GB713542A - Improvements in and relating to burglar alarms - Google Patents

Abstract

713,542. Burglar and like alarms. E.M.G. HANDMADE GRAMOPHONES, Ltd. Nov. 20, 1952 [Aug. 20, 1951], No. 19637/51. Class 118 (1). A burglar alarm for protecting articles in a container such as a show-case, comprises a valve oscillator mounted within the container, a switch system for switching on the oscillator if the container is broken into, and an alarm device including an element disposed.at a substantial distance (i.e., a few feet) from the oscillator and coupled thereto inductively. The oscillator mounted within the show-case comprises a valve V1 whose cathode-heater current is supplied through a switch S which closes, should the case be broken into. The coil L1 is arranged co-axially with a similar coil L2 mounted in the ceiling above the show-case. A signal picked up by the coil being amplified by valve V2, rectified by rectifier D, and the resultant uni-directional potential used to cause the normally non-conducting valve V3 to conduct, thus operating relay R itself connected to control an alarm device, e.g., a bell. The oscillator frequency chosen preferably lies between 6 kc/s and 30 kc/s. The switch S may be provided by a number of switches connected in parallel and attached to plate glass sheets mounted within the show-case so that upon movement of these sheets one at least of these switches is closed. One such plate-glass sheet may be arranged immediately beneath the upper glass panel of the case and switches mounted so that one or more close when the sheet is depressed, while another or others close when it is lifted. Similar sheets may be arranged adjacent the side panels of the case. The Specification discusses also a possible burglar alarm similar to that described above, wherein a small radio transmitter is provided within the show-case for communicating with a receiver including an alarm system. Such an arrangement does not form part of the present invention.