Film reel containing metal cylinder case opening detecting device for metal case monitoring system, has oscillator circuit coupled to metal case, and detection circuit connected to output of oscillator circuit to trigger alarm

摘要

The device has an oscillator circuit (11) coupled to a metal cylinder case (12) to form a resonant oscillator cavity (1) delivering a resonant signal on an output of the oscillator circuit. A detection circuit constituted of a band-pass filter (2) and a power detector (3) is connected to the output of the oscillator circuit to trigger an alarm when resonant frequency of the resonant signal is outside of predetermined frequency range and/or the power level of the resonant signal with the resonant frequency is less than or equal to a predetermined threshold value. An independent claim is also included for a metal case monitoring system comprising a wireless transmission circuit communicated with a central server.