GB1002291A - Improvements in or relating to circuits for detecting alarm conditions - Google Patents

Abstract

1,002,291. Fire alarm systems; selective signalling. PYRENE CO. Ltd. March 19, 1964 [March 22, 1963], No. 11485/63. Addition to 910,920. Headings G4H and G4N. An alarm indicating device comprises a detector, which passes curent in its operated condition, shunted by a constant voltage device and an indicator giving a first indication corresponding to the voltage of the constant voltage device under normal conditions and a different indication when current is passing through the detector. In one form, Fig. 1, a number of fire detectors 10, 11, 12 are each connected in series with a Zener diode 14, 15, 16 having a 5V. reference potential and the series circuits are connected in parallel with each other and with a further Zener diode 13 having 10 V. reference potential and a meter 17 across a 20 V. supply. Under normal operating conditions detector switches 10, 11, 12 are open and the voltage indicated by the meter 17 corresponds to the reference voltage of diode 13 (i.e. 10 V.). Upon closure of one of the pairs of detector contacts, the meter indication is reduced to 5 V. If there is a short-circuit or open-circuit in the lines 6, 7 the meter will indicate zero or 20 V. respectively. Diodes 14, 15, 16 may have different reference voltages, the voltage indication of meter 17 then showing which of the detectors has operated. The Zener diodes may be replaced by batteries, Fig. 2 (not shown). In a modification, the detectors are connected in a loop circuit, Fig. 3 (not shown). In an alternative form, Fig. 4, the detector is in the form of a cable 40, 41 whose resistance changes with temperature connected between Zener diode 13 and the meter 17.