Paulo Feliciano first introduced me to this fantastic instrument, which we also used on On Air. I developed it further and today it's one of my main instruments, used in both the Space program and in one of Sei Miguel's  The Tone Gardens quartets (listen). It features a voltage-controlling potentiometer and a switch to open and close a feedback circuit.

Featured on Space Study 2.
Video of Space Study 2 live

Appears on Space, Space Solo 1 and also on the Noise Precision Library's Space Study 2 mp3 album.