Since I started building circuit sculptures I had in mind to create something interactive. With the don't touch anything mantra of our present time in mind I got the idea of building a piano without buttons/keys. I started by finding a suitable reflection coupler. The CNY70 works quite well since the working distance is a couple of millimeters. Then, as for the tone generating circuit, it was clear that I would use the 555. A square wave is not the most musical sound but at least the circuit is easy to build. With one for each tone it's a must that this is simple. To me a building a circuit sculpture means that the circuit doing something should be the sculpture itself, like this beautiful breathing person. Therefore using a microcontroller for generating the different frequencies was simply a no-go. I used a variable gain amplifier with the reflection coupler to imitate pressing the keys. This allows even dynamics. Here, the square wave input was advantageous because I didn't have to care about linearity. I designed the amplifier such that it saturates at some point. Finally, a simple mixer and amplifying stage.
Of course, this sounds rather simple but it took a lot of hours to complete.
The components list is for one key and the mixer/amplifier stage.
Videos of testing the prototype and of playing the CoViano are in the files section.