With a microcontroller running the eos430 firmware you can connect physical controls such as toggle switches or push buttons to your flight simulator (or other programs) through Helios.
Helios is a program made by Gadroc that allows you to control your flight simulator using a variety of input devices, including DirectX compatible joysticks, virtual panels displayed on a touch screen, and custom hardware.
It supports some popular simulators directly and can also simulate key presses to control anything that has a keyboard shortcut.
EOS is the name of the protocol used by custom hardware to talk to Helios over a serial port.
eos430 is an implementation of this protocol for the "MSP430 Value Line" series of microcontrollers made by Texas Instruments.
The documentation lives in the gh-pages branch of this
repository. You can view it online on GitHub.