How to make LDR circuit | mini project

Hi, in this article you will learn to make your own LDR circuit / Dark sensor. The word LDR stands for Light Dependent Resistor. It is also known as a photo resistor. Its resistance decreases as the light intensity increases. Basically we are going to make a circuit that automatically switches on whenever it senses darkness ( no light particles ). The speciality of my design is –
  1. Easy to make
  2. Safe for beginners

Components required

The components required to make LDR circuit are listed below –
  1. 12 v 500mA transformer or any 12v dc power supply with 0.1 A to 0.5 A current.
  2. LDR ( i.e Light Dependent Resistor )
  3. In 4007 diode ( 1Ampere diode )
  4. 12v Relay
  5. Bc547 NPN transistor
  6. 2200 microfarad 25v capacitor
  7. 33k ohm 0.25w resistor
  8. Vero board
ldr circuit diagram | freak engineer

LDR Circuit

Follow these steps to make your connections –
  1. Connect the 12 v transformer to bridge rectifier ( made using In4007 diode ) as shown in the circuit.
  2. Add a 2200 microfarad 25 v capacitor to bridge rectifier output.
  3. Now connect 33k resistor in series with LDR and then connect its resistor side to positive supply and LDR side to negative supply.
  4. Take bc547 transistor and connect its base pin in between 33k resistor and LDR as shown in circuit.
  5. Add a diode in series with collector pin of transistor and connect it to positive supply.
  6. Connect the emitter pin to negative supply.
  7. Now take a 12 v relay and connect it to circuit as shown.


One wire is similar for both input and output of the circuit and the other wire has to go through relay that will automatically turn on the circuit in dark.

Simple explanation

When light falls on LDR sensor its resistance gets to minimum and it completes the circuit. In this condition no current flows through base of transistor. Whenever the sensor detects darkness its resistance becomes very high. In this condition current flows through base pin that switches on the relay and any load on the output gets powered.

Applications of LDR

  1. Automatic street light.
  • LDR sensor can automatically switch on and off streets lights according to darkness in the surrounding.
  1. Automatic brightness control
  • LDR sensor can be used in devices like mobile, television and computers for automatically changing brightness according to your surrounding.
  1. Automatic home lights on power failure.
  • It can automatically turn on lights or inverter at your home in case of power failure.
  1. Used in security system with LASER.
  • LDR with LASER can be applied to door / windows or can make a secure area bounded by LASER that falls on sensor.
  • When any body suppose human try to cross it. LASER path gets broken and in this case no light will fall on LDR sensor. This can trigger alarm.

Frequently Asked Questions

  1. What is LDR ?
  • The term LDR stands for Light Dependent Resistor. It is a light sensitive device that is mostly used to detect the absence or presence of light in a particular area.
  • In the absence of light its resistance becomes very high ( up to 1Mega Ohm ). In bright areas its resistance becomes zero.
  1. Can I operate this circuit on 12 v battery ?
  • Just replace the power supply part ( that is transformer, diodes or SMPS ) with battery.
  1. Can I use 6V relay ?
  • Yes you can but in this case power supply should also have 6v output. You also need to change the value of resistor.
  1. How can I protect this circuit from Over voltage and current when operating on 230v Ac ?
  1. Can I make this circuit as a project in Polytechnique / Diploma / B. tech ?
  • Yes this circuit can be a good choice for projects in diploma / Polytechnique / B. tech.
  • 9v ldr circuit


Akash Sharma

If you have and any other question. Do comment. For better clarity watch my YouTube video Discover more projects –
  1. 9v ldr circuit
  2. Tesla coil project
  3. LiFi project | Data transmission using light
Visit YouTube
Share This Post

Leave a Comment