Touch Keyboard PCB

Ever since experimenting with copper tape as a “touch keyboard” I wanted something a little more useful on a pcb.  I always quite liked the design of the stylus keyboard for a Stylophone, so that was my starting point.  I’ve not found anything “off the shelf” that wasn’t part of another build, so I thought I’d have a go myself.

This is the result.

Update:

• Find the Build Guide here: Touch Keyboard PCB – Part 2

Tutorials used in this project:

• Arduino MPR121 Touch Piano
• Notes on KiCad for PCB design: Arduino Uno Dual Merge MIDI “Shield” – Part 2

If you are new to microcontrollers, see the Getting Started pages.

The Circuit

The circuit itself is literally just a set of header pins with connections to what will become pads on the pcb.

I did consider the idea of allowing the use of a common MPR121 breakout board, but in the end, I opted to keep this to just the keyboard.

I’ve created two “circuits” as I’m planning on having two “keyboards” within the 100x100mm footprint of the pcb.  I couldn’t decide if I wanted a purely “touch” keyboard, or if I wanted to support the use of a stylus.  So, I’ve provided two designs, one of each.

The “touch” keyboard provides a pin header connection for 13 notes and a GND connection, so 14 in all.  The “stylus” keyboard provides a pin header connection for 13 notes, a GND link and a common connection for the stylus, i.e. 15 in all.  It also has a connection point for the stylus in addition to the 15-way header.

PCB Design

There are several points to note in the design of this PCB.

• I want to provide a full octave of contacts but to stay within a 100x100mm pcb size.
• However, I’d like the option to use several keyboards side by side, so the high “C” pad has a slightly wider gap which hopefully will allow it to be cut off if required.
• One of the designs supports a capacitive touch interface (top), one the use of a common connection for a stylus (bottom).
• The capacitive keyboard has a full layer of solder resist over all the pads to provide an insulating layer between the pad and what will be touching it.
• The stylus keyboard has each pad left uncovered by the masking layer.
• The pads themselves are custom “pad” footprints in KiCad so tracks can be connected anywhere on the pad.
• There are covered pad footprints which don’t include a mask layer, but do include a silkscreen edge pattern.
• And there are “open” pad footprints that include a mask layer to leave the bare copper of the pad exposed (as shown in the lower half of the figure below).
• The placing of the pads was done via calculation to ensure they all line up and have equal spacing.
• I’ve opted for GND fill zones on both sides of the board.

Closing Thoughts

This one will be very experimental as I’m quite out of my depth now in terms of use of ground planes, and large capacitive areas of the board!  But I’ve got to start somewhere.

This is the fourth board being made as part of a round of discount vouchers I’ve had from Seeed Fusion.

Kevin