The bottle toppers are made from air-dry clay that's been mixed with powdered graphite. Graphite is a very conductive molecule and is easy to obtain and work with, and it won't get in the way of your clay setting up.
We'll embed a long wire into the clay of each bottle topper. The wires should be long enough to reach through the holes you drilled in the acrylic and comfortably reach your MPR121 board. Once all the bottles are assembled and the clay is dry, we'll thread the wires through the base and solder them to the MPR121.
Warning - Water Is Conductive
While the clay is wet, it will carry more of a capacitive charge than dry clay will carry. For my prototype castle, I tested all the toppers as I built them, and they all worked great for the first day or two. Once the clay had time to fully dry, however, they became less conductive and I had to remake or re-coat about half of them. I don't want that to happen to you. Be sure to let them dry fully, and test, test, test before final assembly!
Get out your powdered graphite. This stuff is extremely messy and not good to breathe, so wear a particle mask and gloves! (I promise I went and put on gloves right after taking these first two photos.. what a mess. It cleans up easily with soap and water though).
Mix the graphite into your clay. Use a LOT. Add some water if it starts to feel too dry. Keep mixing until the clay is black through and through. You need a solid, unbroken chain of graphite atoms, and the best way to be sure you've got that is if you can't see any white clay at all. Pack as much graphite as you can into that clay.
I sculpted some of my bottle toppers entirely from graphite-impregnated clay. For others, I rolled out the clay and wrapped it around an object -- a small bottle, a shell, or a ball of aluminum foil.
The clay in this photo does not have enough graphite. When this bottle topper dried, the capacitive touch stopped working. That's very sad!
To fix this, I made a slurry of graphite and water, mixed together until it was a paintable consistency. I applied this over the top of the clay to achieve that unbroken graphite atom-chain, so that anywhere I touch on this sculpture will set off the capacitive touch pad.
To carry the capacitive touch signal to the Circuit Playground we'll need to embed our long wire into the wet clay. Strip a few inches of wire and wrap it around your topper to achieve maximum electrical connection. Embedding the wire in a ball of wet graphite clay and pressing it onto your sculpture works well. Make sure there's a lot of contact between the wire and the graphite.
You can hide the wires inside the bottle decorations or just let them run down the backs of the bottles.
Once the clay and graphite slurry are completely dry, mask your bottle - aluminum foil works great for this - and paint the toppers. Metallic paint will not block the capacitive touch powers of your sculpture as long as you keep it to a thin coat. I had success with both spray paint and metallic acrylic paint. I didn't try a non-metallic color, so I don't know whether the metal in the paint helps with the conductivity or not (or whether this paint even has real metal?). I experimented and used what worked! Feel free to do the same. It's your castle.
Once your bottles are painted, it's time for some serious testing. Temporarily attach the other end of each bottle's wire to your MPR121 capacitive touch breakout and touch the heck out of it. Make sure it's reliable and fires the capacitive touch every time, and in all the places you touch. Get someone else to touch it too, to double check your responsiveness.
Test again the next day to be sure that your bottles still work when the clay is fully dry.
My octopus sculptures worked for days, then finally dried out to the point that they stopped working. Instead of starting over, I added a copper tape sticker to the tops of their heads with a line of copper tape attached that wraps around the wire. Copper tape is very conductive, and will not disappoint me by drying out.
It's not cheating to add copper tape if you're having trouble!
Once you're sure your toppers are rock-solid conductors, place the bottles onto your base and thread the wires through the holes. I found it helpful to place the base up on blocks to give myself room to work underneath.
Solder the wires into the holes on the MPR121. Here's how they're laid out in the code we've provided:
- Pin 0: not used
- Pins 1-8: Tone-playing bottles, from low to high
- Pin 9: Octave-shifting bottle
- Pin 10: Off switch bottle
- Pin 11: Demo mode bottle
Once everything is working, glue the bottles down to the acrylic base with strong glue. I used E6000. 2-part epoxy would work as well. Make sure they're solid, since they'll get tapped and touched a lot and you don't want them falling over.
Finish up by using hot glue to secure the electronics in place inside the base. I glued my speakers on the outside back center of the wooden frame, for maximum volume and resonance.