GB1351748A - Electric alarm systems - Google Patents

Abstract

1351748 Alarm systems BARDIC SYSTEMS Ltd 15 July 1971 [16 July 1970] 34545/70 Heading G4N An alarm system responsive to the opening of contacts 21, located at spaced positions, comprises at each of the spaced positions an impedance 20 in series with contacts 21 and means such as transistor 39 and relay 31 responsive to the collapse of voltage across impedance 21 to energize a local warning device 14. As described, each unit 10 may be located in a separate manually operated break-glass fire call unit in which contacts 21 are held closed by a spring plunger so as to open when the glass is broken. The contacts 21 of the respective units 10 are all connected in series with local batteries 17 and impedances 20 in a ring circuit 11. The voltage across each impedance 20 maintains its associated transistor 39 non-conductive. Opening of any of contacts 21 will cause the voltage across each impedance 20 to collapse, allowing all transistors 39 to be turned on, energizing all the relays 31 and operating all local bells 14. Local illuminating or indicating lamps 32, 33 will also be extinguished. The warning may be cancelled by opening the glass frame which closes contacts 26, short-circuiting the break-glass contacts 21. Alternatively, a further pair of cancelling contacts 26/2 may be included so that the illumination of lamps 32, 33 indicates that the unit has been reset. The system is powered by a local mains-operated rectifier 27 arranged to charge local nickel cadmium batteries 17, diode 30 being included so that lamps 32, 33 indicate mains failure. Modifications.-Contacts 21 may alternatively be in the form of thermal switches or flameresponsive photo-electric switches or may be operated by opening a door or window in a burglar alarm system.