Methods, systems and related apparatus are provided to enable an electronic device to operate an external sensor comprising one or more emitters for emitting electromagnetic radiation of two different wavelengths and a detector for generating a response signal based on received electromagnetic radiation of the two different wavelengths connectable to an audio interface by applying a harmonic driving signal to a first contact and a second contact of the audio interface for driving the emitters of the external sensor, receiving the response signal at a third contact of the audio interface, demodulating and demultiplexing the response signal into a first wavelength response signal and a second wavelength response signal, analyzing the first and second wavelength response signals to determine one or more vital signs, and outputting the determined one or more vital signs.