You may have seen the circuit board that is built into the headphones of the Apple iPod Shuffle. David Carne did a great job of reverse engineering what the iPod Shuffle Headphone circuit actually does. He also went as far as to emulate the circuit using a microcontroller on a bread board. There is an interesting frequency that is used at startup so that the iPod can determine what type of head phones are connected.
Via: Hack a Day
“After digging around a bunch, and building an amplified current sensor, I found the power-on detection was actually a 100mV/10uA ultrasonic chirp occurring 8.1 msec after power up. This would seem to coincide with the mention of “ultrasonic” by iLounge. Figure 9 has a blown up view of the chirp. In figure 9, the upper signal is voltage, and the lower is current. The current signal, measured with a 1k resistor, was run through a 5x preamp made from a precision op-amp to bring it up to the level seen in the figure. The ultrasonic chirp is actually a dual-frequency chirp, consisting of 1.5msec of 280khz, followed by 4.6msec of 244khz. [Doing a binary-search by hand using the scope trigger to find the transition point is no fun].”
