| 摘要 |
The circuit detects small changes in field, sensing attempts to force door before it is fully opened. The alarm comprises a sensor consisting of a magnet fixed to the door or window, and an analogue detector capable of sensing a very slight movement of the door and hence magnet (of the order of 1 - 10 mm). The sensor comprises either an electric coil, producing a current related to the displacement of the magnet, or a Hall effect sensor detecting change in the local magnetic field. The electronic processing circuit is compatible with either type of sensor, and comprises a variable gain amplifier to adjust the sensitivity of the device, a low pass filter (below 20Hz) to avoid triggering by sudden shocks, and a threshold detector. These are linked to the conventional elements of an alarm circuit.
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