The Volume / Expression Selector Box allows you to toggle between Volume and Expression functions on your Dunlop Volume Pedal without needing an additional pedal for expression functions.
This box runs on 9v power and powers the LED lights, the Blue light indicates the volume mode and the Red LED indicates that you are in expression mode.

For the expression to work correctly it needs to be a TRS cable from the pedal to the Selector Box and from the Selector Box to the Volume Pedal.
 
Power 9v Center Negative
Power Draw 1mA
Size 3.6 x 1.5 Inches