Light sensor circuit using LDR and 555 timer IC with Adjustable sensitivity

In this tutorial we'll learn how to make a light sensor circuit using LDR (Light Dependent Resistor), 555 timer IC and a few other electronics components. This circuit detects light incident on the LDR and turns on LED whenever the intensity of light is greater than a certain level.

The LED can be replaced by any electronic device like buzzers, relays. DC motors etc. We will learn more about that later in this tutorial. We will also learn how this circuit works and other cool stuff that can be done with this circuit. So let's get started.

A video tutorial is also included at the beginning of this article for easy understanding.

Components Required

  • 555 timer IC
  • Light Dependant Resistor (LDR)
  • Resistors (2*10K, 330R)
  • 100K Potentiometer
  • Light Emitting Diode (LED)
  • Breadboard & Hookup wires
  • Battery / Power Supply (5-12)V

Circuit Diagram

Adjustable Light Sensor/Detector Circuit Diagram

Instructions to Build This Circuit

Step 1: Place 555 Timer IC on the Breadboard. I have placed the IC such that its notch faces left. So the pin-numbering starts from bottom left (Reference: Pin-Outs of 555 Timer)

Step 2: Connect Pin-8 of the IC to Positive Rail & Pin-1 of the IC to Negative-Rail. (We used the holes in top-second row as Positive-Rail and the holes in bottom-second row as Negative-Rail)

Step 3: Connect Pin-2 & Pin-6 of the IC

Step 4: Place a 10K Resistor on breadoard such that one of its terminals is connected to Pin-2 of 555 timer IC. Connect other terminal of the resistor to Negative-Rail

Step 5: Place a 10K Resistor on breadoard such that one of its terminals is connected to Pin-6 of 555 timer IC. Connect other terminal of the resistor to Positive-Rail

Step 6: Now Place an LDR (Light Dependent Resistor) on breadboard such that one of its terminals is connected to Pin-4 of 555 Timer IC. Connect its other terminal to Positive-Rail

Step 7: Place a 4.7K Resistor on the breadboard such that one of its terminals is connected to Pin-4 of 555 Timer IC

Step 8: Place a 100K Potentiometer such that one of its extereme terminals is connected to the other terminal of 4.7K Resistor

Step 9: Now connect the center terminal of 100K Potentiometer to Negative-Rail

Step 10: Place an LED on the breadboard and connect its Anode to Pin-3 of 555 Timer IC. Connect a 330R Resistor between Cathode of the LED and Negative-Rail. (For 5/6V of power supply, use a 220R; for 9V, use 330R; and for 12V, use 470R)

Step 11: Connect the Power Supply and the Circuit is Now Ready!

How this Circuit Works

First things first: The resistance of LDR (Light Dependant Resistor) is inversely proportional to the intensity of light falling on it. It implies that if the intensity of incident light is high, the resistance of LDR will be less and vice versa.

On the other hand, a 555 timer IC gets activated when its reset pin (Pin-4) receives a voltage greater than 0.8V. Once the IC is activated, the voltage at Pins-2,6 need to be between 1/3rd and 2/3rd of the supply voltage, for the output to be ON. For example if the voltage at the reset pin is above 0.8V and the voltage at Pins-2,6 is half the supply voltage, the output turns ON.

In the circuit, we created a voltage divider using LDR and a resistor + potentiometer. It is then connected to Pin-4 (reset) of 555 timer IC. So when it's dark, the LDR's resistance increases and so the voltage at the voltage divider drops below 0.8V, causing the 555 timer IC to turn OFF. When there is enough light, the voltage at the reset pin goes above 0.8V and the IC turns ON.

The two 10K resistors are used to divide the supply voltage by half and provide it at the Pins-2,6. This makes sure that the IC's output turns ON whenever it is activated.

Further Improvements:

We used this circuit just to turn on an LED. But in real life scenarios, you might want this circuit to turn on AC appliances or high power motors etc. In order to achieve that you can directly add a relay at the output and connect AC loads to the relay.

You can also configure the 555 timer to produce an audible frequency and use this as a Light Alarm by connecting a speaker at the output.


  • Used in automatic street light circuits
  • Used to automatically turn on wardrobe or locker lights whenever they are opened
  • In Burglar alarm systems that operate on visible light