I thought I had learned my lesson with the first museum exhibit I developed; don't use buttons because people, especially children, are brutal. So I moved to capacitive touch buttons. And I thought that would solve the problem. A single sheet of acrylic in front of each column of elements would be easy to clean, would provide a nice interface, and wouldn't have any mechanical problems. Well, it does all those things, but presented me with a billion other problems.
I need help figuring out how to get the cap touch to work for the elements. Here's how it's configured now:
- 1/8" acrylic sheet on the full column of elements.
- Photo glossy sheet of paper with the element description and a 'button' that says "Press to Start"
- 1/8" acrylic piece behind the sheet of paper to hold it.
- 3"x1.5" piece of copper tape attached to the back of the acrylic. If you're keeping track, that's 1/4" of acrylic and a glossy sheet of paper between the finger and the foil.
- 4" wire from the foil to the pin.
- 10M Ohm resistor between the two pins on the ATTiny (using the capsense library).
- 22pF capacitor between the sense pin and ground.
- A custom library abstracts away the capsense library so every element gets the same configuration for touch sensitivity.
- There is a ground plane on the PCB, and every element has a cable carrying +5V, +12V, and ground back to an ATX power supply which is connected via a 3 prong power cable into a power strip.
- Acrylic wiped down with anti-static, anti-fog wipe.
- All elements run for a certain amount of time and then stop. During that time they ignore the button. When they stop running, they start looking at the button again for input.
- The cap sense library allows recalibration every N seconds. I have this set to 20.
This works. Usually. It worked in testing just fine. However, when I installed everything at the museum, things went crazy. Here are some of the problems:
- One of the columns is sensitive in such a way that touching anywhere on the acrylic sheet will activate the whole column.
- Some of the elements will activate way too easily.
- Others require a full palm.
- Sometimes they just start on their own.
- Sometimes you can activate them and they'll start, but even if you're standing a few feet away they'll keep going a few more times before they finally stop.
I spent a lot of time playing around with different sensitivity values for this, but I'm worried that it doesn't matter because each element seems to behave differently. I'd really like a consistent configuration because putting them in and taking them out of the installation is difficult, so tweaking each element would be horrible.