Simple disconnection burglar alarm

The micro-power disconnection type anti-theft alarm introduced in this article is very simple, reliable and low-cost. Its circuit principle is shown in the attached figure. Usually, a thin wire is wrapped around valuables or laid on the path that thieves must pass, and the gate and source of the VMOS field effect tube VT (used as a high-power electronic switch here) are short-circuited. VT is in the cut-off state. At this time, the high-loud speaker B with built-in sound source has no power and does not work. The alarm is in a micro-power "waiting" state at this time. The working current of the entire circuit is only 2μA. When a thief breaks the hair-thin enameled wire wrapped around the protected object, VT will obtain gate bias and saturate conduction. At this time, the internal circuit of the high-loud speaker B is powered and works, thereby continuously emitting loud sirens (at this time, only when the broken enameled wire is reconnected, the alarm sound will stop), making the thief panic and flee in a hurry.

In the figure, VT can use VMOS field effect tubes with small size and high power such as 2SK701 and 2SK1748. B uses a high-volume speaker with built-in sound source, and its working voltage can be 6-12V. The thin wire can be removed from some waste power transformers, and its length depends on actual needs. When welding enameled wire, the insulating paint on its surface should be scraped off with a knife. Then use a soldering iron to weld. Battery GB can use four AA dry batteries (should be equipped with a battery box). Since the static power consumption of this alarm is very small, the power switch can be set without setting it. If it is not used for a long time, the battery can be removed.

Since the entire circuit is very simple, all components can be soldered on the back of the internal circuit board of the high-loud speaker B. The alarm and its power supply are preferably installed in a hidden place that is not easily damaged by thieves. When using it, just wrap the enameled wire around valuables or lay it on the path that thieves must pass.