An electronic device is set forth that allows external control of a controllable electronic function of the device. The device includes a function circuit for implementing an electronic function in response to a signal at a function control signal input. The function circuit receives DC power from a power supply that is connected between two battery terminals. The DC power from the power supply circuit is supplied at a predetermined DC voltage level. A control circuit is included in the device and is responsive to a signal superimposed on the predetermined DC voltage level of the power supply at the two battery terminals for generating a function control signal to the function control signal input of the function circuit. In accordance with one embodiment of the present invention, the invention is implemented in the context of a hearing aid.