project namely; power conserving of intensity controlled street lights using LDR.

Power Conserving of Intensity Controlled Street Lights using LDR

Nowadays, the lightening-up of highways is done through HID lamps but, the energy consumption of these lights is high as well as there is no particular mechanism to turn on/off the lights from sunrise to sunset. To overcome this problem, here is an alternative method using LEDs i.e. power conserving of intensity controlled street lights using LDR.

The proposed system is built to overcome the drawback of the present-day HID lamps. The proposed system determines the usage of light-emitting diodes as a light source and its adjustable intensity control according to the requirement. These lights consume less power, as well as the life span of these lights, which is more compared to conventional HID lamps.

The most important feature of this project is, the light intensity can be controlled according to necessity during nighttime, which is not possible in HID lamps. An LDR is used to detect the light and the resistance of the LDR drastically reduces according to the light in the daytime, which forms as an i/p signal to the controller.

A bunch of LEDs is used to make a street light and the microcontroller using in the project is preprogrammed with instructions that control the light intensity based on the Pulse width modulation signals generated. The light intensity is kept high during the nighttime, and as the traffic on the roads tends to fall in peak hours; the intensity also falls gradually till morning. Finally, the LED lights get turned off in the morning at 6 am and continues again at 6 pm in the evening. This process will continue.

Furthermore, this project can be developed by mixing it with a solar panel, which changes the intensity of the solar to the equivalent voltage and solar energy is used to feed the lights on highways.

Thus, this is all about the working principle of the light-dependent resistor, circuit diagram, and its applications and it is one kind of photoresistor. We hope that you have got a better concept through this article. Furthermore, any queries regarding this article or electrical and electronics projects, please give your feedback in the comment section below. Here is a question for you, what is the function of an LDR?

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