GB1486535A - Fire sensor devices - Google Patents

Abstract

1486535 Ionization and semi-conductor smoke detectors NOHMI BOSAI KOGYO CO Ltd 8 Nov 1974 [9 Nov 1973] 04989/77 Divided out of 1485790 Heading GIN [Also in Division G4] A fire sensor device includes sensors 2, 21 differently responsive to at least one phenomenon accompanying combustion and having a common evaluating circuit 7 for initiating an alarm signal in response to predetermined changes in said sensors, each sensor having a respective amplifying element 3, 26, the output signal from one amplifying element being arranged to reduce the bias voltage of the other amplifying element to be exceeded by the response of the other sensor in order that an alarm signal shall be produced. As shown in Fig. 3, ionization chambers 1, 2 are differently sensitive to combustion aerosols or differently accessible to the ambient air and have their common junction voltage amplified by a field-effect transistor FET3, the output of which is arranged to trigger a thyristor 7 which provides an alarm signal. One of two sensors 21, 22 in a bridge circuit may be a semi-conductor gas sensor having a specific sensitivity for e.g. carbon monoxide whilst the other may react to another combustion phenomenon or merely provide temperature compensation. The output of the bridge is amplified at 26 and is arranged to alter the voltage across resistor 28 and so reduce the threshold of amplifying element 3 to signals from: the ionization chambers 1, 2. In Fig. 1, (not shown) details 21 ... 26 are replaced by a temperature-sensing thermistor connected to the base of a single transistor which varies the voltage across resistor 28. In Fig. 2 (not shown) components 21 ... 26 are replaced by a smoke detector comprising a cadmium sulphide photo-resistor and an incondescent lamp stabilized by a Zener diode. In Fig. 4 (not shown) thyristor 7 is replaced by a complementary pair of transistors, components 21 ... 25 being replaced by smoke detector using a light source and solar cell.