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The slide mechanism

A project log for Digibone

A digital instrument with all the flare of a trombone, with the power and flexibility of an MIDI instrument

Craig HissettCraig Hissett 03/16/2018 at 21:502 Comments

This is the area that will need the most thought, investigation and research I feel.

7 distinct note positions, with the areas in between still selectable, all in one smooth and easy to use mechanism.

My first thought was to somehow fabricate a >50cm slide potentiometer to mount in the frame.

Second idea would be to use 7 standard slide potentiometers all in a line, and create a mechanism for the bar moved by the player to move the slide pot as it passes. 

If pot 1 is reading 0 we know the slide is in position 1 and would sound note one on that pitch; any other reading would indicate a pitch bend down in that note (as the slide has been moved out). Once pot 1 hits 1023 we know that the slide has reached position 2 and can start reading pot 2 for the pitch bend value, and so on and so forth.

This second way would take a lot of refinement to get a smooth output from the Arduino but having the range of 7 full potentiometers (compared to the one pot idea) would give a far greater degree of accuracy.

Discussions

Morning.Star wrote 03/20/2018 at 15:12 point

I've thunk a bit about that myself, but not for a trombone lol, not even an instrument... I made a few test pieces that worked out well though.

A handmade 50cm potentiometer is actually really simple, although it would have a considerable end-to-end resistance. Used as a voltage divider for sensing, thats not really a problem though...

All you need is a substrate - bit of plastic, whatever. Graphite, tape and glue. Here's how I made one:

1 Lay two double-thick strips of insulating tape down in parallel to make a trough on the substrate.

2 Mix up some standard (not quick-dry) 2-pack epoxy and blade it into the trough level with the tape.

3 Cover it with graphite powder and tamp it into the epoxy with a rolling pin or something like that. Its a good idea to strip and kink the ends of some wire here and bury it in the graphite.

4 Blade off the excess level with the tapes right away, epoxy down the wires and leave to cure.

5 Run the blade down the tape edges to break the top surface cleanly and then remove the tapes. Burnish the surface with a piece of tinfoil until its shiny for the best results. ;-)


A graphite potentiometer like this measures a few K Ohms per inch and is very hard-wearing as graphite is self-lubricating and doesnt oxidise. It only needs a light brush to make decent reliable contact with it too.

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Craig Hissett wrote 03/20/2018 at 00:01 point

Further thoughts on this one... Building the slide separate from the back section means there's nothing stopping me from having different slide configurations.

I'm tempted to do one using ping sensors that will function like a skewed theremin 😎

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