GB1254130A - Alarm system - Google Patents

Abstract

1,254,130. Burglar alarms. LIGNIN ENG. CO. Ltd. 4 Nov., 1969, No. 54012/69. Headings G4H and G4N. A burglar alarm for a protected enclosure arranged to be pressurized by a predetermined air flow therethrough comprises a switch 10 associated with a time delay 19 so that an alarm 31 operates at a predetermined time after a variation in the air flow associated with switch 10. A flow of air through the protected enclosure is created by means of a fan 1 driven by the mains or from a floating battery. Reduction in air pressure in the region of the outlet 6 due to opening of a door or window results in a vane 7 opening switch 10, so as to release relay 17 which de-energizes the heater of a thermal delay switch 19 so that its contacts open after a predetermined period, releasing relay 32 and initiating a local or remote bell 31. Nullifying devices for initiated persons.-The time delay switch 19 has sufficient delay to enable an authorized person to enter the enclosure and nullify the system by dialling an appropriate code in inhibitor 33 so as to energize the appropriate points of a cross-bar matrix (not described in detail) and have to energize relay 37 and short circuit the contacts of the thermal delay switch 19. A reset button 39 is provided. The inhibitor 31 may be replaced by a key switch. Preventing incorrect operation.-A fuse F is provided, arranged to operate in the event of short circuit of conductors 14<SP>1</SP>. Modifications.-In Fig. 2 (not shown) long and short time delays are included, the latter preventing unwanted response due to temporary draughts &c. Manual test facilities and indicator lights are included. Signalling to and from a central station.- Fig. 3 shows part of a system in a central station and comprises a plurality of oscillators 102, each capable of producing a different frequency or code, but arranged so that oscillation is inhibited except when a core is set to an inward position by means of cam 104 driven by a motor. Each subscriber is provided with a decoder tuned to present a closed circuit only to the particular frequency or code from the central station. Each oscillator in turn transmits a signal over a common line but only one of the decoders responds to any particular signal; each decoder is associated with the alarm contacts so that when the cam 104 is directed towards an oscillator 102 associated with a subscriber having an alarm condition, the signal across resistor 106 will fail and an alarm 109 will operate and the motor will stop.