This project idea came when I wander what are the different mechanisms used to make animated popup cards. I found a (french) blog with rather good descriptions and in the middle of this page some simple technique was illustrated by a lovely piano designed by Maria Voctoria Garrido.

This did not look like a good start for a hackaday.io project... Until two tenths of second later, when I feel like I needed to put some aluminum foil behind the keys to turn them into capacitive touches.

Paper, glue, aluminum foil: all this is "free". How cheap could I make it?

-Is a CR2032 battery enough to power it?

-Could the battery holder be made for free from paper ?

-What are the requirements for the mCU ?

-Can the piezo discs be bought for under a dollar ?

I hoped to stay below 5€, but finding cheap and good looking might be harder than I expected.