Arduino to iPhone Audio Communication

I am working on a project where I'd like to be able to do two things:

  • Send a signal from an Arduino to an iPhone to trigger taking a photo
  • Send a signal from the iPhone to the Arduino to trigger a motor (which is powered externally)

I'd like to do this using a 4-pin audio connector from the iPhone to the Arduino.

Looking into the different options, I found several pages on using Frequency-shift keying (FSK) to transmit audio signals, but I haven't found any working examples with corresponding code. Also, the boards that were discussed are no longer being made (e.g. Audio Jack Modem for iPhone and Android - DEV-10331 - SparkFun Electronics)

Is using FSK overkill for my application (which essentially just needs to communicate on/off in both directions)? If not, does anyone have suggestions for resources to use?

Thanks for your help!

(Also, a related question - I bought a shutter release cable from Amazon and connected it to an Arduino to trigger the camera on the iPhone, similar to pressing volume up or volume down on an inline remote. Anyone know what the circuit is for mimicking this behavior? You can see a working video here:

I'm starting to learn how to do this and want to create some useful documentation for other people wanting to work on similar projects - you can find the documentation here: