| 摘要 |
A vehicle security system comprises first and second conductive loops (A and B) for attachment to a vehicle (C) in substantially parallel relationship with a gap (D) between them. Each loop is connected to an associated first amplifier (1, 2) the outputs from which are connected to a differential amplifier (3) which is arranged to sum the output signals received from the first amplifiers. The output from the differential amplifier (3) is connected to a threshold detector (4) which sets a latch (5) to store an alarm condition when a moving magnetic field is detected. The alarm condition signal is interrogated remotely by, for example, an infrared remote control link (8, 11) to activate an alarm (7). As a moving magnet field is passed across the gap between the two loops (A, B) a voltage is induced in each loop and the first amplifiers (1, 2) detect the induced voltage and the differential amplifier (3) determines a phase shift in the two voltages. This phase shift indicates that a moving magnetic field has passed over the gap between the two loops (1, 2), and indicates the possibility of a magnetic bomb device having been attached to the vehicle. |
