Here is a light-operated light switch circuit built with solid state components that is used to operate a lamp. During darkness the resistance of LDR is in the MΩ range and so the triac does not conduct. When the LDR is illuminated the resistance of the LDR will decrease and the triac will conduct and switched ON the lamp.
The LDR must be placed so that is receiving light from the lamp too because this is the way that the lamps remains ON. Once the lamp is ON it can be switched OFF by interrupting the light falling on LDR. The RFC can be made by winding about 15 turns of 18 SWG wire over an insulated ferrite rod.
Schematic of Light Operated Switch Circuit
Article received from Dakshesh, India. Thanks!