Follow the Steps to successfully build your own Light Sensor. Very useful in automatic lighting system.

Components:

7805 Regulator IC
SL100 transistor
LED (preferably red)
150ohm Resistor
9V supply
Relay(6V)
LDR (commonly available one)
Connecting wires
Breadboard
BUZZER(optional)

Step 1: Connecting Power Supply to Sensor

Hook up 7805 regulator to breadboard. Supply 5v to whole breadboard using its output. Connect the middle terminal to the negative rail of breadboard.

Step 2: RELAY

Connect the relay across the ridge of the breadboard such that three terminals of it come to one side and another two to the other. Keep a small distance between the 7805 regulator and relay.

Step 3: Connecting Relay.

Connect the one of the two coil terminals of the relay to ground. Connect the common terminal of relay to +5v.

Step 4: Connecting Indicator LED

Connect a 150ohm resistor to the Normally connected (NC) terminal of the relay. Connect the LED anode to the resistor and cathode to ground(-ve) in breadboard.

Step 5: Connecting Transistor As Switch

Mount SL100 transistor to bread board. Connect its Collector to 9v supply and its emitter to the unconnected coil terminal of the relay. The NO terminal of relay is kept open. You can connect a buzzer to it.

Step 6: Connecting LDR to Our Circuit

Connect the LDR (it has no polarity) between the base of SL100 and 5v supply. And that's it. Bring it out and focus sunlight upon it. The video shows its working outdoors.