GB1506387A - Alarm indicator systems - Google Patents

Abstract

1506387 Alarm systems BRITISH RAILWAYS BOARD 2 April 1975 [29 March 1974] 14060/74 Heading G4N In an alarm indicator system having audible and/or visual indicating devices for indicating the occurrence of alarm conditions, the system is arranged to receive two or more input signals A, B of which A represents a more urgent condition, the different signals are arranged to produce pulsed outputs of different frequency on a common indicator 11, the system being arranged so that signal A takes precedent over other signals when more than one signal appears simultaneously. As described, the system is applicable to monitoring the security of a plurality of doors. Fig. 3 shows a local indicator system. Signals B are fed to gate G1 which is cross-coupled with gate G2 to form a multivibrator operating with a low mark/space ratio, the pulses from which are used to modulate an oscillator formed by gates G3, G4, the output of which is amplified at TR3, TR4 so as to form a "peep-peep" sound in loudspeaker 11. The more urgent signals A are also fed to transistor TR2 which converts the mark/space ratio of the multivibrator G1, G2 approximately to unity so as to provide a more rapidly pulsating sound in loudspeaker 11. After a time determined by a circuit C3, G5, G6, transistor TR1 is turned on to provide a third mark/space ratio different from the other two. Fig. 4 (not shown) depicts the circuit diagram of a remote station responsive to a plurality of A and a plurality of B signals and including a respective LED for each signal. A further loudspeaker is provided with a volume control which is overridden by any A signal.