LAGHT_ACTIVATED_SWITCH

Published:2009/7/15 22:28:00 Author:Jessie | From:SeekIC

When even a little light hits light-dependent resistor R5, transistor Q1 is turned off because the base has less resistance to the ground than to the positive rail. In that situation, the base is at a negative potential. When the sun sets, R5 is no longer illuminated, giving the transistor's base a high resistance to ground-higher than 100,000Ω. With less resistance to positive potential, the base is biased, turning on Q1. Relay RY1 is then energized and pulls in, connecting the SCR1's anode to positive potential. The 555 timer, IC1, powers up, and its output goes high to approximately 10.67V, which is sufficient to energize RY2. Relay RY2 then pulls in, keeping the ac bulb on the whole night, and turning it off again at sunrise.



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