Introduction: 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.
Step 1: VIDEO
Watch the video to know full procedure
Step 2: Component Required
1) LDR—–1No. http://amzn.to/2rNr33i
2) 50KOHM RESISTOR———1No.
3) 1KOHM RESISTOR———1No.
4) BC547 TRANSISTOR———1No. http://amzn.to/2rNxpjm
5) 9V BATTERY———1No.
Step 3: Circuit Diagram
Step 4: 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.
Step 5:
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.
Step 6: 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. data sheet
Step 7: Procedure
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.

Participated in the
Explore Science Contest 2017
3 People Made This Project!
- safadi63 made it!
- JudeJeevan made it!
- Gilian ReinalieC made it!
31 Comments
Question 5 months ago on Step 1
What if we have to increase the no. of led
Like if I add 2-3 more led .Do I have increase voltage of battery accordingly?
Please help
Question 1 year ago
If I wanted to use a solar panel to charge the battery, what voltage solar panel should I use?
Question 3 years ago on Step 7
How can I apply this to a grid support solar installation. To switch battery charging from solar panels source to grid source when sunlight is low and battery voltage down to low limits.
Answer 2 years ago
if you had a relay with the coil resistance of 1000 ohms use it in place of the resistor and the common to battery with normally closed to grid charger and normally open to
solar panels
Reply 1 year ago
what does circuit look like
Question 3 years ago
Hello, thanks for your information. Just one question, there is any possibility to ADD one more Led at your circuit? but just for stay ON when there is light, when it darks then goes OFF and the other LED turns ON. Thanks
Answer 1 year ago
You can, but it will have to be using another photoresistor. You also must use another transistor.
2 years ago
What is the forward voltage and mA of the LED?
Question 3 years ago
there is no 50k reistor
Question 5 years ago on Step 4
Photoresistor's resistance get lowered when light of adequate freguency strikes on it.But in this circuit photoresistor is in reverse function.Why and how?
Answer 3 years ago
The direction you connect your resistor will determine if it will detect light or darkness
ie connecting a resistor before or after the LDR will give different results
Answer 4 years ago
This job is done by the transistor.
Question 3 years ago
sir in replace of two 50kohm resistor i used 100kohm resistor it didnot work
Question 3 years ago on Introduction
Sir it didnt Work.When it dark led is on And if we give a light led Off.Give Me what is the my mistake
Answer 3 years ago
The circuit as drawn is a darkness sensor. Swap the position of the 50kohm resistor and LDR and it should light the led when it detects light.
Question 4 years ago
I want to connect a buzzer to the circuit such that when the LED glows the buzzer makes a sound.Where should I connect the buzzer in this circuit?
Answer 3 years ago
I want LDR bout
Answer 3 years ago
Listen , led as well as buzzer has both positive and negative pins , so in order to make them turn on at the same time just connect positive leg of the buzzer to where the positive leg of led is connected and do same with the negative leg also. You will get it working .
4 years ago
can i have the block diagram
4 years ago
In the components required please mention LED,Breadboard and connecting wires so that people can use it as a list while buying the components.