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Light Detector using LDR

A Light Detector or a Light Sensor is a device or circuit that detects the intensity of the light incident on it. Different types of light detectors are LDRs (or Light Dependent Resistors), Photo Diodes, Photo Transistors, etc.

All these devices are called as Photoelectric Devices as they convert light energy to electric energy. These Light Detectors or Sensors can detect different types of light like visible light, ultraviolet light, infrared light etc.

In this project, we have designed a simple Light Detector using LDR. When the light falls on the LDR, the light stays off and when the light stops falling on LDR, the LED glows. We have designed two types of circuits: one using an OP – AMP (Operational Amplifier) and other using just transistors.

Light Detector using LDR Image 4
Light Detector using LDR Image 1
Light Detector using LDR Image 3

Light Detector using LDR and OP – Amp

Circuit Diagram

Light Detector CircuitComponents Required

  • LM358 OP – Amp IC
  • Small LDR
  • 10 KΩ Resistor
  • 10 KΩ Potentiometer
  • White LED
  • 220 Ω Resistor
  • Connecting wires
  • Power supply (9V Battery)

Working of the Circuit

The main components of the project are LM358 Op – Amp and LDR. First let us see about LDR (Light Dependent Resistor). An LDR, as the name suggests, is a type of resistor, whose resistance changes depending on the intensity of the light surrounding it.

Typically, when light is incident directly on the LDR, its resistance will be very low and when there is no light i.e. in darker conditions, its resistance jumps to few mega Ohms.

We will use this feature of the LDR is our project to detect light and turn on an LED. For this we have used an Operational Amplifier. The Op – Amp is configured in Comparator mode i.e. it will compare the voltages at inverting and non – inverting terminals and correspondingly generate a HIGH or LOW output.

We have connected the wiper terminal of the 10 KΩ Potentiometer to the inverting terminal of the OP – Amp. To the non – inverting terminal, we have connected the junction of a 10 KΩ Resistor and the LDR. These two will form a potential divider feeding its output to the OP – Amp.

When the light is incident on the LDR, its resistance decreases. Hence, the non – inverting voltage is lower than the inverting voltage and as a result, the LED remains OFF.

When there is no light falling on the LDR, the resistance of the LDR becomes very high and as a result, the voltage at the non – inverting terminal will be more than the voltage at inverting terminal. The output of the Op – Amp will be high and the LED is turned ON. 

Construction and Output Video

 

Light Detector using LDR and Transistors

Circuit DiagramLight Detector using Transistors CircuitComponents Required

  • 2 x BC547 NPN Transistors
  • Small LDR
  • 1 KΩ Resistor
  • 10 KΩ Potentiometer
  • White LED
  • 330 Ω Resistor
  • Connecting wires
  • Power supply (9V Battery)

Working of the Circuit

In the previous circuit, we have seen a simple Light Detector using LDR and an Op – Amp. If you don’t have an Op – Amp, then the above circuit will be helpful. It uses just two transistor to perform the light detection operation.

In this circuit, we have used two transistor in the form of Darlington Pair. Even though a single transistor would be suffice, Darlington Pair ensures that there is enough current at the output.

The construction of the circuit is very simple. A 10 KΩ Potentiometer and the LDR form the biasing resistors of the first Transistor. When there is light incident on the LDR, its resistance will become less.

Hence, there will not be any biasing voltage to the first transistor. As a result, it will not be turned. As there is no output from the first transistor, the second transistor will not be turned on and hence, the LED stays OFF.

When the light incident on the LDR is removed i.e. under darker conditions, the resistance of the LDR will become very high. Hence, the voltage from the POT will take a less resistance path i.e. it turns ON the first transistor.

This will in turn ON the second transistor as its input is connected to the output of the first transistor. As a result, the LED, which is connected to the output of the second transistor will be turned ON.

Applications

  • Simple Light Detector Circuit can be used in many applications like automatic switching of appliances up on detecting light, security systems, etc.
  • This circuit can be used in alarm system, where increasing intensity of light will trigger the alarm.
  • It can also be used in cupboards or wardrobes. When the door is opened, the light will automatically turned on. 

 

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