[CS3 AS3] Controlling A Circuit?

Hello,

im trying to find a way to control an external circuit, using at least flash to do the base control.
this is for a desktop app that would always run on the same computer.

ie, someone hits a button in the flash app… and it turns on a LED light in a sign.

Anyone know of anything like this? Trying to find the right direction to go in.
Thanks!