It is very convenient to light a lamp in our absense in the eving when it gets dark. This automatic night lamp circuit can be used to light staircase light, porch light etc automatically using domestic powersupply. It is very in expensive in construction and we dont have to employ or depend anybody to put on the lights when we are out of station. We can connect a CFL in place of filament lamp by making necessary alterations to the circuit.
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An SCR and LDR together plays the role of automation in this circuit. In figure, the resistance of LDR is low during daytime and high during night. So the required trigger pulse is developed across the SCR when it becomes dark and it is applied acrosss the gate cathod terminals and SCR is triggered and the circuit is closed.