Protect the Home Locker with this simple Alarm. When somebody opens the Locker, it sounds an alarm till the locker is closed again.

Idea of the circuit is simple. When the circuit is placed inside a locked locker, darkness in  its inside increases the resistance of LDR, So, Zener diode will not get its breakdown voltage and remains off. The buzzer driver Transistor BC 547 also will be off. When the door of locker opens, light enters and LDR gives current flow to charge the 100 uF capacitor, When its voltage increases above 5 volts, Zener conducts to switch on the transistor. Buzzer then sounds .

A natural doubt will arise. What will happen if the locker is opened in the night? Its answer is simple. User needs room light to use the locker. A Burglar uses a torch to search for valuables. In both cases, LDR gets light and alarm sounds. So it works day and night.