Many of us travel alone today and sometime we fell threatened or require an emergency assistance. So in that case you have to just press the switch S1 and this will catch the attention of other people because its sound is loud enough. Advantage of this circuit-
This safety alarm circuit is very helpful for the women who are travelling alone. The advantage of this circuit is its size. Its size is very small as you can easily carry this in your purse or pocket and whenever you fell danger just press the switch. This circuit is very simple and you can assemble it on general purpose PCB with readily available components. Alarm will sound for some time and become off. Can be operated with 3- 6V power supply and consume no power in standby mode.
Fig. 1: Prototype Of Safety Alarm Circuit On Breadboard
This circuit is based on UM3561 IC. When you press the switch S1, capacitor C1 connected to start charging and with few seconds it charge equal to supply voltage. And this will provide base current to the transistor T1. This voltage causes the transistor T1 to start conducting which in turn charges the capacitor C2 and transistor T2 connected to it start conducting. And collector voltage of transistor T2 becomes zero. Because of that transistor T3 which is an Npn transistor stats conduction (because emitter voltage is more in comparison to base voltage). Now pin 5 of IC1 gets the voltage and start sounding which can be heard through speaker.
We have used a zener diode at pin 5 of IC1 because excess voltage more than 3.3V will destroy the IC and capacitor C3 is used to maintain the breakdown voltage of zener diode. If battery is remove after pressing the switch then also alarm will sound for 2- 3 minutes because capacitor C1 take some time to discharge completely and it will continue to provide supply to transistor T1 which will be in conduction mode. Similarly T2 also conduct because of C2 and when C2 discharge completely then alarm will become off as this will remove the power supply from pin 5 of IC1.