The Working Principle of this Automatic Bulb Switch same as My previous Project Automatic Transformerless Light Switch. But Here Used A Triac At 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 Voltagr 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 transistor become increase (above 0.7v) and now transistor turn ON and current starts flow between Emitter and COLLECTOR. Therefor the Voltage increasing at GATE of TRIAC BT136. As voltage increase at GATE then TRIAC get Triggered . After Triac triggerd it Turn on and Passes the Power Supply between terminal 2 to terminal 1 and Bulb Switched On.
You can use 2 LDR for More Sensitivity.
Here is a Modified Complete circuit diagram of Automatic Light Operated bulb switch. In this circuit An Optocoupler IC MOC3021 is used . This Optocoupler isolate 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 first circuit.
This is automatic Light operated Bulb Switch circuit using LDR, Optocoupler and TRIAC.