From LoRNix … Earphone remote in sound jack as X input:
Those 'special' headphones or earphones which can be used on specialized devices to control media players, volume and mute usually have FOUR connections on the plug, versus the typical THREE a normal headphone output jack has.
The usual three are Left Channel, Right Channel and Ground (common), while the fourth is often set up as a multi-value resistance, each button when pressed presents a particular resistance on the fourth wire (+ ground), which the media device can sense and from that determine what function is needed. Pretty slick method of getting several buttons to work off one wire without resorting to expensive digital signal generators and stuff (all packed in that little blob on the wires!).
Four buttons might use four resistances (of any unit):
volume up: 1 ohm volume down: 2 ohms stop: 4 ohms play: 8 ohms
If this looks suspiciously like a binary encoding scheme... it is!! (You're so smart!!) Using values similarly ratio'd, you can sense 16 different outputs, even handling multiple keys pressed at the same time. Taa Daa!
There will be 16 different values, not necessarily in an ascending or descending value... the various values can be sensed. It's only 'binary' in the sense that it's a matrix of 4 buttons with either on/off positions.
Photo by roberthunt1987 - http://flic.kr/p/87jKin