GB1428685A - Method and apparatus for actuating an electric circuit - Google Patents

Abstract

1428685 Anti-theft alarms T F THOMPSON 13 June 1973 28099/73 Heading G4N [Also in Divisions H3-H5] A tuned circuit attached to an article to be identified or protected from theft is subjected to an intermittent electric field, and its resulting ringing detected only after the field has been given time to decay fully. The electric field may be electrostatic, applied to a protected enclosure by plates 10, 12, Fig. 1, or electromagnetic applied by coils (100, 102, Fig. 6, not shown), or radio applied by an aerial (140, Fig. 8, not shown). The circuits which produce the intermittent field and the receiver 58 which detects the ringing are controlled by squared waves from clippers such that the receiver is actuated only after collapse of the field is complete, leaving the latter part (T4, Fig. 2, not shown) of the decaying ringing oscillation to be detected. In addition, the electrostatic arrangement has its plates 10, 12 shorted out by valve 40 to assist field collapse. The radio system (Fig. 14, not shown) uses transistor oscillator and receiver circuits, the receiver output being blocked by a gate (280, Fig. 15, not shown) while the oscillator is transmitting. The tuned circuit (Figs. 3, 4, not shown) consists of a metal loop (66) between paper sheets (62, 64) and an insulated overlapping segment (68) to form the capacitor. A fusible link (72) enables the circuit to be disabled by application of a strong A.C. field, when a purchased article is to be removed from a shop. Polyiron particles (78) increase the inductance. Three tuned circuits may be used (Fig. 10, not shown) and a receiver with three tuned stages (174, 176, 178, Fig. 9, not shown) employed for improved identification. Two differently tuned circuits (Fig. 11, not shown) enable different frequencies for transmitter and receiver to be used, one tuned circuit causing the other to resonate. A tuned circuit may have its frequency raised or lowered by destroying a fused link in one of two appropriate connections (Figs. 12, 13, not shown).