Patent ID: 8897429

Claim:
A communication circuit of a first communication device, comprising: an interface unit; the interface unit outputting a first voice signal to a second communication device and receiving a second voice signal from the second communication device; a DC control circuit providing a constant current for the communication circuit; the DC control circuit connecting to a voice output circuit comprising a voice input terminal and a control signal input terminal and a voice receiving circuit comprising a voice output terminal; and wherein in DC control circuit comprises a first transistor, a second transistor, a first zener diode and a plurality of resistors; the first transistor and the second transistor both comprises a first conducting terminal, a second conducting terminal and a control terminal; the first conducting terminal of the first transistor is electrically connected to the interface, the first conducting terminal of the first transistor is electrically connected to the control terminal of the first transistor via a first resistor, the control terminal of the first transistor is electrically connected to the voice output circuit, the second conducting terminal of the first transistor is electrically connected to the second conducting terminal of the second transistor, the second conducting terminal of the second transistor is electrically connected to the control terminal of the second transistor via a second resistor and a third resistor in series; a cathode of the first zener diode is electrically connected to the first conducting terminal of the second transistor, an anode of the first zener diode is electrically connected to the interface unit; wherein the first voice signal into the voice input terminal is coupled to the DC control circuit by a first optocoupler and the interface unit outputs the first voice signal to the second communication device; and the second voice signal is transmitted from the second communication device via the interface unit and is coupled from the DC control circuit to the voice output terminal by a second optocoupler.