Here’s a look at the TRRS cable that Android phones use. [Rich Kappmeier] want to control the music player on his Nexus One while driving. It’s not necessarily a safe endeavor if you’re staring at the screen and poking away with one hand while trying to stay in your lane. A little bit of research helped him figure out how the hardware in a headphone controller worked and he decided to incorporate that into a connector cable for the car.
The control signals rely on a specific resistance between the TRRS function ring and ground. Once he worked out the chart above and targeted the correct resistance values he built a rocker switch for Fast Forward and Reverse, as well as a Play/Pause button into the connector cable. You should be able to use this for more than just music control. Take a look at our Android Development tutorial and see what else you can come up with.