Building the box

It's important to have an airtight box aside from the sound hole(s). It's good to get all the joints as solid as you can get them. I believe, the more uniform the material, the better it will be able resonate. I designed this with a removable back and bike inner tube gasket for serviceability, but if I wanted a better sound, I could have sealed the bottom permanently. Fortunately, with the effects and amplification added, it's not too important to get the design perfect. It's important that the area around the bridge is perfectly flat, otherwise you'll have buzzy tongs.

It's made of discarded acrylic, laser cut with a 30w engraving machine. I used an online tool to generate an SVG for a basic box, and finished the design work in Inkscape(a real pain for CAD-type work). Next time, I'd like to design in a cad program. The fractal sound hole was designed in ContextFree. The current design files do not include holes for the teensy, battery holder, or power switch. I had to drill those on this version.

I epoxied 3 of the pieces together first, then 2 more, then the back plate, simply holding them together until the 5 min epoxy cured enough. Next time, I'd like to get a longer working time epoxy and use clamps or elastic to keep it held together.

Building the bridge

The bridge is made from 2 pieces of scrap hardwood that I dumpstered. They needed to be as flat and strong as possible. First I marked one of the pieces by holding it up to the holes etched through the box. I drilled a hole the size of the bolt through both pieces of wood. Then I put the first bolt through to keep the wood aligned for the next holes. On the inside piece, I drilled halfway through with a larger bit, then, using an alan socket, I punched the hole into a hex shape for the nut to tighten against.

With the bridge in place, I put a spacer against it so I could consistently run a file just below the bridge. This made a shallow notch for the steel welding rod(bench ground to length) to rest. It's important the notch is even and cups the rod well, so that the sound can transmit well.

Making the tongs

The tongs are made of this spring steel that some kalimba makers use from the website https://www.kalimbamagic.com/shop/accessories . In the future, I'll be trying to source the steel in bulk here in Portland, where I live. I've heard of people using the steel strips from auto wind shield wipers.

The steel comes in a roll, so I started by bending that straight by hand. I made lengths of about 9cm to 14cm. To cut them, I used a bench grinder to make a notch. I immediately bent the metal at that notch and it broke away. I then smoothed each one out on the grinder. To get a second row of tongs up above, I used a bench vice to hold 3 tongs at a time at equal lengths while I used a block of wood to bend them evenly. I wanted to start with a bend about 1cm from the playing tip, then about 1cm further, make the opposite bend.

Putting in the components

I got a pack of contact piezo mics from Amazon and broke apart the plastic casing they were in to expose the bare mic. It is a metal disc with a very fragile layer on top that a wire is soldered to. I chose to scrape off some of the residual glue, which was risky because these things are so fragile. Do not let the mic touch the bolt, else it will have noise.

I used an 18650 for the battery. These are easy to find online or vape/hobby shops, as it's a very common size of lithium ion cell. I could've gone with a rechargeable AA or AAA pack, but I had this on hand, and they hold more juice. I cut off 2 of the fins of the holder because I knew it would be too hard to remove the cell otherwise.

The 1/8" jack was hot glued into place with it's wiring pre soldered.

Wiring it all up

I soldered wires to header pins to make all the connectors I needed. If someone wasn't able or willing to solder (aside from the teensy), you could get away with buying lots of female header pins and krimp on electrical connectors. The cheap 1/8" audio jacks I used forced me to solder quickly, else they'd melt and fail. I added 2 extra wires to the 1/8" jack to go straight to the 1/4" jack.

I usually do my wire stripping with a lighter because it's very gentle on the wire. All the pots share grounds and positives, and I was able to use the lighter to remove stripping inline along the wires, which I tinned and attached to the pots with only 2 continuous pieces of wire. It's good to give the back plate's wiring harness plenty of length to make it easy to work on. It's good to ziptie it at the case, and evenly near the pots(so that tension on each of the wires is equal) to keep everything stable.