Bela Trill Flex

[BJG145]

Every now and then I pull one of my half-finished (OK, 10%-finished) projects off the shelf and look at it. I've still got a couple of Misa Tri-Basses with faulty touchscreens knocking around that I wonder about repairing or rebuilding, and I recently pulled the fretboard sensor off one of them.



It's a neat design; a compact 60-contact I2C-based capacitive-touch fretbard. But the connections and communication are undocumented and would be too difficult for me to figure out. Besides which, using this to rebuild a Teensy-based Tri-Bass with new code, touchscreen and wiring would be a lot of effort to go to for a one-off component. So I've been looking around for something else to use...and I think I just found it.

https://bela.io/products/trill/

I've been aware of the Bela Trill range for a while, but I hadn't clocked the fact that the Trill Flex is based around the idea of creating your own custom capacitive touch sensors, with up to 30 contacts. The Tri-Bass fretboard has 60, but I'm thinking I could join two in the middle.





There's a step-by-step guide on how to do it, so I've decided to have a go at this. Just ordered my first gadget of 2023...a Trill Flex..

[Wonks]

I'm not quite sure how that will work for a bass unless it's monophonic. The Trill flex will give you positioning along the fretboard, but obviously the strip positions are equispaced, so will require complete position re-learning for a traditional bass player. Your Tri-bass has the sensors reducing in size as they move up the fretboard.

The Trill Flex also has overlapping sensor areas, so you can easily trigger two sensor strips at the same time. If you are dealing in standard semitone intervals, which one do you pick? You'll need to have some sort of wide fretwork arrangement on top of the strips so you can't touch two strips together.

There's no side-to-side positioning on those strips, so I'm not sure how a 4x4 keypad will help trigger more than one sound at a time (if you were planning to use it for note triggering rather than for instrument control functions). For monophonic use you could have four touch pads (or as many as you want the instrument to have strings), so those provide the string selection and note triggers.

Otherwise I suppose you could try having say three strips per fret position, one per 'string', (the tri-bass was a 3- rather than 4-string instrument) and rely on slanted finger positioning to determine which of the three 'strings' you are fingering, but you still run into the 'one finger touching two strips together' issue so lots of mistriggering.

[BJG145]

Hmm, I'd been thinking of cloning the Tri-Bass layout with a custom Trill design but I haven't read the small print yet so perhaps that's not possible. The above pics are just examples though, I think you can design whatever 30-touch layout you want...? So a couple of these should do the trick...maybe.

[Wonks]

A custom PCB could do it, and give you a 3-string 20 fret bass using two of the modules. Getting the PCB connected to the two modules would seem to be the biggest task, the thin edge connectors used being more suitable for the bendy all-in-one PCB of the flex kit than a solid PCB. And if you manage to find someone in the UK to make you a custom bendy PCB at an affordable price, then you still have the problem of getting two sets of connectors out of the end of a pretty narrow PCB.

It's probably not impossible if you widen the PCB as it runs up the neck like a real bass. That Flex PCB is 22mm wide, so you could probably make the neck wide enough to get two sets of 32-pin connectors and the boards side by side at the end of the neck.

I was wondering if the Trill Craft might be better for a solid PCB design as the connecting wires would probably be easier to connect to a PCB.
https://shop.bela.io/products/trill-craft/

[BJG145]

Spent an hour or two with the Trill Flex today, and it's quality stuff. The Bela range generally seems to be well designed and documented so I able to get up and running fairly quickly.

The Trill Flex, or your own custom design, clips into a small PCB which is then easy to connect to Teensy/Arduino via I2C. This is a simple way to connect and daisy-chain devices, needing just +5V, Ground, and a couple of data lines, and Bela have produced introductory code examples. I was pleased how well these reported position and also pressure for the basic Trill Flex, this being calculated from coverage, or surface area.

Will now proceed to step 2, which is where I have to gen up on KiCad to create the fretboard layout...

[Folderol]

Interesting.
... but too many ideas, too little time

[Wonks]

So does it give a discrete strip position so that you get a '1' reported for each of the zig-zag segments touched as well as a pressure figure for that segment, or does it somehow calculate a continuous position figure for the whole length based on the segment or segments touched and the amount of finger coverage per segment?

Just trying to understand what sort of info you get back from it.