Electronic – arduino – 5V Relay and Arduino to control a 6v nerf gun

Project Sentry Gun looks to be what you want.

This project autonomously tracks, aims, and shoots at targets, using:

• An airsoft or paintball gun
• A webcam to find targets
• A computer to process the video feed and aim the gun
• Servo motors to aim the gun and squeeze the triger
• Arduino to interface between the computer and the servos

Build instructions and the source code are posted on the website.

You want jack "C" at the bottom of this link: http://en.wikipedia.org/wiki/Phone_connector_%28audio%29

The tip and sleeve switches are electrically isolated from the plug so signal is transmitted uninterrupted.

There are 1/4" jacks here (http://www.minute-man.com/acatalog/1_4__Stereo_Jacks.html) that do this. I'm sure they have 1/8" jacks that do the same thing, but I didn't look.

I was looking for the same thing, but for switching inputs based on whether the plug was inserted in the effects return on a tube guitar amp. Can't be puttin' that signal level into the common when someone has it in their hands!