GB1192298A - Fire Alarm Devices including Field-Effect Transistors - Google Patents

Abstract

1,192,298. Fire alarms. CERBERUS A.G. 9 Aug., 1968 [9 Aug., 1967], No. 38248/68. Heading G4N. [Also in Division H1] A fire alarm comprises a detector element responsive to a phenomenon consequent upon combustion to yield an electrical signal, a fieldeffect transistor arranged to amplify the signal and a cover normally preventing external contact with the gate electrode of the transistor together with a switch operable in response to displacement of the cover to short circuit the gate electrode to another electrode of said transistor and so prevent breakdown of the field effect transistor due to spurious charge introduced on to its gate. As shown in Fig. 1, the detector comprises a measuring ionization chamber 10 having access to ambient air via perforations 22 in a cover 21 and a reference ionization chamber 11 sealed from the atmosphere by housing 20. The gate 14 of an insulated gate field effect transistor 12 is connected to the common electrode 17 of the two ionization chambers. Spring-biased contacts 15 are normally maintained open by a pin 22 operated by a flange 26 on the cover 21, but closed when the cover is removed so as to short circuit the gate/drain electrodes of transistor 12 and so protect it against spurious charges. In Fig. 2a, contacts 15 are replaced by a reed switch 31 which is closed by a magnet 32 when the cover 21 is removed but become opened by the interposition of a ferro-magnetic screen 34 when the cover 21 is in place. In Fig. 3 (not shown) the ferro-magnetic screen 34 is replaced by a bucking permanent magnet (32). The ionization chambers may be replaced by a photo-cell or temperature-dependent resistor.