Method and electrical interface circuit enabling multiplexing

An electrical interface circuit is disclosed. The circuit comprises a microphone circuit (100); a battery charger circuit (200); and an electrical connector (300) for connecting said electrical interface circuit to an external device. The electrical connector has a pin (302) on which signals are multiplexed for connecting either the battery charger circuit to an external supply voltage or the microphone circuit to an external microphone. The battery charger circuit comprises an amplifying circuit (202) for controlling voltage or current to a battery (400) at battery charging and a p-type power transistor. The pin is connected to the microphone circuit and to a source of the p-type power transistor, and when a voltage applied to the pin exceeds the battery voltage, the p-type power transistor will provide current from the pin to the charger circuit, and otherwise the charger circuit and battery is disconnected from the pin. A method of multiplexing signals on the electrical interface circuit is also disclosed.