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Light sensitive switch circuit

Light Activated Switch Without Relay

Light/Dark Activated Switch circuit without Relay

The Working Principle of this Automatic Bulb Switch same as My previous Project Automatic Transformerless Light Switch.  But Here Used A Triac At the place of Relay. In the circuit, There is no used Relay for switching. 

This is Automatic Light Controlled Bulb Switch With LDR Using Without Relay 

When Light fall on LDR in DAY then the Resistance of LDR goes LOW And at this situation the maximum voltage Dropped With Negative supply (Ground) by cross through LDR and Voltage at the base of transistor Q1 is Very LOW.  When the Voltage goes to very LOW (Below0.7v) at BASE of Q1 then Transistor not get ON and no Power flows between COLLECTOR and EMITTER. Gate Voltage of TRIAC is LOW.  Now TRIAC Remain OFF Because It not TRIGGERED.

At Night Time when no light incident on LDR then the internal Resistance of LDR comes HIGH and Very less amount of Power cross through LDR to ground, then the Voltage At base Terminal of a transistor becomes increase (above 0.7v) and now transistor turns ON and current starts flow between Emitter and COLLECTOR. Therefore the Voltage increasing at GATE of TRIAC BT136. As voltage increase at GATE then TRIAC gets Triggered.  After Triac triggered it Turns on and Passes the Power Supply between terminal 2 to terminal 1 and Bulb Switched On.

You can use 2 LDR for More Sensitivity.

Circuit Diagram

Light sensitive switch circuit


Here is a Modified Complete circuit diagram of Automatic Light Operated bulb switch. In this circuit, An Optocoupler IC MOC3021 is used. This Optocoupler isolates the High AC voltage from the whole circuit. And then this circuit is Isolated from High AC current. Only Triac and some portion of the circuit flows AC supply. Now, this is more Protected and Less dangerous than the first circuit.

This is automatic Light operated Bulb Switch circuit using LDR, Optocoupler, and TRIAC. 

Light sensitive switch circuit




Optocoupler MOC3021BT136 TRIAC Pin Diagram and Symbol

BC 547 transistor. Transistor BC547 Pin configuration Diagram

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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