I'm fiddling with this module based on the popular bluetooth module CSR8635(). All I would like to do is create a simple AC Voltmeter over the Mic input from this bluetooth module. The end goal would be a way to tell whether the user has the headphones connected or not. In the above image, we can see that pin 8 is "Line_AP" or, according to the table, "line input positive, channel A". What I would like to do is basically turn this bluetooth circuit into an AC Voltmeter. I would like to be able to measure voltage differences between pin 7 and pin 8, no matter what I connect them to. This voltage measurement should be communicated back as the MIC input The first image I've posted (where Line AP is compared to ground) does not work because the AP voltage is being measured relative to ground. I need to measure voltage from variable A (skin point A) to variable B (skin point B). Just telling whether the user is connected is only the beginning of my applications, but measuring the voltage between two arbitrary skin points is the final application. Skin outputs AC voltage. Nothing I've tried so far is getting my Android device to record back voltages.