A design for a small synth created by Rob Hordijk. The Benjolin consists of two voltage controlled oscillators, one voltage controlled filter, and a circuit Hordijk invented called a Rungler. The Rungler is a quasi-random control voltage generator whose output can be routed to the VCOs or the VCF. The Rungler itself is driven by the VCOs, so there is a feedback relationship between the two, creating a variety of patterns that seem to repeat but seldom actually do.
Hordijk publishes the Benjolin design as open source, and a huge variety of variations exist. The Benjolin is designed to be built by DIY'ers, and Hordijk frequently hosts workshops in which the participants build a Benjolin in the process of learning about synthesizer circuits.