1/4" Jacks for Dummies

[Inventor]

OK, so it sounds very basic but to be honest I'm not very familiar with 1/4" jacks. Aside from the simple two-terminal mono jack, we have a contact on the tip which may be open or closed with no plug inserted. Some of my questions include:

What is the extra terminal used for? I presume it is something like detecting when a plug is present and/or providing ground to the input when not in use.

For the Digi-Key part number SC1123-ND, it is described as "double open circuit", what does this mean?

Thanks for educating me on these details.

[DrJustice]

Here's the Switchcraft page, there's a link to a datasheet with a schematic drawing. As you can see, when the plug is inserted, the normally closed contact disengages from the plug terminal, and the plug makes contact instead. You get them in both mono and stereo/balanced versions, the latter using two such contacts for the tip and the ring. Switchrafct call the switches 'shunts' - haven't heard that before, but it makes sense.

The extra, normally closed contacts, can be used for grounding an unused input, detecting an inserted jack or disconnecting a normalized input signal - but you've got all of that already

DJ
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[Uncle Krunkus]

The two normally closed switches are(were) most commonly used to short out the dropping resistors which you don't want in circuit when you don't have your headphones plugged in.
So it follows that the third connection is the right hand channel of your headphones. ie; Tip/Ring/Sleeve -> Left/Right/Ground

BTW Of course, if you're using mono plugs, the third connection is only connected to ground when a plug is present. That's why stomp-boxes use it as the ground connection for the battery.

[Inventor]

OK, thanks guys. You know, it's funny we can understand complicated stuff but sometimes the simplest of things escape us, or me anyway. Cheers!