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4 pin 3.5mm (2.5mm) plug connector layout
4 pin 3.5mm (2.5mm) plug connector
at the iPhone (2.5mm or 3.5mm, depends on model)
iPhone headphone (handsfree)
Pin
Number
Pin
Name
Description
1 Tip Left audio
2 Ring Right audio
3 Ring Common/Ground
4 Sleeve  Microphone

Pressing the headset button shorts Microphone to Ground

Inserting a 3-pin plug into the iPhone's 4-pin receptacle would also short Mic to Common and the L/R earphones will work correctly.

Using 4-pin Headsets provided by certain Phone Carriers also work with the microphone connection and certain remote functions.

Using iPod Earphone that use a microphone with a Cell phone may not provide the mic connection, and wil likely use the phone's internal mic instead.

The iPhone requires a certain impedance on the microphone pin for the input to be detected and used.  1600ohms is a good choice as it is very close to the mic on the standard Apple headphones.  So, if you are running an output from a PC or other amplified device into the iPhone mic input you will need to put a 1.6kohm resistor in series for it to work.  Tested on iPhone 3GS.

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Pinouts.ru > Headsets, Headphones, Handsfree - pinouts of mobile phones and other equipment  > Pinout of Apple iPhone headphone jack adapter
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