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What are internal switches?

I've been searching out parts on digikey, and noticed that a lot of the power/audio jacks had an "internal switch", with three conductors and four contacts.  I've never heard of these before, and none of the datasheets were helpful, nor ten minutes of googling, in educating me.  So what are these mysterious internal switches?  I presume they somehow switch between two states depending on if there's a plug in the jack.  But why is this helpful or necessary?  And more importantly, how does this affect how to wire the jack?


kelseymh6 years ago
You can get these at Radio Shack, among other places; I make use of one of them in my own Instructable.

They allow you to set up your circuit in such a way that when you plug in the jack, it automatically switches from one mode to another: in your example, plugging in the jack would disconnect speakers and connect to headphones instead; in mine, it removes the battery compartment and uses the wall wart for power.
76543216 years ago
like relays?
No, they're entirely mechanical. If you look at

https://secure.wikimedia.org/wikipedia/en/wiki/TRS_connector#Configurations_and_schematic_symbols

the illustrations and explanations for C and D explain this fairly well.

The power jack with internal switch is similar. The barrel of the plug, in pushing on its spring contact, also pushes that contact away from (and disconnects it from) another contact.

As to what you'd use them for: An obvious example would be a system which runs from external power when that's plugged in, and otherwise runs from a battery.

If you aren't using the switch, just ignore the extra contact(s) and wire the ones needed to make the connection to the plug.
Ahh, thanks