[sdiy] MIDI to solenoid/optoisolator/relay interface?

[Glen]

I've seen plenty of products and projects that interface MIDI in various
ways. There are key switch scanning circuits that add MIDI outputs to older
keyboard-based instruments. There are MIDI to CV converters, to add MIDI
inputs to modular synths. You get the idea.

What I'm having trouble researching on the internet is circuitry that
accepts an incoming MIDI stream and controls either solenoids,
optoisolators, or relay contacts. I have a few uses for such a conversion,
but have trouble finding much on the internet.

Part of the problem is not knowing what to call this sort of MIDI
interface. I appreciate any pointers. I'd prefer to build my own project,
but I might also be interested in existing commercial interfaces.

thanks,
Glen