Locker Guard or Locker Security Alarm

This tiny circuit can protect your Locker or Almirah from a burglary. If the locker opens in the armed state, the circuit generates a loud police siren to thwart the attempt of burglary. The siren stops only when the locker door closes again. The circuit is too compact and can be powered by 3 volt Lithium cell.

Working of the circuit

This Locker Security Alarm circuit uses the Siren generator IC UM3561 to generate the police siren. UM 3561 is the ROM IC with an internal oscillator to generate 4 sirens like police siren, ambulance siren, fire brigade siren and gun sound.

Fig. 1: Image of IC UM3561

The frequency of oscillation and hence the tone depends on the resistor R2 connected its pins 7 and 8. A 220K resistor is the ideal one for the good tone. The selection of siren is based on the pin connection of IC. IC UM3561 works off two pen cells or 3 volt lithium cell. It is the widely used tone generator in toys.

A Normally Closed (NC) Reed switch is used to control the working of IC1. The NC reed switch remains closed and opens when a magnet is close to it. When the door of the locker is closed, magnet and reed switch remains very close and the reed switch opens. So IC1 will not get power and the circuit remains stand by. When the door opens, reed switch closes and IC1 gets 3 volts through the reed switch and it starts oscillations. The tone from the output of IC1 is amplified by T1 and is heard through the speaker. Mylar speaker is used in the circuit to make the unit compact. It is the common Mini speaker used in toys.

Fix the bar magnet (a speaker magnet can do the job) on the door and the reed switch on the door frame of the locker so as to open the reed switch when the door closes. The unit can be placed inside the locker .Switch S2 is used to power off the circuit.

Components

Resistors

R1 470 ohms, R2 220K, R3 100 ohms

Capacitor

C1 100 uF 25 V

Integrated circuit

IC1 UM3561

Transistor

T1 BC548

Switches

S1 Reed Switch Normally Closed, S2 Toggle switch

Mylar speaker, 3volt lithium cell, magnet.