GB1315602A - Fire alarm system - Google Patents

Abstract

1315602 Fire alarms NITTAN CO Ltd 10 Aug 1971 [10 Aug 1970 (2) 20 Aug 1970 31 Aug 1970] 37574/71 Heading G1N [Also in Division G4] In a fire alarm system an indication is given as to whether a fire is detected or an ionisation smoke detector forming part of the system is operating erroneously. As shown an ionisation detector including series connected open and closed chambers 20, 10 and a FET 6, is supplied with an operating voltage. A detection output of the detector is supplied to the base of a bi-polar transistor 31, whose emitter collector is connected in series with a Zener diode 32 across the voltage source, to render the transistor conductive and thereby reduce the voltage supplied to the chambers 10, 20 to the Zener voltage which is insufficient to enable smoke detection but sufficient for the detector to still provide an output when the F. E. T. fails or due to insulation breakdown of the chambers. Thus, when smoke is detected, the transistor 31 and thus a relay 16 is operated intermittently, whereas a continuous operation is provided due to malfunction. The relay 16 is used to operate a discriminator circuit, Fig.7 (not shown), which discriminates between intermittent and continuous operation to energise an appropriate alarm device. The discriminator may be a pulse counter. Other embodiments for reducing the operating voltage, upon receiving a detection output, include a switch operated by the relay 16, to switch to a lever voltage source, Fig. 5 (not shown), or utilizing the impedance of the relay element, Fig. 6 (not shown).