A very simple but interesting magnetic reed switch alarm circuit can be constructed using this circuit diagram . This magnetic reed switch alarm can be used for home or handbags. If it is installed for home it can be placed on door or windows, and if it is installed for bags or handbags it will placed in the bag.
This circuit consists of a small magnet and a reed switch. If the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound.
So this circuit is very simple , requiring few common components , thing that make it suitable for an low cost anti-theft alarm .

This circuit uses a complementary transistor-pair which is wired as a high efficiency oscillator, directly driving a small loudspeaker with an 8 ohms impedance . Low part-count and 3V battery supply enable a very compact construction.
When the circuit is in stand-by mode ( alarm is not active ) current drawing is less than 20µA , and when the alarm is active , the current consumption is about 100mA.
For this magnetic alarm circuit you will need a 3 volts power supply ( you can use a 3 volts button cell battery or two 1.5 volts button cells ) .

Do not use a power supply that exceeds 4.5 volts .