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Curious Inventor - Store: 1/8" Panel Mount Mono Switching Jack
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in stock - price: $1.29
1/8" Panel Mount Mono Switching Jack
Eighth-inch-switching-jack

short description:

This jack accepts standard 1/8" audio plugs, and it features a mechanical switch that can be used to tie the signal line to ground when a plug is not present.

more information and instructions:

This fits into a 1/4" hole. It has three solder ringlets: signal, ground and a third that gets connected to the signal line when a plug is not present. The picture below shows this "switch". The spring both holds the plug in place, and also closes a switch between the middle and right connections when the plug is missing.