How to make LDR Darkness Sensor Circuit Simple DIY

How to make LDR Darkness Sensor Circuit Simple DIY

Hello Friends

In this post we will see how to use LDR to make a Darkness sensor circuit.

Basically a darkness sensor circuit turn on a light in darkness and turn off light in brightness.



Circuit drawing



Component Required

1) LDR-----1No.

2) 50KOHM RESISTOR---------1No.

3) 1KOHM RESISTOR---------1No.

4) BC547 TRANSISTOR---------1No.

5) 9V BATTERY---------1No.



What is LDR ?


LDR is a photoresistor is a light-controlled variable resistor.

The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity.

A photoresistor can be applied in light-sensitive detector circuits, and light- and dark-activated switching circuits.



A photoresistor is made of a high resistance semiconductor.

In the dark, a photoresistor can have a resistance as high as several megohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms.



BC547 Transistor

BC547 is an NPN bi-polar junction transistor. A transistor, stands for transfer of resistance, is commonly used to amplify current. A small current at its base controls a larger current at collector & emitter terminals.
BC547 is mainly used for amplification and switching purposes. It has a maximum current gain of 800. Its equivalent transistors are BC548 and BC549.



​Follow the step as shown in the Video

50K ohm resistor one end is connected with Positive (9V) of battery and other end is connected in series with LDR and LDR another point is connected with negative of battery

here in video I have used 2 100Kohm resistor in parallel because i dont have 50K one.

1K OHM resistor one end is connected with Positive (9V) of battery and other end is connected with anode (positive) of LED 

Cathode (negative) of LED is Connected with C (collectr) of BC 547 Transistor

Emitter (E) of transistor is connected with the negative of battery

Base (B) is connected Junction point of the 50K resistor & LDR

it is a simple and powerful concept , which uses transistor ( BC 547 NPN) as a switch to switch ON and OFF the LED LIGHT automatically .
It automatically switches ON lights when the light goes below the visible region of our eyes. ( e.g in evening after Sunset ). it automatically switches OFF lights when light fall on it ( e.g in morning ) , by using a sensor called LDR (Light Dependent Resistor) which senses the light just like our eyes.


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2 Responses

  1. Hi Sandeep thanks for this project.
    I prepared the complete circuit but it functioning in reverse order.LED turns off in the dark & gets on in light.

    Help me out please if possible share me any other contact info so that I can send u a pic of my circuit.

    Ajay Kumar

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