Harp guitars are a neat weird instrument that never really took off:
You play it like a normal guitar, with six fretted strings, but there are also some number of extra harp strings you can pluck.
I see four main downsides:
They're bulky and hard to transport.
You need to tune the strings to match the song.
You don't have many note choices.
Guitars are worse than mandolins.
We can fix all of these: an electronic harp mandolin! It wouldn't need to be much bigger than a regular mandolin, it's electronic so easy to switch between tunings, and narrow sensors can let us have two axes to choose notes from instead of one.
I had previously been thinking about simple switches, but it really would be nice to have something velocity-sensitive.
Here's the idea for the pluck sensor:
The circle is a cheap little piezo disc, the same kind I used in my stomp sensors. I was thinking of maybe 3D printing the angled bit, but was worried about durability. Instead I went to the hardware store and wandered around until I found a 1/16" aluminum offset angle:
The cross section is an 'L', 3/4" on one side and 1/2" on the other. I used a hacksaw to cut it down to the width of one of my piezo discs, and filed the rough bits and sharp corners. I epoxied the disc onto the angle, and put extra epoxy on top to protect the solder joints:
Then I cut rectangles of sorbothane I had left over from a previous project into rectangles, and used a cyanoacrylate adhesive to attach them to the metal angle and the base plate:
After letting it dry for a day I hooked up some old 1/4" cables and plugged it into an audio interface for testing:
Here's what I get for the two signals when plucking one of the two teeth with a 0.6mm pick:
This is really good news! There's a clear sharp peak on the channel corresponding to the tooth I plucked, and a tiny signal on the tooth I didn't pluck. The one I plucked is also nicely damped, so I think I won't need to worry about distinguishing multiple plucks in short succession vs continued vibration. You can see the 60hz hum of the power grid as well, but it's nice and small relative to the signal.
One very interesting thing is I get clearly different signals when plucking downward vs upward:
Another pices of good news! This lets it be bisonoric, like a diatonic button accordion or harmonica with different notes on the in and the out, for twice as many note options in the same space.
Here's what it looks like if I move my pick in a circle, alternating down on one and up on the other as fast as I can:
There's still a lot more work to turn this into an instrument. Physically, I need to figure out exactly where I want these on the mandolin, shape a base plate, and attach it. Then comes the electronics, figuring out what chip I want to use, making a circuit to get the output in the right range, and wiring it up reliably. And then the actual coding, interpreting these signals so I can reliably say which one was plucked, in which direction, and how hard.