Birthday Card (2013)¶
This was a quick and dirty hack to create a musical birthday card. I had made a simple light up valentines day card using a coin cell battery and a LED. This was an attempt to see if I could do something more sophisticated.
The core of the project was a spare ATtiny13A microcontroller. The sound output is generated with a very simple piezo element. The battery and wiring are from a Bare Conductive greeting card kit. The battery is a simple coin cell with leads which make it easy to attach to paper. The wires are created with conductive paint on the back side of the face.
The Bare Paint Pen that came with the greeting card kit was interesting to work with. It's basically a paint pen, only the paint is conductive. This allows you to draw electrical traces on paper. It certainly felt like a simpler solution than trying to use wires or a some type of proto board, especially considering that I mounted the components to paper. The one surprise was how long it took for the paint to dry. It's not very conductive when it's still liquid, so you can't really test out any traces until it dries completely. I was worried that my traces would be too thin, so I put down a lot of paint. This, of course, had the side effect of taking a long time to dry, meaning I had to let the paint dry over night before I was sure it would work.
While it worked very well for a first attempt, there were a couple of mistakes I made along the way. First, the music is very quiet. In the open air, the piezo is not very loud, and I didn't come up with a good way to mount it to the card to increase the volume. If I held it tight to the card with my fingers, it was much louder. Second, the pitch of the notes was much worse on the card compared to my prototype. I developed the circuit using USB as the power supply, so I suspect the battery wasn't able to provide enough current at peak demand. The other possibility is that the resistance of the traces caused problems.
Source code as well as minimal circuit information is available on GitHub