I put together a first prototype for a touch keyboard which at the moment sends MIDI through USB. Sometimes when i’m at school i want to write MIDI to recorded songs so the idea is to make a really small portable keyboard for MIDI. But i’ll also probably put together a touch keyboard with CV output for testing my modules easily without taking up too much space with a regular keyboard size.
I used copper tape and the MPR121 breakout for the prototype. There’s a variety of companies selling MPR121 breakouts, i use the one from Adafruit and would recommend them because of my prior experience with their products/quality.
There’s a basic code example for the MPR121 so you could get away without having to program too much but if you want to adjust sensitivity etc you will have to do some testing with different codes.
The MPR121 works through L2C which makes the number of pins required minimal. The Adafruit breakout features 12 inputs which gives one octave (C-C#-D-D#-E-F-F#-G-G#-A-A#-B).
Dimensions and sensitivity
The first design on the picture has 20mm (without the gap) wide keys and 12mm wide black keys. But i would recommend making smaller sized keys since these felt awkward to play. I tried using overhead-paper(transparent plastic paper) on top of the copper surface to test conductivity through the material and with the standard configure on the mpr121 it could sense touches through 3 overhead-sheets (300 microns).
You could probably get thicker by adjusting sensitivity and having just one sheet of 300+ microns (i just bent a 100 micron sheet 3 times (as layers) so the contact between them was far from perfect).
I used a wide copper tape across the whole surface and then cut it with a “design knife” (a really sharp knife). I ended up cutting some of the surface underneath the tape so i would suggest to cut the tape before putting it on the surface and planning it a bit more. I put copper tape with conductive-adhesive on top of the dupont cable connectors to attach the cable to the “keys”. A warning that some copper tapes don’t have conductive adhesive which only gives you one surface of conductivity to work with. I put another strip of tape on top of the dupont cables to keep them steady against the case.
Theory after practice
It would probably be easier to use a scissor in this scenario. I used non-permanent marker to draw the dimensions. You could easily find better solutions to applying your design to a surface but i like the hands-on technique just to spark motivation for making a better design (since no one probably will get it right the first time anyway).
Ideally you would want to place the conductive material (in this case the copper tape) on a sturdy material unlike my “on top of a case-intended-for-a-iphone-case”. Some copper tape can work well with soldering and that would be ideal way for attaching the wires to the keys.
Also it is obviously really easy to mistakingly play two notes instead of one since the keys do not have any depth/height
In the drawing for the next design I marked the parts of the key that i would actually need to have conductive and the parts that would benefit from not being conductive. This would hopefully help to avoid play double notes when intending to play one. It would also make it easier to play in case you had bigger fingers. I also scaled down the keys.