Automatic Dusk to Dawn (Evening on to Morning Off)


Uploaded on April 14, 2012 by Efxkits

Model No: 203 This project is based on LDR for automatic lights flicker due to the change in light intensity at dawn and dusk. Hence the compact fluorescent lamps are unsuitable in such circuits as it may damage the electronic circuits within these lamps. The circuit described here can solve the problem and switch ON the lamp instantly when the light intensity decreases below a preset level. The circuit uses a timer IC as a Schmitt trigger to give the Bistable action.

The set and reset functions of the comparators within the IC are used to give the instantaneous action. The inputs of both the threshold comparator and the trigger comparator are tied together and connected to the voltage divider formed by LDR and potentiometer. The voltage across LDR depends on the light intensity. In day light, LDR has low resistance and the input voltage to the threshold comparator goes above and its output becomes zero, which resets the internal flip-flop of IC. Thus lamp remains ‘OFF’ during the day time.

At sunset, the resistance of LDR increases, and the voltage at the input of the threshold comparator decreases below and that of the trigger comparator goes below. As a result, the outputs of the trigger and threshold comparators go high, which sets the flip-flop. This changes output of IC from low to high. Hence, LED glows to indicate the high output state of IC and the bulb glows. For more details visit or

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