GB1499581A - Fire detector - Google Patents

Abstract

1499581 Ionization alarm systems PITTWAY CORP 3 Jan 1975 [7 Jan 1974] 171/75 Heading G1N [Also in Division G4] A fire alarm monitoring system employing an ionization chamber 34 and a source of alpha particles 40 is energized from a 12v supply and comprises, a L.E.D. indicator 80 which is illuminated when the system is working in a proper state of readiness, and an alarm horn 68. In the event of a fire the products of combustion increase the impedance of the chamber 34 which causes the conductivity of a MOS-FET transistor amplifer 46 to increase, thereby lowering the voltage at the junction of the resistors 50, 52, and at the base of transistor 54. When this signal or junction voltage, which is the voltage from the MOS-FET amplifier drops below the reference voltage at the base of the transistor 56 forming part of a differential amplifier or voltage comparator, the transistor 56 will be rendered non-conductive and the transistor 54 will be rendered conductive to forwardly bias the transistor 66 and energize the horn 68. The system may be adjusted for sensitivity by altering the potentiometer 64. Should the line voltage at 42 fail, then the voltage across the transistor 54 and the voltage across the diode 59 is no longer sufficient to hold the transistor 72 which then turns off, becomes non-conductive and extinguishes the L.E.D. 80. Should the potentiometer 64 be improperly adjusted to provide too high a voltage then the transistor 54 would be rendered conductive to sound the horn 68. If the potentiometer 64 provides too low a voltage the horn would not sound but the voltage across the transistor 54 and the diode 59 would be less than the required 0.9 volts and the transistor 72 made non-conductive, thereby extinguishing the L.E.D. 80.