Jeff at Totus Terra has been working on an Arduino based sequencer. He has a three part series on his blog.  Part 1 takes control of the SX-150 pitch control.

Part 2 is a build on the basic sequencer note-on/note-off control.

Part 3 is where it comes together with a hardware-and-Arduino based 4-step sequencer for the SX-150

Jeff has included complete instructions, explanations, schematics and Arduino sketch files for this build.