If you feel unsafe or need help right away, just set off this panic alarm. It will get the attention of people who can help right away. After three minutes, the alarm will stop going off. It is small enough to fit in a pocket or a handbag and is especially helpful for women who are traveling alone.

The panic alarm circuit is simple and can be put together with easy-to-find parts on any general-purpose PCB. Basically, it is a transistorized timer that uses the ability of a capacitor to charge and drain to set the time delay. Panic alarm circuit & operation When switch S1 is briefly pressed, capacitor C1 fills up to full battery voltage and gives transistor T1 base current. When the voltage at T1's base goes up, the transistor begins to work. C2 gets power from T1, which is an emitter follower. T2 conducts when the voltage across C2 goes up. This lowers the voltage at the collector of T2 to zero, which makes the pnp transistor T3 work. The Zener diode ZD1 makes sure that the voltage going to IC1 is always 3.3 volts. The breakdown voltage of ZD1 is kept by capacitor C3. Limiters of current are R1, R3, and R4. The siren generator is IC1 (UM3561), which has an oscillator built in. When transistor T3 gives IC1 power, it oscillates with the help of resistor R6 (220 k). Transistor T4 then amplifies siren tone pulses at pin 3 to make an alarm sound. As capacitor C1 slowly empties, the base current of transistor T1 drops until it stops conducting. But T2 still uses the charge from C2 to do its work. When the charge on C2 goes down, T2 stops, making the base of T3 positive. T3 immediately turns off, taking the power away from IC1. This takes about three minutes, and the alarm will sound during that time. Make the circuit as small as possible on a matrix board or general-purpose PCB and put it in a cabinet. The circuit can run for a long time on a 9V battery. To make the unit small, use a small Mylar speaker.