My friend and I are trying to design a remote controlled car and one of the aims is for the controller to be a smartphone. The original idea was, have a transmitter plugged into the audio headphone jack which would interpret audio signals sent from a controller app and then send an appropriate signal.
My problem is I don't know what sort of signal is appropriate. All that needs to be controlled is forwards/backwards motion and turning left or right. Ideally it would be able to have speed control - not just forwards and backwards.
My initial idea was using something like this for radio transmission: https://www.sparkfun.com/products/10534, but then it would mean doing analog -> digital and sending digital data with some kind of protocol, so then I thought of sending just the analog data but I'm not sure how that would separate the forward/backward and right/left controls.
I am a beginner in this field and would like some direction - what approach do I take that will allow me to keep the cost low (e.g. no bluetooth modules)?