A method for sensing the on/off status condition of a power switch in an appliance includes applying a series of alternating current (AC) voltage pulses to an appliance with an on/off switch that are synchronized to a source of AC power, sensing the presence of any series of AC current pulses that result...http://www.google.com/patents/US5506573?utm_source=gb-gplus-sharePatent US5506573 - Remote sensor and method for detecting the on/off status of an automatically controlled appliance
