Alarm system with detectors and signaling devices on the same cable pair

摘要

A single zone system which may use a single cable pair for both detectors of nonstandard conditions and audio/visual alarms. The audio/visual alarms are polarity responsive and in response to an alarm detection, comprising an increase in line current, the polarity is reversed and the line current supervision circuit disconnected. Some detectors or signaling devices may be polarity sensitive so that they present a high impedance under normal and reverse polarity conditions, and a lower impedance on abnormal conditions and prior to polarity reversal. Other detectors may be polarity insensitive and include a relay which may be activated from line potential subsequent to detection of an abnormal condition. The relay may be used to operate door closers or to initiate other appropriate action. A feature of the invention is that a nearly unlimited number of the polarity insensitive detectors may be used because the line current supervision circuit is removed and a hard power supply connected to the line after the first alarm is received.