Antitheft alarm system for large area displays - uses detector to sense variations in resistance of wire connected to all protected objects

摘要

The antitheft system for a large area display exhibition uses a cable connected to each of the objects to be protected. It also uses a detector which detects the variation in electrical resistance of the cable. The system also has an indicator controlle by the cable resistance variation detector. The cable used is a steel cable with a high linear resistance. The cable consisting of multistrands of steel wire. The detector has a resistance bridge and an amplifier followed by a step device. The detector has two parallel stages for detection of positive and negative variations. These stages are connected at their output by an OR gate whose output is connected to the indicator. The indicating uses a thyristor for maintaining alarm indication even after disappearance of the alarm cause.