is a 1/8-inch or is an audio socket that accepts a 3.55mm male pin or audio plug. A small round connector for accepting the pin-shaped plug from a standard pair of music headphones. Below schematic shows the typical diagram of .

3.5mm Diagram

3.5mm Headphone Jack Schematic Diagram

1 – tip : audio out left channel;
2 – 1° ring : audio out right channel;
3 – 2° ring : microphone line in;
4 – 3° ring/sleeve : ground – common for microphone and audio out.

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