Patent ID: 7091728

Claim:
A detection circuit, comprising: a transformer primary circuit comprising: an alternating-current signal (AC) source; an impedance in series with said AC source; a transformer primary winding in series with said AC source and said impedance and establishing an electrical circuit by completing a signal path back to said AC source, wherein said transformer primary winding further comprises a primary winding on one of a ferrite core, an iron core, and a non-ferromagnetic core; and a detector positioned across one of said impedance and said primary winding; and a transformer secondary circuit comprising: a transformer secondary winding magnetically coupled to and electrically isolated from said transformer primary winding; a plurality of load devices switchably connected across said secondary winding a first load comprising a first load impedance and a call cord detection switch, wherein said first load impedance and said call cord detection switch are wired in series across said transformer secondary winding; and a second load comprising a call cord activations switch wired across said transformer secondary winding.