At the HAM Radio fair in Friedrichshafen, I found a pedal for musical instruments at a good price at the flea market. Since I had always wanted to connect such a thing to a synthesizer, I bought it for a few euros. It was a volume pedal, but I thought, nothing is too difficult for an engineer, I’ll figure out how to convert it somehow. This is what the piece looks like:

The circuit diagram of the Roland EV-5 served as a template for the conversion. The minimum volume potentiometer (50kOhm) was irrelevant to me; when connected to modern keyboards, the effective range is configured more flexibly anyway:

The conversion was not that simple after all. Of course, a logarithmic potentiometer is installed in a volume pedal, so the effect when moving the pedal was completely unusable: At first, nothing happens, then the value changes too quickly. So the potentiometer had to be replaced with a linear one. I thought it would be easy, but then the next problem became apparent: The built-in potentiometer has only a rotation angle of about 45 degrees! Fascinating, a kind of special manufacturing for pedals! A quick search through Google and the usual musician forums quickly made it clear: You can't get something like this just anywhere; rather, you can’t get it at all. Cynics might think that pedal manufacturers deliberately do not want to offer this type of potentiometer on the market. After all, buying a new pedal brings more revenue than a repair. Not sustainable at all, and even more of a reason not to give up now.

In the search for a solution, it first seemed interesting to see what the other side of the keyboard looks like. Here is an excerpt from the circuit diagram of an old Roland U-20 keyboard. The potentiometer safely receives 5V at the upper end of the resistor track, ground at the lower end, and the wiper is captured by an analog-to-digital converter. Additionally, there is a pull-up resistor to 5V. The circuit is conservatively designed, and shorts between the connections or misconnections can’t harm it:

A measurement on a Roland FA-06 revealed that the developers had probably switched to the typical 3.3V by now. A look at the EV-5 circuit diagram shows that a 10kOhm "load" is apparently tolerated without any problems. That would be 330uA. The idea: That’s enough for a low-power operational amplifier! If you use a rail-to-rail variant, like the TS912, then the following circuit can be realized, which compensates for the limited rotation angle and allows the use of a "normal" 270-degree potentiometer. The unused half of the TS912 is connected in such a way that it remains in a defined state:

[Circuit for Rotation Angle Compensation]

The circuit also fits onto a small PCB in the pedal, here still without the diodes shown in the circuit diagram:

The entire circuit draws just around 300uA, so it's no problem for keyboards that operate on the principle shown above and supply at least 2.7V but no more than 16V (the TS912 can’t handle more than that). R4 was included just to be safe. Overall, the circuit cannot pose a "danger" to the keyboard by design. Although it is active, it cannot supply additional power or increase the voltage. Somewhat against the rules, decoupling capacitors were omitted to prevent energy storage in the circuit. Since it's only about very low frequencies, this shouldn’t be a problem. The diodes conduct in case of reverse polarity and thus protect the OP. Also, no danger for the keyboard, since it must tolerate a short circuit, as always happens in the event of a cable break with contact between the wires.

Calibration is simple. First, loosen the screw on the lever at the axis of the potentiometer and set the pedal to the non-pressed position. Then turn the potentiometer's axis to the ground side and tighten the screw. Now, press the pedal fully down and adjust the output voltage to the maximum value using the trimmer R3.

Have fun building it! Even though the risk is very low: Of course, I offer no guarantee! In any case, the output voltage of the keyboard should be checked with a multimeter beforehand.