Darkness Detector using LDR

Darkness Detector or Dark Detector is a circuit that detects darkness or absence of light. In this project, we have implemented a simple Darkness Detector Circuit using the simplest of all light sensors: the LDR (Light Dependent Resistor).

Darkness Detector circuits like this can be used in applications where we can automatically turn on lights when it becomes dark.

In addition to the LDR, we have also used the good old 555 Timer IC in Astable Mode to generate the required square wave. There are some passive components like capacitor and resistors. We have used a Piezo Buzzer as an alarm to indicate darkness.

The aim of this simple project is to detect darkness with the help of LDR and activate the buzzer.

Darkness Detector using LDR Image 1
Darkness Detector using LDR Image 2
Darkness Detector using LDR Image 3
Darkness Detector using LDR Image 4
Darkness Detector using LDR Image 5

NOTE: As this is a simple circuit, we haven’t implemented any automatic light on systems but just a buzzer.

Circuit Diagram of Darkness Detector

The following image shows the simple circuit diagram for Darkness Detector project implemented using LDR and 555 Timer IC.

Darkness Detector using LDR Circuit Diagram

Components Required

To implement this project, we need the following components.

  • 1 x 555 Timer IC
  • 1 x LDR
  • 1 x Piezo Buzzer
  • 1 x 10 KΩ Resistor (1/4 Watt)
  • 1 x 2.2 KΩ Resistor (1/4 Watt)
  • 1 x 1 MΩ Resistor (1/4 Watt)
  • 1 x 1 µF Electrolytic Capacitor (50V)
  • 1 x 0.1 nF Ceramic Capacitor (Also called 100 pF with code 101)
  • 1 x 9V Battery
  • 1 x Mini Breadboard
  • Jumper Wires

Component Description

IC 555: 555 Timer is an 8 – pin DIP IC and it is one of the commonly used Timer ICs for different applications like timing, pulse generation, OP – Amps, etc.

The 555 Timer IC is used in this project in its Astable Multivibrator Mode of operation (with a slight modification). The following is the pin diagram of the 555 Timer IC.

555 Timer Pin Diagram
LDR (Light Dependent Resistor): LDR or Light Dependent Resistor is one of the commonly used light sensors. In this project, we are using an LDR to detect darkness i.e. when the intensity of light decreases.

Circuit Design

We will design the circuit with respect to 555 Timer IC. As mentioned earlier, the 555 IC has 8 pins. Connect the pins 8 and 1 to 9V supply and GND respectively. Connect a 2.2 KΩ Resistor between the 9V supply and pin 7 of 555 IC. Now, connect a 10 KΩ Resistor between pins 7 and 6.

Pins 6 and 2 are shorted and a capacitor of capacitance 1 µF is connected between pins 2 and GND. Here, the positive lead of the capacitor is connected to pin 2 of 555 and the negative lead is connected to GND.

Connect a bypass capacitor of 0.1 nF (100pF) between pins 5 and GND. A piezo buzzer is connected between the output pin 3 and GND.

We have connected the buzzer directly to the output pin of the 555 Timer IC as it is a small buzzer. If you are not sure whether the 555 can drive the buzzer or not, you can use a transistor to drive the buzzer.

In case you are using a transistor (like 2N2222 or BC547), connect the output pin of 555 to the base of the transistor through a current limiting resistor. Connect one end of buzzer to supply and other end to the collector terminal of the transistor.

The emitter terminal of the transistor must be connected to GND. The following diagram shows this connection.

Buzzer with TransistorWorking

A simple dark detector or darkness detector is designed in this project. The project is implemented using very simple components like 555 and LDR (few passive components as well). The working of the project is explained here.

First, we will start with the 555 Timer. It is configured in Astable mode but the RESET pin is controlled by the LDR and Resistor network. When there is ample light around the LDR, its resistance becomes very low.

In our lab setup, it came down to around 2 KΩ. In this condition, the voltage divider formed by the 1 MΩ resistor and the LDR will produce almost 0V at its output. As this is given to the RESET pin of the 555 timer IC, the 555 Timer IC is Reset. As a result, you won’t get any output at the output pin.

When we block the LDR with an obstacle or hand, the light falling on it will decrease. The resistance of the LDR will increase and in our case (lab setup with studio lighting) the resistance increased to around 120 KΩ. This will pull up the reset pin and the Astable Mode will be activated.

Since we connected a small buzzer to the output pin of the 555 timer IC, the buzzer will be activated. Hence, when there is enough light on the LDR, the buzzer will be off and when it is dark, the buzzer is activated.

NOTE: In place of 1 MΩ Resistor, you can actually connect a 1 MΩ Potentiometer so that you can adjust the level of light that the LDR will detect.


  • It is a very basic darkness detector with very simple hardware components and circuit.
  • There is no need for any complex microcontroller circuit or programming to implement this project.


  • Since the system is not controlled by any microcontroller, the results might not be as accurate as expected.


  • This project can be implemented in applications like automatic switching on of lights when it becomes dark.
  • This circuit can be part of a bigger circuit or project like home automation or home security system.

4 Responses

  1. Thank you for the circuit. I have a project due in 4 days and i’m using your circuit to create a doorbell for my dorm door. I am using a 90db (12V) piezo buzzer and roughly all the other components from your circuit. I want the buzzer’s sound to be stronger. Would it injure the circuit if I connected a 12V or 14V battery instead of a 9V one? If it does, do you know any small modifications that could help me?

    Thank you.

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