GB704779A - Improvements in and relating to intruder-operated alarm devices - Google Patents

Abstract

704,779. Burglar and like alarms. E.M.G. HANDMADE GRAMOPHONES, Ltd. March 3, 1952 [March 2, 1951], No. 5164/51. Class 118 (1). An alarm device responsive to the presence of an intruder comprises an electric conductor to which are applied radio-frequency oscillations and receiving means connected to the conductor for detecting variations in the transmission characteristics of the conductor occasioned bv the proximity of an intruder. This invention is particularly suited for the protection of large regions, e.g. an airfield. In such a system, Fig. 1, a transmitter 11, generating radio-frequency oscillations having a frequency of the order of 200 Mc/s, is connected by a length of wire 10 to a receiver 12. The oscillations generated at 10 are modulated at 1,000 c/s and the signals received at the receiver 12 are detected to extract the 1,000 c/s modulating signal whose amplitude varies with presence or movement of persons near the wire 10. The 1,000 c/s signal is passed through a rectifier to give a unidirectional signal and this is applied via a differentiating circuit having a time constant between “ second and 5 seconds, to a valve circuit operating a relay which controls an indicator lamp or alarm bell. Due to the differentiating circuit slow changes in the transmission characteristics of the wire 10 caused, for example, by changing weather conditions do not actuate the indicator while rapid changes such as those produced by movement of a person near the cable cause the indicator to be actuated. A discontinuity (e.g. an abrupt change in the direction of the conductor or the insertion of an insulator therein) may be arranged in the conductor at points where increased sensitivity is required, the field strength being greater in the region of such discontinuities. Signalling locality of intrusion to central station. Fig. 4 illustrates a system giving an alarm at a central station 50 indicating the locality from which it has been received. The system comprises three transmitter-receiver combinations, 44 and 45, 46 and 47, 48 and 49, each employing respective distinctive transmission and modulation frequencies. Transmission is effected by means of a pair of conductors 10 which are also used to supply power to the transmitters and receivers from the central station and to convey signalling currents from the receivers back to the central station. Each receiver when actuated by the presence of a person near the corresponding length of conductor 10 transmits a distinctive signal which operates a respective one of the alarms 52, 53 or 54 at the central station. The Provisional Specification refers to Specification 688,644.