Schematic Diagram

Description               Circuit showing a Burglar Alarm.Here we have used a ldr and a switching transistor for making this circuit
.When the light coming towards the ldr during the period the ldr have low resistance so the buzzer will on.When the light going away the ldr during the period the ldr have high resistance so the transistor will off.Here you need a 12 volt power supply

 Components Required      Resistor

                   10 k(preset)

      Transistor
                   BC 548

       LDR
        Buzzer

Source by : http://www.electronics-circuits.in/2012/03/burglar-alarm-ldr-bc-548.html