…ribbon controllers are touch-sensitive strips that let you control pitch continuously (i.e., not fixed to the pitches of a keyboard). The original Martenot instrument of the 1920s had an early ribbon controller, but rocker Keith Emerson probably gets the most credit for popularizing the design.. Here are some DIY instructions for building your own ribbon controller. These aren’t just any old ribbon controller, either: not only are they much cheaper than commercial models, but they can play two notes at once. Link.
DIY Ribbon Controller