Mini Alarm Circuit Schematic

This circuit, housed in a compact plastic enclosure, is designed for portability and can easily fit into 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 sudden bag theft, the magnet loses contact with the reed switch, causing SW1 to open. This action initiates oscillation in the circuit, resulting in the loudspeaker emitting a piercing alarm sound. A complementary transistor pair is configured as a high-efficiency oscillator that directly powers a small loudspeaker. The low component count and 3V battery supply facilitate a very compact design. If utilized as an anti-theft device for bags, SW1 can be substituted with a 3.5mm mono jack socket, while the magnet can be replaced with a 3.5mm mono jack plug that has its internal leads shorted. This jack plug will connect to the thin cord.

The circuit operates on the principle of magnetic contact interruption. The reed switch (SW1) is a crucial component that detects the presence of the magnet. When the magnet is in proximity, the reed switch remains closed, allowing the circuit to remain inactive. Upon a sudden movement, such as a bag being snatched, the magnet is pulled away from the reed switch, causing it to open. This action triggers the oscillator circuit, which consists of a complementary transistor pair (typically NPN and PNP transistors configured in a feedback loop) that generates a square wave signal.

The generated oscillation drives a small loudspeaker, producing a loud alarm sound intended to deter theft and attract attention. The use of a 3V battery allows for a lightweight and compact design, making the device easily portable. The low part-count not only simplifies the assembly but also enhances reliability due to fewer potential points of failure.

For enhanced versatility, the option to replace the reed switch with a 3.5mm mono jack socket allows for different configurations. The shorted jack plug acts as a substitute magnet, maintaining the functionality of the device while providing flexibility in its application. This design is particularly suitable for personal security, providing a discreet yet effective alarm system for bags and handbags.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 osc

illating 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. If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3. 5mm mono Jack socket and the magnet by a 3. 5mm. Mono Jack plugs having its internal leads shorted. The Jack plug will be connected to the tiny cord etc. 🔗 External reference