| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a magnetic tape-detecting device that can detect not only the presence or absence of a magnetic tape being used for preventing a book and merchandise from being illegally taken out, but also the number of the magnetic tapes. SOLUTION: A detection coil 4 is provided at the rear side of first and second plates 1a and 1b being provided while being inclined for composing a reference plate 1, and a bias coil 5 is further provided at the rear side of the detection coil 4. A bias circuit 8 supplies an AC current with a frequency of 300 Hz to the bias coil 5. By the AC current, the bias coil 5 generates an AC magnetic field of 5-6 gausses. When a magnetic tape 2 is subjected to the AC magnetic field, the magnetic tape 2 generates a pulse magnetic field of 600 Hz that is the half cycle of the AC magnetic field being energized by magnetic characteristics of the magnetic tape 2. The pulse magnetic field of 600 Hz is detected by the detection coil 4 with an energized AC magnetic field of 300 Hz, and a pulse magnetic field signal is selectively detected by a detection circuit 7 for outputting.</p> |
