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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.

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closed as not a real question by Anindo Ghosh, Keelan, Brian Carlton, Nick Alexeev, Olin Lathrop May 30 '13 at 18:10

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ First you mention "iPhone that will produce an analog signal that I want to send ...", then follow up with "if the iphone has an ADC ...". Since an ADC will take analog input and deliver digital output, there's a conflict here: Which is it, are you generating analog and sending it out through the headphones, or getting analog signal into the phone and then digitizing it in code? It's difficult to tell what is being asked here. \$\endgroup\$ – Anindo Ghosh May 30 '13 at 16:32
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    \$\begingroup\$ @Passe: I read the first sentence that way too, but something called a "headphone jack" will be a output, not input, so this still doesn't make any sense. \$\endgroup\$ – Olin Lathrop May 30 '13 at 18:10
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    \$\begingroup\$ @Passe: If this iPhone "headphone jack" can also be a input, then the OP should have mentioned that. Perhaps this dual purpose is common with iPhones (I wouldn't know), but in general anything called a "headphone jack" can be reasonably assumed to be output-only unless you explicitly say otherwise. \$\endgroup\$ – Olin Lathrop May 30 '13 at 18:45
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    \$\begingroup\$ @OlinLathrop that can generally only be reasonably assumed, if you lived somewhere where cell phones of every single brand for the last decade don't exist. The real general assumption of cell phone is that the headphone jack does both audio out and mic in, not just for iphones, but for almost every brand. \$\endgroup\$ – Passerby May 30 '13 at 18:55
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    \$\begingroup\$ @Passe: Then it should be called "audio jack", not "headphone jack" or "microphone jack", although the latter would have at least been clear in this context. \$\endgroup\$ – Olin Lathrop May 30 '13 at 19:01
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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.

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  • \$\begingroup\$ thanks for the response-despite the responses above, i did have a question, and it was why use a microprocessor in this setting-i apologize to everyone for the misuse of the term jack-i'm not an engineer, and I'll make sure i use terms appropriately next time! \$\endgroup\$ – jeremyw May 30 '13 at 18:57
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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.

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  • \$\begingroup\$ The question is not clear, and the answer makes things clear as mud :-) \$\endgroup\$ – Anindo Ghosh May 30 '13 at 16:36

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