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Automatic Night Lamp Using LDR

LDR darkness sensor circuit

dark sensor circuit

 This Automatic light operated light switch circuit is Based on LDR. LDR is a Light Dependent Resistor. It is also called Photoresistor.


Introduction of LDR

LDR (Photoresistor) is a type of variable resistor which Increases its Resistance when the Light is Decreases and its Resistance Decreases When Light is Increases. It means in a dark place or when no light falls on LDR, the resistance of its is very HIGH  and in Sufficient light, the resistance of LDR is very LOW. 

This resistor works on the principle of photoconductivity. These devices depend on the light. The most obvious application for an LDR is to automatically turn on a light at a certain light level. 

Construction And Working Of Circuit

Construct the circuit as shown in fig. Two NPN transistors are used to switching operations. The LDR is connected between the supply voltage and Base terminal of transistor Q2. The Emitter terminal of this transistor is connected with the ground through a variable resistor or potentiometer (POT).  This variable resistor is used to adjust the sensitivity of the circuit.

LDR circuit diagram

ldr circuit diagram

At Day When  Light falls on LDR then conductivity increased and current flows between supply to the base of the transistor and then transistor Q1 will on. Transistor Q1 And Transistor Q2 are connected as NOT Gate. When transistor Q1 is on by conducting even small current on base, The output of transistor Q2 is LOW (OFF). 

At night when there is no light falls on LDR, then high resistance will create and there are no current flows on the base of transistor Q2.  As there is no supply comes on the base of transistor Q1, this is in OFF state, When this transistor has LOW input then the output of Transistor Q2 will HIGH   When transistor Q2 is in HIGH mode then is energized (turned on) the relay connected with its collector and Electric Lamp will be Switched ON. 

Assemble the circuit at any general PCB board and set the adjustment of pot according to required sensitivity. Fix the LDR out of the cabinet for detecting the light.


LDR (medium size)-1








Relay 6v -1




Also read

What is DIAC 

Remote controlled  switch 

About the Author


Hi, My name is Aman Bharti, I am interested in making and study of Electronics, circuit diagram, PCB designing and layout etc. I like to share the knowledge and all ideas with people which I get from My Experiment & from different sources. I try to provide any circuit details deeply with test results as possible. If you want to give a suggestion or comments on anything, please leave your comment in the comment box of the related page.

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