GB1320651A - Fire alarm systems - Google Patents

Abstract

1320651 Alarm systems; fire alarms NITTAN CO Ltd 19 April 1971 [31 March 1970] 25235/71 Headings G4H and G4N [Also in Division H3] In a fire alarm system, each of a plurality of fire detector units, connected in parallel through a pair of conductors to a central station, responds to fire by producing an oscillation with a frequency characteristic respective to the unit, this being discriminated by the central station. General.-A plurality of heat or smoke detectors are connected by a common pair of lines to a central station. On detection of fire, a detector sends an AC frequency specific to the detector, and a D.C. level, to the central station where the D.C. level actuates a relay via a low-pass filter to light a lamp and actuate a sound generator, and the A.C. frequency is passed by a high-pass filter to a bank of parallel band-pass filters one of which lights a respective lamp via a rectifier to identify the detector. Details of detectors.-In Fig. 2, a bimetallic element 21 responds to heat to close a switch 22 thereby altering the D.C. signal across the pair of lines 1, 2 and changes the biasing voltage of transistor 51 to cause blocking oscillator 5 to apply A.C. of a characteristic frequency to lines 1, 2. In Fig. 3, an ionisation smoke detector has two ionisation chambers 61, 62 respectively closed and open to the air, smoke causing increasing conduction in FET 63. If sufficient, this triggers zener diode 65 to make SCR 66 conduct, altering the D.C. across the lines 1, 2 and actuating a tuning-fork oscillator circuit 7 which feeds A.C. of a characteristic frequency to lines 1, 2 via a resonant-circuit amplifier 25, 26. Fig. 5 (not shown), shows a modification of Fig. 3 in which the oscillator circuit 7 and circuitry to the right of it are omitted, and the secondary of a transformer is inserted in series with SCR 66, the transformer primary being fed from a continually-running oscillator. In Fig. 6, A.C. from oscillator 30 normally does not pass to transformer 32 due to diodes 33, 34, but closure of switch 37 (corresponding to 22 in Fig. 2, or SCR 66 in Fig. 3) causes a D.C. flow between lines 1, 2 via diodes 33, 34 which permits the A.C. to pass to transformer 32 and so to lines 1, 2. An audible alarm may be given at the detector, fed from a second secondary on transformer 52 of Fig. 2 or 32 of Fig. 6 or from an extra transformer inserted in series with SCR 66 in Fig. 5 (not shown).