Posted on Aug 31, 2012

This circuit has stable threshold characteristics from its dependence on the-photo diode current in the Ll4Cl to generate a base emitter voltage drop across the sensitivity setting resistor. The double phase shift network supplying voltage to the ST-4 trigger insures triac triggering at line voltage phase angles small enough to minimize RFI problems with a lamp load. This eliminates the need for a large, expensive inductor, contains the dV/dt snubber network, and utilizes lower voltage capacitors than the snubber of rfi suppression network normally used.

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The addition of a programmable unijunction timer can modify this circuit to tum the lamp on for a fixed time interval each time its environment gets dark. Only the additions to the previous circuit are shown in the interest of simplicity. When power is applied to the lamp, the 2N6028 timer starts. Upon completion of the time interval, the HllC3 is triggered and turns off the lamp by preventing the ST-4 from triggering the triac. The SCR of the HllC3 will stay on until the Ll4Cl is illuminated and allows the 2N6076 to commutate it off. Because of capacitor leakage currents. temperature variations and component tolerances, the time delay may vary considerably from nominal values.

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