In this project, a variable resistor is used to set the sensitivity of the LDR since LDR the resistance of LDR changes in response to the ambient light. The resistance decreases as the intensity of incident light increases and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light.
An IC 555 timer is also used. When the light falls on LDR (Light Dependent Resistor), its resistance becomes very low and as a result IC1 doesn’t work. So the voltage at PIN2 becomes zero. Due to this there will be no output at PIN3 as a result the lamp remains switched OFF.
When the LDR is not lighted, its resistance becomes high. As a result voltage across LDR becomes high enough to make IC1 work. Due to this PIN2 of IC1 receives a positive pulse or a high output that could be enough to activate the relay. So PIN3 receives high voltage and drives the lamp in ON state.