"The internals are quite simple: eight solenoids triggered by a Raspberry Pi via a relay. To make the project interactive, I created a browser-based step-sequencer. The sequencer is available on GitHub, so you can build your own junk drum machine or modify it to fit your needs. Be sure to check out the video".
If you'd like to make your own Junk Drum Machine, there's a detailed guide on Instructables.com