Anti-theft detector for stores - has two inductive loops operated in anti-phase to produce dead zone whose position is varied by de-tuning

摘要

The anti-shoplifting alarm for stores detects magnetic markers and consists of two oscillatory circuits, one either side of a gangway or exit, which comprise open coils (2, 2') and parallel-connected capacitors (4, 4'). The resonant frequency of the circuits is in the region of 10 kH. The circuits are energised via switches (6, 6') operated by means of a control unit (8). The switches are operated at 2, 5 kH. Since the coils are excited in opposing phase, the lines of force produce a pattern with a dead zone in the centre line between the coils. In each circuit are additional determining elements (10) which can be switched in and out of circuit so that the blind spot between the coils is continuously shifted.