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Dark Sensor Circuit

Dark/Light Activated Circuit using LDR

The dark sensor circuit is very useful to Automatic Switch ON and OFF any appliance by Detection of Light and Dark. LDR is a photoresistor that is used for the detection of light. When Light intensity high on LDR, The Resistance through it Decrease and When Light intensity is Low on LDR, then the resistance through it will  Increase and Go very High. This Property is very useful in the operation of Dark Detector circuits. LDR is Light-dependent Resistor. LDR is a special type of Variable resistor which value can be increased or decreased according to light falling on it. As its specified property LDR act as the light and Dark sensor. When Light falls on LDR the Resistance of it goes very high about in MEGA OHM.


Here is Simple Dark sensor Circuit which can be used as automatic Switch On And Switch Off Any Load Using Relay. Here I am presenting simple circuit using an only resistor, LDR and Transistor but Many Types of circuits can be constructed using LDR for Dark detected Switching. This Simple and best Dark Activated circuit used One NPN transistor To On Off according to light sensed by the LDR. You can use Variable Resister at the place of 50K for the adjustment of sensitivity.

When Light Fall on LDR then the internal resistance of LDR will minimum and all voltage come through R1 dropped with the ground and become 0V, then no sufficient power at the base of transistor get to turn on and light is switched Off. At Dark Resistance will be Very High and very less power dropped with the ground because of LDR holds by high resistance and transistor gate terminal get sufficient voltage to turn ON and the Light is switched ON

At Day-When Light intensity High the connected load remains to Switch off. At Night- when the light intensity is negligible, then the light is Automatic Switched ON. 



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Schematic diagram 1

dark detector circuit

If you want to connect any high load or AC Then you need to use Relay, Below the circuit diagram shows the Relay connection with the Dark Activated automatic Light switch. Use DC supply 0f 9-12v for the circuit operation or use any adaptor or can be used homemade AC to DC converter for circuit input. If BC547 get Hot And not able to handle the relay then you can use 2N222A transistor Or SL100. But If you use Small size Pcb Relay then BC547 can handle easily. These circuits are very useful in the automatic street light.

Schematic diagram 2

light activated automatic light switch circuit


Transistor Bc547 or 2N222A-1

Resistor 50k-1 or Preset 100k-1, 1k-1

Relay-1 (9v) 6A


Diode 1N4007-1

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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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