GB1270388A - Improvements in or relating to detectors of smoke and the like - Google Patents

Abstract

1,270,388. Transistor pulse circuits. GRAVINER (COLNBROOK) Ltd. 22 July, 1969 [28 June, 1968], No. 30871/68. Heading H3T. [Also in Division G1] In an apparatus for detecting smoke, in the absence of smoke, resistor 78 of the detector circuit 17 is adjusted to make the outputs from the photo-cells resulting from a pulsed light source 18, equal but, as the output circuits are dissimilar, positive pulses are produced and these are amplified by the transistors 82, 84, 86, 88. Between energizing pulses for the source 10 transistor 20 is non-conducting. Therefore transistor 98 is conducting, line 94 is at the same potential as line 38, and transistor 96 is hence unaffected by the amplified output from transistor 88 between light source pulses. If this output exceeds a predetermined value, transistor 96 and hence transistor 103 are switched off and transistor 102 is rendered conducting. Thus, a pulse of charge is applied to capacitor 104 which is thereby maintained at a sufficient charge level to reverse-bias diode 112 and hold transistor 114 conducting. Transistor 116 is therefore non-conducting, holding transistor 120 conducting and energizing relay coil 122. When smoke obscures the path to detector 12 beyond a predetermined level the amplifier pulses no longer switch off transistor 96. Transistor 103 conducts and transistor 102 does not. Thus capacitor 104 discharges, resulting in coil 122 becoming de-energized to produce a warning signal. This warning signal is also actuated as a result of component failure, as the whole of the detecting circuit is active to maintain the relay coil 122 energized. If the push-button 126 of a test relay 124 is depressed, resistor 76 is shortcircuited and the amplitude of the pulses applied to transistor 82 is thereby decreased, resulting in relay coil 122 being de-energized if the circuit is operating correctly.