I have an "Expression pedal" that worked fine on my old keyboard (Gem WX2+) but works almost like a digital switch on my new keyboard (Roland VR-09).
The recommended expression pedal is a Roland EV-5 which has the schema at the bottom right of this page:
I measured the pins of my old pedal and the schema is probably like this (have not opened it yet):
An important difference is that the Roland pot is Lineair, while mine seems to be logarithmic. When close to the heel position I can easily navigate close to a wanted resistance. That becomes increasingly difficult moving closer to the toe (fully pressed) position.
I think it should be possible to tweak it to a more linear like curve by making an adapter, adding a resistor between the tip and the ring to pull the curve up somewhat. And while doing so I can just as well add the minimum volume feature as well. This is what I have in mind so far:
I'm not an expert at electronics, so I would really appreciate a peer review of this idea before I order the parts. I'm not sure if the value of the curve correction pot is ideal? At the maximum setting, the total resistance would be 9.6K which is a little lower than the 10K of the Roland pedal...
In fact I'm not sure if this idea would work at all, but since a new pedal is quite expensive and has a smaller "throw/range/angle" than my old one, I'd really like to keep my trusty old pedal with a hack if possible. Any other suggestions are welcome off course!