This is partially me sharing this technique, and partially me learning how to use Instructables. If there are issues with my documentation of the technique or my use of Instructables, please let me know in the comments - thanks!
I needed a long row of switches in a pinch, and didn't have nearly enough switches in my tool box for this project. This is a way to use copper tape to create many switches, quickly, in order to create a digital selection pad.
For this project you'll need
*A Sheet or roll of copper tape or other conductive foil. You can find this at any hardware store, or in any craft store. (Probably other places too - feel free to leave suggestions in the comments.)
*A roll of breadboard gauge (or other Arduino-compatible) wire
*A piece of balsa wood (cardboard works in a pinch, but balsa seems to work better, as it "molds to" the shape of the wire somewhat, helping it stay in place without sliding out.)
*A roll of Scotch tape
*One 220 ohm resistor for each distinct selection you want to allow (that is, if you want to make 9 buttons, use 9 resistors)
*Something to interpret your input voltage levels from the selection pads (in my case, an Arduino Diecimila with some code performing digital reads - though sadly, the Arduino is limited to 14 distinct input pins; one could an additional circuit to allow for more inputs but that's beyond the scope of this tutorial).
Step 1: Step One - Assemble Materials and Create Components
For each selection pad/button, first
*cut a piece of balsa wood in the size and shape of the desired button
*cut a strip of copper tape long enough to cover both sides if you wrapped it along the short end
- that is to say, if your balsa wood piece looks like this
make your tape strip look like this
*cut a wire and strip both ends of the wire - one end will go into your input (say, an Arduino digital in pin) and the other end will be attached to the copper tape.
You can lay your pieces out and count them to make sure you have enough before going to step 2.
Step 2: Building Each Selection Button
For each selection button:
*lay an exposed wire end onto one side of the piece of balsa wood and fold your copper tape down over the wire, leaving a bit of balsa wood exposed on one side (so the tape will stick to it)
*now, tape the wire and copper tape together around the bottom of the piece of balsa wood (being sure to leave some copper exposed on both sides at the top of the piece)
The pictures below will hopefully show it better than my words can.
Step 3: Turn Each Selection Pad Into a Switch
Lay each of the selection pads down on your mounting surface (wood/balsa wood/wall/etc, whatever your project needs) and secure the pads with tape, nails or by any other means (so long as you leave the copper tips exposed.)
Now, connect each selection pad via a 220 ohm resistor to a long, master strip of copper tape. I found one quick way to do this was to hook the resistor into the exposed part of the pad's copper tape by slipping it under the tape where it folds over the edge, and then taping it down to the surface to avoid it from moving before connecting the other end of the resistor to the master strip. It isn't *that* secure, but for my stationary, temporary and flat project, it did the trick.
Now connect that "master strip" of copper tape to ground.
Finally, plug the unused wire end from each pad into an input pin on your project.
What this does is process ground every single input selection pad in a way that still lets each pad be applied a voltage independently frrom each other. The resistors allow all of the pads to share a common ground, while still being able to be individually "turned on" by applying 5V to the selection pad.
Step 4: Begin Pressing Buttons
Now, how can we press these buttons? Well, any voltage source will switch a selection pad's voltage to from "low" to "high" - so we can just use any mechanism we want to apply voltage to a selection pad to turn it on.
For my purposes, I literally just stuck an exposed wire into Vcc (5V or 3.3V on my Arduino, or "High" on your breadboard) and begun pressing it against selection pads to test whether they worked.
And that's it! Copper tape takes some time to secure properly, and it's no substitute for a good soldering - but when your entire point of making a device is to create a physical manifestation of a cheap pun (in this case, a "United State Machine), it does the trick, creating solid electrical connections quickly and cheaply.