First post here, hello everyone!
I thought I could give some information I've found regarding DIY expression pedals used with helix. After reading through a few threads discussing how HX compatible expression pedals are put together, I thought I'd try to build my own. I ordered a wah enclosure kit, momentary foot switch, TRS jack, and 10k B potentiometer (specifically one from Dunlop that is meant for a wah pedal). I'm using an HX Stomp XL in case that matters.
I used alligator clips to experiment with wiring before I soldered anything. I could only get the potentiometer to work correctly when wired to the ring of the TRS jack, not sure why. I had the EXP/FS Tip and Ring settings configured correctly for each type of wiring, also had the EXP positions set to global.
I also ran into an issue when combining the expression pedal with the footswitch. My enclosure is very similar to a crybaby, where pressing the footswitch tilts the treadle "past" toe position. The HX recognized that maximum forward position from hitting the footswitch and assigned it to 100%. Therefore, when resting in the conventional toe position, I would only get to 65% or so. I tried assigning a max limit, but that did the opposite of what I wanted (where a properly working toe position would result in 65% position on the hx).
I had initially wired lug 2 to ring, and lug 1 to sleeve. I finally found that wiring lug 3 to the ring, and lug 2 to the sleeve of the jack made it work; especially after tweaking with what portion of the potentiometer the pedal swept through. I measured the resistance at toe and heel position: toe was 9.4k ohm and heel was 0k ohm. I don't know how consistent this is across units, YMMV. My hypothesis is that HX is designed with a max resistance/voltage that allows for that extra travel in order to use the footswitch. The way I had wired the pot the first time put that resistance at the heel end rather than the toe end. Not an electrical engineer, so I'd love to hear others' thoughts.
Hopefully this helps out anyone looking to make their own Helix/HX Stomp compatible expression/footswitch combo.