Patent GB-1228931-A (GB)
IPC: G08B17/06
Filed: 1968-06-26
Abstract: 1,228,931. Smoke detectors. CERBERUS A. G. June 26, 1968 [June 26, 1967], No. 30558/68. Heading G1N. [Also in Division G4] A fire alarm, Fig.1, comprises a plurality of sensors M1, ., Mn connected in parallel with a line 1, 2 to a central station SZ having means to feed line 1, 2 with alternating voltages, each sensor M1 . . . Mn having means (such as switch 7) responsive to a phenomenon accompanying combustion so as to change the impedance offered to respective half cycles of the alternating voltage, and means within the central station SZ respectively responsive to one or all sensors M1 ... Mn being in actuated condition. As shown in Fig.9, an alternating voltage in the form of bi-polar pulses, trapezoidal or sine waves are fed from a source 100 via line 1, 2 to the sensors M1 . . . Mn. The line 1, 2 is terminated in the transistor multivibrator, Fig.7 (not shown) which provides pulses of higher frequency than the source 100 and so provides the check on the continuity of line 1, 2, The sensors, e. g. Fig. 5, have diodes 3 and 4 which ensure that the impedance is different during line pulses of opposite polarity. The line current is monitored across resistor 102 by diode 105 and detector 103 during positive half cycles, which provide a checking phase, a fault alarm being emitted if the current during the positive half cycles is excessive. The line current is monitored by diode 106 and detector 104 during negative half cycles, a fire alarm being emitted if the current is excessive. Filters 109 separate the higher frequency (line-testing) pulses and emit a line fault alarm in their absence. Ionization chamber smoke detector circuits. Fig.5 shows sensing and reference chambers 20, 21, coupled to a normally non-conductive field-effect transistor (FET) 22. During normal operation, capacitor 29 charges to the voltage of negative pulses from line 1, 2 hence the impedance across the line is high during negative pulses. A bi-stable circuit comprising transistors 69, 73 is conductive via diode 3 during positive pulses, so that a low impedance is exhibited at that time. Capacitor 52 is charged via diode 53. If smoke enters ionization chamber 21, FET 22 and transistor 58 become conductive during negative pulses thus introducing a lower impedance across the line and turning on transistors 64, 68 of a further bi-stable circuit and enabling a gas discharge indicator 67 to light when larger amplitude positive pulses are supplied. Testing. A test signal can be sent along line 1, 2 by increasing amplitude or frequency of the positive pulses so that the voltage across capacitor 56 changes sufficiently to turn on transistors 22, 58, 64, 68 to turn off transistors 69, 73, hence the impedance across line 1, 2 is high during positive pulses. Modifications. In Fig.6 (not shown) the lower electrode of ionization chamber 21 is connected to line 2 thus substrate of FET 22 is connected to the tapping between resistors 33, 34. In Fig. 3 (not shown) the FET 22 becomes conductive during positive pulses in the absence of smoke and has positive feedback around a PNP transistor so as to provide bi-stable action. A Zener diode is included to allow positive pulses of reduced amplitude to be used for checking. The ionization chambers 20, 21 may be replaced by photo-electric smoke or flame sensing devices or by temperaturesensitive elements.