Robin Newman has taken a Raspberry Pi, a RasPiO Pro HAT and an Adafruit MPR121 touch capacitive breakout and created a touch-sensitive keyboard for Sonic Pi. He’s documented the whole process over on his blog along with all the installation instructions and scripts you need to create your own. Read more here. As he points out, you can do this in a variety of ways. You can see it in action controlling Sonic Pi on a Mac below: