audioiphone
I'm trying to emulate via Arduino the volume down button of the remote mounted on iphone earbuds, and I can't find anywhere how that function is achieved; I imagine that certain passive components housed in the earbuds are shorted to the mic-gnd TRRS pins (upper ring and sleeve), but I can't really figure which ones.
Thanks.
There is a project developed by David Jurick, Adam Stolarz, Damien Stolarz that is described in the book iPhone Hacks. They use the audio jack as a Frequency Shift Keying (FSK) modem that operates at about 1200 baud. Their project interfaces with a Cypress PSoC microcontroller to convert the audio signal to RS232. Source Code can be found at the www.perceptdev.com site.
A similar project using an Arduino to convert the signal can be found here.
Another project called HiJack harvests power and bandwidth from the iPhone's audio jack.
Line levels are 0.5-2.0V range. Mic inputs are expecting millivolts. You just need what is called an L-pad attenuator. Your drop should be 40-50 dB.
If you are using a consumer line level, you will need about 40 dB attenuation. (Professional audio line input would need about 50 dB.)
Attenuation in dB = 20 * log10((R1 + R2) / R2)
Try R1 = 10k and R2 = 100.
If you get accurate measurements of your line source and a know good mic signal for the iPhone, you will be able to calculate more accurate values. Or you can put a 500 ohm trimmer resistor in place of R2 to allow varying of the signal if the line out doesn't have adjustment.
Best Answer
I found this form to have some answers. It looks like pressing different buttons shorts the signal pin and ground with different resistor values. In this diagram, that would be pin 3 for GND and pin 4 for signal.