Just a quick think-out-loud about how to read the slide positions for my pitch bending shenanigans.

I'm pretty much set on a slide frame with a freely moving handle inside. 30cm of travel would be great, as it would give me the following positions:

1st position - 0cm

2nd - 5cm

3rd - 10cm

4th - 15cm

5th - 20cm

6th - 25cm

7th - 30cm

It's just under half the size or a normal tenor trombone length, but it will be perfect for the compact build I am after :)

So far, I have two options for tracking positions on the slide:

1) encoder, belt & pulley

I like this idea; encoder at one end, handle connected to the belt to rotate the encoder. Would be easily enclosed, plus end switches at position 1 & 7 would help calibrate it.

2) Ultrasonic sensor

Fixed at the top, offset from the handle. The handle would have a small 'paddle' opposite the sensor.

Care would need to be taken to not have the hand on the handle affecting readings. This would make it like a shoulder mounted Theremin. I like theremins, ha ha ha.

I'm open to more suggestions :-)