Light sensitive switch circuit

Light Activated Switch Without Relay

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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

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2 thoughts on “Light Activated Switch Without Relay”

  1. i would like to ask for help, my optocoupler had only 4 legs and 1 pair is cathode anode for the diode part and other is Emitter and collector.
    Collector acts as the output while the other 3 as input. i would like to ask for sample circuits i can make use to ensure my optocoupler is working right

    1. Your Optocoupler may be PC817 or MCT2E or 4N35. These optocouplers has internally light-activated transistors and these do not use for controlling direct AC. You Must use MOC3021/3022 or other this type of optocouplers. This optocoupler has an internal optoTriac and two pins of these can directly be connected to control the AC.

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