| 摘要 |
At least one interference coil (19, 21, 24, 25) is provided inside the shop (7), but outside the monitoring zone (11-13) after the check-out till (8-10), in which a check is made for the presence of an alarm device (22, 23). Each coil transmits on a frequency close to the resonance frequency of the alarm device and the coil frequency is chosen so that it causes the alarm device to transmit a return signal substantially different from the return signal normally sent out by the alarm device in response to the magnetic field transmitted in the monitoring zone. An alarm device is used to receive an alternating magnetic field sent out by a transmitter and to transmit a return magnetic field signal, without the need for any energy input. The alarm device has a circuit with a coil, capacitance diode and resistor arranged in series in a closed circuit, and a capacitor connected in parallel via the resistor. The coil and diode form a resonance circuit with a resonance effect at the magnetic field frequency when the voltage at the diode is zero volts. The alarm device returns a periodically varying magnetic field. A receiver is also provided in the system. |
