"Automatic" Light Bulb Changer

Posted on Aug 6, 2012

The circuit presented here guarantees that if bulb Lai `gives up the ghost,` bulb La2 will take over its tas

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k. In series with Lai is triac Tri2. Resistor R3 and C2 form a delay network. As soon as the voltage across C2 rises above about 30 V, diac (gateless triac) D2 is switched on, which causes Tri2 to conduct so that Lai lights. The control circuit of La2 is parallel to that of Lai, but because R2/C1 has twice the delay of R3/C2, Tril will not be triggered when Tri2 conducts; CI discharges so that Tril cannot be triggered. When, however, Lai is open-circuited, a voltage is across both RC networks via La2 and Rl. Again, Tri2 will be triggered first, but because the current is smaller than its holding current, it will cease to conduct almost immediately. Capacitor CI will then continue to charge and after a little while Tril is switched on. Because the time constant for La2 is somewhat longer than that for Lai, La2 will always be slightly less bright than Lai. It is, of course, possible to give La2 a slightly higher wattage than Lai to ensure equal brightness. Without heatsinks, the triacs can handle up to 100 W each; with heatsinks, powers of up to 1000 W can be accommodated. It is not recommended to use bulbs with a wattage below 25 W, because these can flicker. The triacs can be any type that can handle at least 400 V at no less than 5 A. The M types used in the prototype can handle 600 V at 5A.

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