I am building an external device for the iPhone that will produce an analog signal that I want to send through the headphone jack. From all my reading, all data sent through the jack is in analog form, yet when i've read up on other hacks doing similar things(like hijack) everyone is including a microprocessor as part of the external device. if the iphone has an ADC(which it does) why bother with the microprocessor? It seems to me all you would need is a signal amplifier/rectifier and then use the output from the native ADC to create your end result code. Meaning basically treating the analog signal like any other signal from the headphone set and normalizing the results to create the effect/outcome you are looking for. What am i missing? any answers greatly appreciated.
closed as not a real question by Anindo Ghosh, Keelan, Brian Carlton, Nick Alexeev♦, Olin Lathrop May 30 '13 at 18:10
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What you're missing is that the microphone input on the phone is not just a bare ADC. It's intended for receiving audio signals, which means it has isolation, DC offset removal, filtering, amplification, normalization, etc. then an ADC. Microcontrollers are used to modulate the raw sensor input into a signal that looks like audio, because an unmodulated DC signal wouldn't get through.
Of course, you could do the modulation the old-school way without a microcontroller. A microcontroller just happens to be convenient since it can be used for both analog and digital sensor data.
It depends on what kind of device you're building what you need.
If you want to process the data from the iPhone or so, you'd be helped a lot with a microcontroller.
However, if you just want to blink an LED when the voltage level on the jack reaches a certain level, a simple comparator circuit/chip would do.
Because most devices are more complex, microcontrollers are used almost always.