Automatic Lights On/Off with LDR

Hello, 

wouldn't it be cool if the lights could be automatically turned on when it is dark and turned off when it is bright?

The circuit is made with very few components and works nicely, responding to the light intensity on the LDR sensor, also known as Light Dependent Resistor.

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There is no coding required as there is no microcontroller in the circuit.

Things you'll need:

1. Breadboard

2. LED (red, green or blue)
3. NPN Transistor (BC547)
4. LDR sensor
5. Resistance (100k ohm)
6. Battery 9V & battery cap
7. Jumper cables

Working Principle of LDR:

Light Dependent Resistor

A Light Dependent Resistor (LDR) or a photo resistor is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light sensitive devices. They are also called as photo conductors, photo conductive cells or simple photo cells. They are made up of semiconductor materials having high resistance.

A light dependent resistor works on the principle of photo conductivity. Photo conductivity is an optical phenomenon in which the materials conductivity (hence resistivity) reduces 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 band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. 

Hence when light having enough energy is incident on the device more & more electrons are excited to the conduction band which results in large number of charge carriers. The result of this process is more and more current starts flowing and hence it is said that the resistance of the device has decreased. This is the most common working principle of LDR.

Step 1: Making the Circuit

Circuit Diagram of the project

1. Connect the BC547 transistor on the breadboard carefully noting the emitter, base and the collector terminals.

2. Connect the negative terminal of the battery to the emitter terminal of the transistor and to the one end of the LDR sensor.

3. Connect the other end of the LDR sensor to the base terminal of the transistor.

4. Connect one end of the 100k ohm resistance to the base terminal of the transistor and the other end of the 100k ohm resistance to the positive of the LED (the longer leg).

5. Connect the negative of the LED (the shorter leg) to the collector terminal of the transistor.

6. Connect the positive terminal of the battery to the positive of the LED.

Step 2: The Dark Sensor LED

Now, turn off the lights or simply cover the LDR sensor with your hand such that it is dark on the sensor surface. You should now see the LED glowing. Turn the lights back on or remove your hand from the sensor, you should now see the LED turned off.

Repeat this while slowly moving your hand towards and away from the LDR sensor, you shall notice that the brightness of the LED is increased or decreased in proportion to the darkness that is around the LDR sensor.

You may also read : Wireless LED Light Operated With Smartphone

Stay hungry! Stay foolish!