Boss

The FV500L from Boss is a both a stereo volume pedal and an expression pedal for use with keyboards and other low impedance instruments. When the pedal is plugged in between a keyboard and an amplifier (or an input on a recording device), rocking the pedal back and forth will raise and lower its volume. If you have an amplifier or an effect that can be controlled by an expression pedal, the FV500L has an output that enables it to be used as an expression pedal.