I found that quite frequently I accidentally left the laser on my red dot sight turned on. This led to more than one dead battery when I needed it most. With this remote pressure switch, the dot only turns on when I need it, instead of running down before it's necessary. It also adds a professional "look" to my paintball marker, adding to the "milsim" theme.
The principle of operation is quite simple. There are two conductive pieces of copper foil that are separated by a spongy foam, and when the foil is pressed together, they connect through a hole in the middle of the foam.
I was able to make this switch and wire it into my scope in under two hours, using only materials I had on hand. Granted, had I not been documenting the process, it probably would have taken an hour or less.
Also note that the accessory can be returned to its normal operating mode by simply removing the battery isolator, shorting the switch.
Step 1: Materials
+A scope/laser/light/etc of your choice
+An old car cell phone charger cord - If you prefer a curly cord, just like the commercial ones have, that is. Bonus points if the cable is colored black. An old telephone cable should work, too, but having a straight segment at the switch end makes life a lot easier. Also, you only need two wires to make the switch work.
+A small rectangle of plastic stock 1.5" x 0.5" - The rectangle will form the semi-rigid switch body. This can be anything from part of an old credit card to a cutout from a Chubs Wipes container. I used the latter.
+Heat shrink tubing - Black is best, as it doesn't stand out as much. You'll need at least 0.75" tubing to cover the switch itself and some smaller sizes to seal the cord end. I used a piece about 2" long and trimmed it once it was shrunk. This can be found anywhere from Fry's electronics (they have a great variety box) to Radio Shack.
+Light-duty epoxy to seal the seams between tubing segments (thanks Spork) or (in my case)
+Copper tape - Two strips of tape form the switch contacts. I used some leftover tape from shielding my bass guitar. Aluminum could be used, except that it doesn't solder. Any conductive, solder-able tape or foil will work.
+Thin packing foam - This is what keeps the switch off when there's no pressure on it. I used a leftover sheet of pink foam, the kind that is made of very small bubbles. Whatever you use, it has to be thick enough to hold the conductive tape apart but thick enough to not need too much force to activate.
+Small strip of paper - Depending on the model of scope/sight/laser/light you're using, it might be necessary to isolate the sides of the battery from the compartment. The idea is to only have the battery connected when the switch is pressed.
+Solder of your choice