Emergency Alarm

If you feel threatened or need an urgent support in a crowd, use this alarm to catch the attention of others. It generates a loud siren simulating that of the Fire brigade which is enough to get support from others. A single press on the switch can activate the siren for 2 minutes and then it automatically stops. It is too small enough to keep in the pocket or hand bag. This circuit is quite compact and is powered by a button cell. The battery life is reasonable too and hence, it can be used for longer times without any power hassles.

Two ICs are used in the circuit. IC1 is the CMOS version of 555 which works at low voltage as low as 3 volts. It is wired as a short duration monostable timer and with the timing components R1 and C1, IC1 gives 2 minutes high output when the trigger pin2 is grounded via push switch. The high output from IC1 is used to drive IC2.

IC2 is the siren generator used in Toys and Alarm systems. Its maximum power rating is 3.1 volts so a Zener regulated 3 volts is provided to the IC. UM 3561 is aROM IC with an internal oscillator and the frequency of oscillation is determined by the 220K resistor connected to its pins 7 and 8.By changing the pin connections, this IC can produce different sirens like Police siren, Ambulance siren, Gun sound, Fire brigade siren etc. Since the output tone from IC is too feeble a single transistor amplifier based on AC 187 is used to give a loud alarm.

Use two 3 V lithium cells and a Mylarspeaker (one used in Toys) to make the gadget handy. Enclose the circuit in a matchbox sized case or Keychain typecase. A small button sized press switchwith lock can be included as the On/Off switch. Push switch should be a small press switch without lock.

Since, this alarm goes OFF after a regular interval of time it can also be used as time-alert bell. The speaker used is not that powerful and hence, the alarm can be fairly restricted to personal usage too.