mini alarm

This circuit, housed in a compact plastic enclosure, can be conveniently stored in a bag or handbag. A small magnet is positioned near a reed switch and is attached to the individual carrying the bag via a thin cord. In the event of a sudden theft, the magnet detaches from the reed switch, causing SW1 to open. This action initiates an oscillation within the circuit, resulting in the loudspeaker producing a piercing alarm sound. A complementary transistor pair is configured as a high-efficiency oscillator, which drives a small loudspeaker directly. The low component count and a 3V battery supply facilitate a very compact design.

The described circuit functions as a personal security alarm system designed to deter theft of bags or handbags. The reed switch (SW1) is a crucial component that remains closed when the magnet is in proximity, allowing the circuit to remain in a dormant state. The magnet's attachment to the user ensures that any sudden movement or theft results in the magnet moving away from the reed switch, triggering the alarm.

The oscillator circuit is built using a complementary transistor pair, typically comprising an NPN and a PNP transistor. This configuration is advantageous as it allows for efficient switching and amplification of the oscillating signal. When the reed switch opens, the transistors begin to oscillate, generating a square wave signal that is sent directly to the loudspeaker. The speaker is designed to produce a high-decibel output, ensuring that the alarm is audible in various environments.

Powering the circuit with a 3V battery not only ensures a low power consumption but also contributes to the compactness of the overall design. The low part count indicates a simplified assembly process, making it accessible for hobbyists and engineers alike. The use of a plastic enclosure protects the internal components from physical damage and environmental factors, while also making the device lightweight and portable.

In summary, this compact alarm circuit serves as an effective deterrent against theft, utilizing a magnet-reed switch mechanism and a high-efficiency oscillator to produce a loud alarm sound in the event of unauthorized removal of the bag. Its design emphasizes portability, simplicity, and effectiveness, making it an ideal solution for personal security.This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound.

A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply allow a very compact construction.. 🔗 External reference