Somewhat related to this, several years ago I have successfully used FSK[0] to transfer data[1][2] from an AVR microcontroller to iPhone through its headphone jack.<p>[0] - <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Frequency-shift_keying" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Frequency-shift_keying</a><p>[1] - <a href="http:&#x2F;&#x2F;www.creativedistraction.com&#x2F;demos&#x2F;sensor-data-to-iphone-through-the-headphone-jack-using-arduino&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.creativedistraction.com&#x2F;demos&#x2F;sensor-data-to-ipho...</a><p>[2] - <a href="https:&#x2F;&#x2F;github.com&#x2F;ezefranca&#x2F;FSK-Arduino-iOS" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;ezefranca&#x2F;FSK-Arduino-iOS</a>