Light Dependent Resistor (LDR) 

Working Principle of Light Dependent Resistor (LDR)

Light Dependent Resistor work based off of the principle of photoconductivity. Photoconductivity is an optical phenomenon in which the material’s conductivity is increased when light is absorbed by the material. When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the bandgap of the semiconductor material to make the electrons jump from the valence band to the conduction band. Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in a large number of charge carriers. The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased. 
Characteristics of Photoresistor (LDR)
Photoresistor LDR’s are light-dependent devices whose resistance is decreased when light falls on them and that is increased in the dark. When a light dependent resistor is kept in dark, its resistance is very high. This resistance is called as dark resistance. It can be as high as 1012 Ω and if the device is allowed to absorb light its resistance will be decreased drastically. If a constant voltage is applied to it and the intensity of light is increased the current starts increasing. The  showing in figure the resistance vs. illumination curve for a particular LDR.
Photocells or LDR’s are nonlinear devices. Their sensitivity varies with the wavelength of light incident on them. Some photocells might not at all response to a certain range of wavelengths. Based on the material used different cells have different spectral response curves. When light is incident on a photocell it usually takes about 8 to 12 ms for the change in resistance to take place, while it takes one or more seconds for the resistance to rise back again to its initial value after removal of light. This phenomenon is called a resistance recovery rate. This property is used in audio compressors. Also, LDR’s are less sensitive than photodiodes and phototransistors. (A photo diode and a photocell (LDR) are not the same, a photo-diode is a pn junction semiconductor device that converts light to electricity, whereas a photocell is a passive device, there is no pn junction in this nor it “converts” light to electricity).

Light Dependent Resistor specifications 

• Typical values vary between 1 MΩ or more in the dark and 100MΩ in bright light.
• Maximum dissipation (50mW – 1W).
• Maximum voltage (600V).
• Spectral response.
• The typical response time of an LDR is in the order of one tenth of a second.

Types of light Dependent Resistors

• Intrinsic photoresistors 
• Extrinsic photoresistors

Intrinsic photoresistors (Undoped semiconductor)

These are made of pure semiconductor materials such as silicon or germanium. Electrons get excited from valance band to conduction band when photons of enough energy fall on it and the number charge carriers are increased.

Extrinsic photoresistors

These are semiconductor materials doped with impurities which are called dopants. These dopants create new energy bands above the valence band which is filled with electrons. Hence this reduces the bandgap and less energy is required in exciting them. Extrinsic photo resistors are generally used for long wavelengths.

Construction of a Photocell

The structure of a light-dependent resistor consists of a light-sensitive material which is deposited on an insulating substrate such as ceramic. The material is deposited in a zigzag pattern in order to obtain the desired resistance and power rating. This zigzag area separates the metal deposited areas into two regions. Then the ohmic contacts are made on either sides of the area. The resistances of these contracts should be as less as possible to make sure that the resistance mainly changes due to the effect of light only. Materials normally used are cadmium sulfide, cadmium selenide, indium antimonide, and cadmium sulfide. The use of lead and cadmium is avoided as they are harmful to the environment.

Simple Circuit  of a Light Dependent Resistor

When the light intensity is low, then the resistance of the LDR is high. This stops the current flow to the base terminal of the transistor. So, the LED does not light. However, when the light intensity onto the LDR is high, then the resistance of the LDR is low.So current flows onto the base of the first transistor and then the second transistor.Consequently the LED lights.Here, a preset resistor is used to turn up or down to increase or decrease the resistance.

Light Dependent Resistor advantages

• High sensitivity (due to the large area it can cover).
• Easy employment.
• Low cost.
• There is no union potential.
• High light-dark resistance ratio.

Light Dependent Resistor disadvantages

• Narrow spectral response.
• Hysteresis effect.
• Low temperature stability for the fastest materials.
• The variation of the value of the resistance has a certain delay, different if it goes from dark to illuminated or from illuminated to dark. This limits the use of LDRs in applications where the light signal varies rapidly.
• Slow response in stable materials.

Applications of Light Dependent Resistor

• Photoresistors (LDRs) have low cost and simple structure and are often used as light sensors. 
• Detect absences or presences of light like in a camera light meter.
• Used in street lighting design (can be combined with a good Arduino starter kit to act as a street light controller)
• Alarm clocks
• Burglar alarm circuits
• Light intensity meters
• Used as part of a SCADA system to perform functions such as counting the number of packages on a moving conveyor belt

Light Dependent Resistor Real-life example 

Security System Controlled by An Electronic Eye

This security system controlled by an electronic eye project is based on photo sensing arrangement. The proposed system uses a 14-stage ripple carry binary counter to sense the intensity of light using LDR. The o/p makes a relay and buzzer for the required action. This project is very useful to deter burglars from malls, banks and shops. This project or systems  uses a light dependent resistor. When light falls on the LDR sensor, then the resistance of the sensor decreases, which lead to activate an alarm to give an alert to the user. This project is suitable in the application of providing security system for different types of area or place  like lockers, cash boxes,  malls, shops. The circuit of this project is placed inside of the cash box in malls or inside of the lockers  in such a way that, when a burglar opens the cash box or locker and uses a torch light to search the valuables. When the light falls on the circuit which includes an electronic eye and gives a command to the ripple counter. This triggers the alarm and shows a burglary attempt. A lamp is also used to indicate the theft when light falls on the sensor. And  this project can be  interfaced to send an SMS to the user in case of burglary

LDR Based light Intensity Control for Street Lights

In the system, generally the lighting up of highways is done through High-intensity discharge lamps. Because, the energy consumption of these lamps is high. This project uses an LEDs to overcome the drawbacks of High-intensity discharge lamps. This project demonstrates the usage of light emitting diodes as a light source. These lights consumes low power and its life is more as compared to High-intensity discharge lamps.A light depending resistor is used to detect the light. The resistance of the LDR drastically reduces according to the daylight. A bunch of LEDs are used to make a street light. The microcontroller comprises programmable instructions that controls the light intensity based on the Pulse width modulation signals generated. The light intensity is kept high during the peak hours, and as the traffic on the highways tend to decrease in late nights: and the light intensity also decreases till morning. Finally, the street lights completely shut down at morning and continues again at evening 6pm and this project  developed by connecting it with a solar panel, which converts the intensity of the solar into corresponding voltage, and this energy is used to feed the street lights on highways.

Dusk to Dawn Lighting Switch or   Lighting Switch from Sunset to Sunrise

This sunset to sunrise lighting switch or Dusk to Dawn Lighting Switch is designed to control the light illuminated on the LDR sensor. The resistance of the LDR sensor changes with the change in intensity of light falling on LDR. This sensor output is given to IC 555 timer connected in bistable mode. The output of the IC 555 timer is used to control the prompting of load through a TRIAC. Hence, this circuit switches on the load in the sunset and switches off the load in the sun rise automatically.
The sunset to sunrise lighting switch or Dusk to Dawn Lighting Switch project is used where the light will continue on only from sunrise to sunset (dusk to dawn). Nowadays, the electricity cost is very high and it is advisable to use electricity carefully. The applications of this project mainly include where more electricity is required in the night time like highways, complexes, colleges, parks, and industries. The proposed system gives a solution for these applications and automatically the lights will be on at sunset and turn off at sunrise. So, conserving money, energy, human interference is also excluded.

Time Programmed Sun Tracking Solar Panel

Time Programmed Sun Tracking Solar Panel project uses an 8051 family microcontroller and the motor is motivated by an interfacing an integrated circuit as the microcontroller is not capable of controlling the requirements of the power of the motor. Furthermore, this project can be developed by using a real-time clock to track the sun. This helps in keeping the necessary position of the solar panel even if the power is disturbed for some time
Time Programmed Sun Tracking Solar Panel project uses a solar panel which is fixed to a stepper motor to track the sun so that full sunlight is made incident upon the solar panel at any time of the day. This is best compared to the light detecting method that may not always be correct. The solar panel is used to convert solar energy to electrical energy is very famous, but due to the moving of the sun from east to west, the solar panel may not be able to produce ideal energy. To overcome this problem, this project is implemented to track the sun using a solar panel. The movement of the solar panel is done by connecting a stepper motor so that the panel keeps its face always vertical to the sun to produce maximum energy. This is attained by using a pre-programmed microcontroller to send stepped pulses in periodical time breaks for 12 hours for the motor to switch the mounted panel. Here solar panel rotates in one direction & then comes back to the start point.

Light Dependent Resistor Projects List 

The list of LDR project ideas for engineering students is listed below.

• Solar Lights Circuit in Outdoor Garden
• Solar Tracker using LDR Automatically
• LDR based Light Detector
• Op-amp & LDR based Light Detector
• LDR& Transistors based Light Detector
• LDR based Darkness Detecting
• Smart Bulb Holder using LDR
• Controlling of Street Light using LDR
• LDR based Lamp Circuit 
• Battery Protector of Emergency Lamp
• Automatic Street Light Circuit using LDR
• LDR based Pocket Synth
• IoT testing of Mbed using LDR
• LED Lighting in Aquarium using LDR
• LDR based pot
• Laser Piano using LDR
• Light Switch Circuit for Automatic Parking using LDR
• Gate Lamp Circuit using LDR
• MicroSynth using LDR
• LDR based Garage Light
• LDR based Sensor Stick
• Appliance Switch Controlled by Hand Gesture using LDR
• LDR based Monitoring of Environment
• Alarm Circuit for Refrigerator using LDR
• Emergency Mini LED Light Circuit
• Starburst using LDR
• Flashing LED using LDR
• Switch Circuit of Pole Light using LDR
• Desk Lamp for PC using LDR
• LDR based Voltage Stabilizer
• Siren Circuit Activated by Light
• Switch Activated through Light using LDR
• Charger circuit using LDR
• LDR based Automatic Lawn Light
• Alarm Circuit for Light using LDR
• LED Lights Circuit for Christmas using LDR
• Luxmeter Design using LDR
• Light Sensor Circuit using LDR & Arduino
• LED Fading with Arduino Uno & LDR
• LED Brightness Control Circuit with LDR & Passive Components
• LDR Sensor with Arduino Uno
• Visitor Counter using LDR
• LDR based Automatic Lamp in Night
• LDR & Transistor-based Light Sensor & Darkness Detector Circuit
• LDR based Current Variation
• LDR & Relays based Street Light Controlling
• CD4027 & LDR based Wireless Switch Circuit
• Security System Controlled by an Electronic Eye using LDR
• Digital Object Counter using LDR
• Fence Circuit for Light through Alarm
• LDR based Smart Electronic Candle
• Arduino Mood Light Controlled by Smart Phone through Alarm
• Real-Time Oscilloscope based on LDR & Arduino
• Power LED Auto Intensity Control with LDR & Arduino
• Alarm Circuit for Laser Security using LDR
• LDR &AVR Microcontroller based Measurement of Light Intensity
• Indicator Circuit for Dark & Light using LDR
• 555 Timer & LDR based Detection of Dark
• Staircase Light using LDR Automatically
• Dark Detector using LDR and 555 Timer IC
• An Efficient & Intelligent Light Control System Design
• Power Failure & Fuse Indicator using LDR
• White LEDs & LDR based Twi-light
• Electronics Counter using LDR
• Door Guard using LDR
• LDR & Thermistor based Fire Alarm
• Twilight Switch based Flashing Light
• Interruption Counter using IR & LDR
• LDR based Controlling of Dual Motor in Robots
• LDR based Sunset Lamp
• Sun Tracking Solar Panel Using Arduino
• Lighting Device Circuit with Simple Keyhole
• Color Mixing Lamp with Arduino, LDR & RGB LED
• LDR based Arduino Light Sensor
• Emergency Light using Raspberry Pi with Darkness & Detecting of AC Power Line Off