The above circuit is a light sensitive automatic switch which functions in the absence of light. We are using IC 4060 in this circuit which works as an oscillator and the generated signal is applied to the base of T4 transistor.Phototransistor T3 is in conduction by existence of light and it keeps the T5 transistor’s base at ground potential.
When no light reaches the phototransistor, it gets in insulation. T3 reaches positive potential through 22kohm. T4 and T5 both gets in conductance. The oscillator signal reaches the optic coupler and lamp(s) shines.
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