If you feel threatened or need emergency assistance, simply activate this alarm. It will catch the attention of others for immediate help. The alarm will sound for three minutes and then cease. Especially useful for women travelling alone, it is small enough to pocket or carry in a handbag.
The circuit is simple and can be assembled on any general-purpose PCB with readily available components. Basically, it is a transistorised timer using the charging/discharging property of a capacitor to give the time delay.
When switch S1 is pressed momentarily, capacitor C1 charges to full battery voltage and provides base current to transistor T1. When voltage at the base of T1 rises, the transistor starts conducting. T1 is an emitter follower and provides charge to C2.
When the voltage across C2 rises, T2 conducts. This reduces the collector voltage of T2 to zero to make pnp transistor T3 conduct. Zener diode ZD1 regulates this supply voltage to IC1 as 3.3 volts. Capacitor C3 maintains the breakdown voltage of ZD1. Resistors R1, R3 and R4 are current limiters. IC1 (UM3561) is the siren generator with an internal oscillator. When IC1 gets power supply from transistor T3, it oscillates using R6 (220-kilo-ohm), and siren tone pulses available at its pin 3 are then amplified by transistor T4 to sound an alarm.
As capacitor C1 discharges slowly, the base current of T1 drops and it stops conducting. But T2 still conducts using the charge available from C2. When the charge from C2 drops, T2 cuts off making the base of T3 positive and it turns off immediately, removing the power from IC1. This process takes about three minutes, during which the alarm will be heard.
Assemble the circuit on a matrix board/general-purpose PCB as compact as possible and enclose in a cabinet. A 9V battery can power the circuit for a long period. Use a small Mylar speaker to make the unit compact.