Darkness Detector / Light Sensor Using LDR

In this tutorial, I will show you how to make a darkness detector sensor that can detect darkness using the cheapest light sensor which is LDR (Light Dependent Resistor). Darkness detector sensor circuits can be used in applications where the light will be going off suddenly.

We have also used the 555 timer IC in astable mode to generate a square wave pulse. There are some passive components that we also used i.e. capacitor and a resistor. We have used a piezo electronic buzzer as an alarm to indicate the darkness.

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darkness detector using 555 timer

Circuit Diagram

darkness detector circuit

Components Required

  • 555 Timer Ic
  • LDR (Light Dependent Resistor)
  • Piezo Buzzer
  • 10KΩ Resistor
  • 2.2KΩ Resistor
  • 1MΩ Resistor
  • 1uF/25V Electrolytic Capacitor
  • 9V Battery
  • Breadboard
  • Jumper Wires

Circuit Connection 

The 555 IC has 8 pins. Connect pins 8 and 1 directly to the 9V power supply and ground respectively. A 2.2KΩ resistor is connected between the 9V power supply and pin 7 of 555 timer ic. Now, connect a 10KΩ resistor between pins 7 and 6. 

Pin 6 and 2 are shorted with each other and a capacitor of 1uF/25V is connected between pin 2 and GND. Here, the positive terminal of the capacitor is connected to pin 2 of the 555 ic and the negative terminal is connected to the ground. 

A piezo buzzer is connected with the output pin 3 and another negative terminal is connected directly to the ground. 

The buzzer is directly connected to the output pin of the 555 timer IC.

Working Principle of Darkness Detector

First, we will start with the 555 timers. It is configured in astable mode. When there is a light falling into the LDR, its resistance becomes very low.

In this condition, the voltage divider will produce almost 0V from its output formed by a 1MΩ resistor. In this situation, the 555 timer IC will be reset. As a result, you won’t get any output from the output pin.

When we block the LDR with an obstacle, the light falling on it will decrease. Then the resistance of the LDR will increase. This will activate the reset pin and the astable mode will activate.

Since we connected a small buzzer to the output pin of the 555 timer ic and 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 on.

Application of Darkness Detector

This darkness detector sensor project can be used in applications like automated switching of ON-OFF light when it becomes dark.

This circuit can be a part of a home automation or security system.

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