automatic light switch circuit schematic

220V Automatic Light Switch Circuit

This light sensitive automatic light switch circuit is intended to be connected at the main 220V supply. The circuit will connect a 220V lamp at the nightfall and disconnects it in the morning light. The switching is made without a relay in order to avoid problems with the electric arc and noise caused by inductance coil and contacts.

The automatic light switch is powered from the 220V mains through R10, C4, D3, D2 and C3. A reference voltage source, D1, feeds with 8.2 V the circuit for measuring light, R2-P1. At lower light intensity, LDR’s resistance, R2, increases and therefore the P1 voltage falls, so will fall the gate-source voltage of FET’s T1.

When the switch S1 is closed, R3-C2 time constant makes that the gate voltage of T1 to vary slower then R2 resistance. This is necessary to prevent the circuit reaction at rapid changes of ambient light intensity.

Sensitive Automatic Light Switch Circuit Schematic

automatic light switch circuit schematic

T1, T2, R4, R5, R6 and R8 forms a trigger Schmitt. Normally, T1 is open and T2 is blocked. When the gate voltage of the FET goes below a certain level, T2 starts to conduct and so does T2 which will provides the necessary gate current to boost the triac Tri1 in order to connect the load (220V lamp).

Caution! Because there are many points that are connected at 220V it is essential to apply a good insolation. Do not work on the circuit when is connected to the mains.

Components list
R1 = 2.2K
R2 = LDR
R3 = 150K
R4 = 15k
R5 = 10k
R6 = 27k
R7 = 560Ω
R8 = 1.2k
R9 = 1.2M
R10 = 470Ω
R11 = 100Ω
C1 = 4.7µF/16V tantalum
C2 = 47µF/16V
C3 = 1000µF/16V
C4 = 470nF/250V~(630V)
C5 = 100nF/630V
D1 = 8.2V zener
D2 = 1N4001
D3 = 15V/1W zener
D4 = LED
T1 = BS250
T2 = BC557B
T3 = BC547B
Tri1 = TIC226M
F1 = 5A


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