A remotely-controllable relay which is controlled by 175 Hz ripple voltage superimposed on the mains supply voltage in an electric-power distribution network contains an oscillator generating a signal at 173 Hz. This is mixed with the network voltage so that, when ripple voltage is present, a beat note...http://www.google.es/patents/US4232298?utm_source=gb-gplus-sharePatente US4232298 - Remotely-controllable relays and filters therefor
