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LIGHT BULB TIMER

2015-03-02 18:18  
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LIGHT BULB TIMER
Occasionally you might have a need to keep a light on for a certain time, usually a few minutes, and be sure that it switches off even if you forget to turn off the switch. This could be useful in a cellar or in a closet. The circuit will switch on a light bulb simply by pressing the push button. After a time of 3-7 minutes it will switch off automatically. The long delay is achieved by partly using the leakage current between anode and gate of the scr. This current is dependent almost on anything: voltage, temperature, lamp power, scr device, etc., this is the reason why the timing is not constant but for the intended application it is not important. If the delay is too short you may increase the 220nF capacitor up to 470nF. Too high a value will keep the light always on. It will work with incandescent light bulbs only. Its operation with electronic lamps is erratic and the delay is only 1 or 2 minutes. The scr must be the sensitive gate type and no other type was tested except the TIC106N. The circuit is rather small and could be housed in the same case as the push button, if there is enough room. Of course you have to substitute the standard switch with a push button. Operation with a 110Vac mains has not been tested although I expect that a 100μF 100V capacitor instead of the 47μF capacitor should do the trick. It goes without saying that you must know what you are doing as working with the household mains can be dangerous and remember to switch off the mains breaker before doing any work on the electric wiring. Do not attempt to install this circuit if you have doubts on its operation, connections and relevant safety measures.