I've got a small device that I have not yet opened. It sends some unknown sequence of voltage and current through a medium for a few seconds thereby calulating and then displaying a result value on a display. During the measuring time it also displays some small waiting animation on the screen. I guess that one small chip will be handling all these activities.
Now I'm wondering how I could grap the result and import them into a smartphone through the audio jack. I want to loose the integrated display of the device and only use the smartphone screen. What would be the smartest way to go?
Grabing the display signal doesn't seem to be feasable because it are multiple digitally coded lines and not one signal with ampiltude. Could I maybe listen to the measuring signal through the audio line and then try to do my own reference measurements, or might this disport the actual measurement? Is there maybe some other way to grab this information to provide means of interoperability?
What voltage (& current?) can be measured my a mic in line and how must the signal be modulated? Some fixed frequency or can microphone electronics also analyse frequency?