Switch Circuit Diagram - Dark-activated switch needs only three components
	When you light the photocell, its voltage drop is lower than the triggering level of the threshold voltage, and Q1 is locked By Abel Raynus, Armitron International, Malden, MA, USA
Assume that you have a device that receives its power from the main 120 or 220V-ac line and you need to add a switch between the ac line and the device so that the device works only when it is dark. Although you may think this task would be trivial, it is difficult to find a workable approach.