I was looking at 3.5mm stereo headphone jacks on Digikey and saw this part that piqued my curiosity. I do not understand the function and connections for pins 4-9:
enter image description here

The red-circled part is the one I'm looking at. In particular, what is the meaning of the different connections between 4-5 and 7-8 for example for "plug not inserted?" And the different, but symmetrical, connections for "plug inserted?" I can't find any obvious reference to these anywhere else on the datasheet.

Pins 1, 3, 11, 10, and 2 make sense to me from the diagram (in red circled area), but these others are puzzling. Can someone shed some light on this, please?


  • \$\begingroup\$ it's a DPDT switch \$\endgroup\$
    – jsotola
    Commented Feb 26, 2022 at 21:31

3 Answers 3


There is pretty common use case of the 3.5mm stereo jack where you want to turn off the main speakers when the headphones are plugged in.

What you see here is a stereo jack socket that has this function built-in, implemented in the simplest and time-honored mechanical way.

You wire the main speakers to pins 10 and 11, the amplifier output to 2 and 3 and you use pins 4-9 to switch different circuits (e.g. volume control, equalizer) between the headphones setting and the main speakers setting


That particular device includes internal switches, which change as a plug is inserted.

With no plug inserted, 4-5 are connected, as are 7-8.

With plug inserted, 5-6 are connected, and 8-9.


Some of the connection do not make electrical contact with the actual TRS jack so they are presented in a separate table. It's effectively a DPDT switch that is actuated by the jack, so it does make a physical contact to the jack.

  • \$\begingroup\$ The CUI SJ-357XX -devices in the question. \$\endgroup\$
    – Ralph
    Commented Feb 27, 2022 at 21:01

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