Abstract: An expansive and expandable monitoring system which in one of its broadest embodiments includes a multi-sector, multi-condition monitoring system including radio linked alert monitors, smoke monitors and an expandable range of activity monitors such as door-opening monitors and toilet-flushing monitors and including method and apparatus for identifying a particular monitor within a sector; wherein each sector stands itself as a unique sub-system built about a two-wire bus on which both power identifying signals are simultaneously conveyed along the same two wires; and within each sector, the plurality of monitoring devices receive operating power from a common power source and convey identifying signals to a common discriminator (identifier) arrangement all along the common two-wire bus, each monitoring device being constructed with a unique arrangement of current modulating and signal/voltage isolation elements to facilitate the common bus--common power source--common discriminator scheme of the present inve