Some sound card audio jacks can now inform recent Windows OS's that an audio device is plugged in. Anyone know how it does this? I'm thinking it uses some kind of voltage comparator or resistance measurement.
This question was asked at Superuser. My gut feeling is that the circuit is not as trivial as one of the participants is suggesting, but I'm a bit rusty on circuits.
Answer
While audio jacks often have switches that can be used for plug detection, most codecs now implement jack sensing that measures the impedance and other characteristics of the plugged in device and make that information available to the processor they are connected to.
This is also very helpful in preventing an audio amplifier from trying to drive speakers outside its impedance range and damaging the amp or the speakers.
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