Got no purdy pictures yet. (Still in working prototype stage anyhow -- maybe pics after I assemble the first proper PCB).
What I've got, is a box that sends a MIDI note-on message on receipt of signal from a keychain-type RF transmitter. The original intent was so an actor with a prop gun could trigger a gunshot sound remotely. Practically, I've been using it mostly to trigger an on-stage keyboard while I gain-stage the FOH mix and walk the monitors!
Simple wiring. Got the transceiver pair I'm using now from "ColdfusionX" -- http://cgi.ebay.com/RF-Radio-Controller-Wireless-Control-for-Basic-stamp_W0QQitemZ250238926508QQihZ015QQcategoryZ1200QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I'll be experimenting with a transceiver pair from SparkFun next.
MIDI connector and the basic code was adapted from ITP: http://itp.nyu.edu/physcomp/Labs/MIDIOutput
I also built the box with a Big Red Button, so I could in theory mount it right on the set and let the actors trigger a doorbell sound (always seems to work better if the actor presses the doorbell button, rather than have a sound operator run the cue). I chose MIDI as the protocol because there is some MIDI already around most theaters; samplers, rack-mount sound modules and keyboards, even many light boards and some special effects units speak MIDI. These days I mostly run my theater sound designs off of the Mac freeware Qlab -- http://figure53.com/qlab/ and a reasonable fee unlocks MIDI control for that, as well; an easy solution to stream any length of sound effect off hard disk on command.
(The Qlab functionality includes hot cues that can be fired at any arbitrary time; these include not just sound files, but fades, cross-fades, group cues, and even transmission of MIDI sequences to external devices. Plus the ordinary cue list functions -- go, stop, pause, etc., can be addressed via MIDI. Basically, you could sit in the audience and run the entire show from a keychain remote.)
I'm wondering about the next steps, though. Going through MIDI does add to the component count if I'm just firing a cue from laptop. I might think about MIDI-thru-Serial, or even let the Arduino simulate a USB keyboard and make use of Qlab's hotkey functions. The intent is as much as possible a plug-and-play solution for theater, and adding additional licenses and interfaces is not a good way to go. Also, the whole design has been to not confront operators -- and especially actors! -- with too many choices. Bare-bones, one obvious button, direct result is the way I want it to be.
But, of course, the existence of a micro-processor moderated MIDI event generator opens up all sorts of interesting options for an interactive sound design. For instance; adding an audio input or a piezo trigger could help it solve the "breakaway bottle" problem (breakaway glass looks great on stage, but sounds really fake. A trick used in the past is to smash the bottle against a hidden mic and use that input to trigger a pre-recorded "breaking glass" sound.) As a more subtle idea, frequently I've designed outdoors ambience, street traffic, etc., that is faded up when a door or window is opened on set, and faded down when it is closed again. Seems to me that could be triggered automatically as well!
The question is what I can shove into a box before it stops being a no-frills, plug-and-play solution, and turns into an electronic music project with all sorts of dangling sensors and ancillary software to set sensitivities and other parameters.
I'd love thoughts from anyone, particularly other theater people. This will of course be an open-source, open hardware project, as my intent is to make these options available to the theater community.