Protect your Locker or Briefcase from theft using this pocket sized gadget. It gives a loud police siren to catch attention.
The circuit uses a single IC and a few discrete components. It works on the principle of light detection by LDR. When the unit is in dark, LDR cease to conduct and the alarm generator does not get power supply and it remains idle. When some one tries open the door of the locker, C1 charges via LDR. When the voltage in C1 raises to 3.1 volts, Zener conducts and gives power to IC UM 3561. It is a ROM IC that can generate different tones based on its pin connections. Here a police siren is selected by keeping pin6 floating. R1 determines the oscillation of IC. 220K (R1) is a must to give correct tone.C1 provides a lag of two minutes, so that the user gets sufficient time to disarm the unit.