9v ldr circuit | mini project for ece

This article is a tutorial for darkness sensor using LDR ( 9v ldr circuit ). The word LDR stands for ( Light Dependent Resistor ), this sensor changes its resistance according to the intensity of light falling on it. When light falls on LDR its resistance decreases and in darkness it has very high resistance.

We will use this function of LDR to make a darkness sensor circuit which will automatically turn on light in darkness and switch it off when there is sufficient light.

Working of 9v ldr circuit

Light Dependent resistor is made up of high resistance semiconductor. When light falls on LDR the photons in light gets absorbed by the semiconductor, this gives energy to the bounded electrons to jump to the conduction band.

Due to which the free electrons in semiconductor make it conductive and its resistance get reduced.

When there is light falling on sensor its resistance is few ohms and in darkness it is counted in mega ohms.

The change in resistance of LDR will get detected by the circuit and the load ( LED ) will switch automatically.

When LDR has very less resistance, the current flows through first loop in the circuit that contains battery, 47 KΩ resistor and sensor. The second loop is open.

When LDR has very high resistance, the current heads towards base of transistor and switch it on. In this case, the current flows from collector to emitter that activates second loop in the circuit that contains load ( LED ).

Components Required

The components required are –

  1. LDR ( Light Dependent Resistor )
  2. 9 V battery
  3. Green LED
  4. BC 547 NPN transistor
  5. Resistor – 1k Ω, 47k Ω
  6. Battery connector
  7. Connecting wires
  8. Breadboard or Vero board

Circuit Diagram

ldr circuit diagram


Follow these steps and make connections step by step

  1. First of all, make a series connection of 47k Ω resistor with LDR.
  2. Connect positive wire of battery connector to 47k Ω resistor and the negative wire to LDR as shown.
  3. For second loop, connect 1k Ω resistor to positive terminal. Connect the other side of resistor to positive terminal of the LED and the negative terminal of LED to collector pin of transistor.
  4. Connect the emitter pin of transistor to negative supply.
  5. The base pin of transistor is connected in between the series connection of LDR and 47k Ω resistor ( in the first loop)
  6. The circuit is now complete. Connect battery in the battery connector and circuit will start working.


Application of the LDR circuit are –

  1. Automatic lights / Street light
  • It can automatically turn on lights at your home / street in case of power failure.
  1. Used with LASER in security alarms
  • LDR with LASER can be applied to doors / windows that will make a secure area bounded by LASER light that falls on LDR sensor. When any matter suppose human try to cross it. LASER path gets broken and in this case no light will fall on LDR. This will trigger alarm circuit.
  1. Automatic brightness control in electrical / electronic devices.
  • LDR sensor can be used in devices like television, mobile and computers for automatically adjusting brightness according to the surrounding light.

Frequently Asked Questions

  1. My LDR circuit is not working ?

  • Sometime room light may not be sufficient. Use flashlight on the sensor or make a proper shadow in reverse case.
  1. Can I make this project as mini project in school / collage / Polytechnique / diploma / b tech ( mini project for ece ) ?

  • This project is 100 % working and one of the most famous project among collage students because of its cool applications.
  • mostly used as mini project for ece branch.


Akash Sharma

Discover more projects –

  1. 220v LDR circuit
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