Ground wire adaptor

A ground wire adaptor for operatively connecting a low-voltage control unit (e.g. a thermostat) requiring a three-wire connection to a three-wire system-under-control (e.g. a HVAC system) using a two-wire conductor. A ground terminal and an output control signal of the control unit are interconnected such that the output control signal, in the form of a half-wave rectified signal, is superimposed on the ground. The ground wire adaptor detects the half-wave rectified signal superimposed on the ground connection and generates a control output signal to be provided to the system-under-control responsive to the presence of the half-wave rectified signal (i.e. the control signal from the control unit). In another aspect of the invention the ground wire adaptor can be used to connect an 'N' function control unit to an 'N' function system-under-control using a two-wire conductor.