This is a simple and efficient circuit of a light sensor switch or light activated relay switch circuit using a 555 timer IC. The circuit can activate any AC or DC device connected with the relay when light falls on the LDR.
It is using only five components due to which it can be easily built

in less then 30 minutes if all parts are available with you. An LED can also be connected between the pin3 and ground with a 470 ohms current limiting resistor to indicate the activation of relay, make sure the LED and LDR should be in opposite position and not see each other or the circuit will not work properly.

The circuit is using an LDR (light dependent resistor) or photoresistor to sense the light. When light falls on the LDR its resistance starts decreasing due to which the output of the NE555 at pin3 goes high and activates the relay switch. The sensitivity of the circuit can be adjusted with the 15K variable resistor.   

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Light Activated Relay Switch Using 555