This is a basic radio panel that works with X-plane.

The hardware is an Arduino, connected to an LCD display 
and a matrix keypad. 11 keys are used to implement the
six radio choices: com1, com2, nav1, nav2, adf1, adf2,
to switch active and standby frequencies and to increment
and decrement. I use two buttons for increment, two for
decrement, as I don't yet have suitable rotary encoders.

The Arduino side is completely dumb. All interactions are
sent to the X-plane plugin, which sends all D values back.

The plugin is written in Python and requires Sandy Barbour's
python interface plugin for X-plane, and it requires the 
PySerial library to be installed. 

This was written on a Mac. If you want to work on another 
machine, you'll need to edit the port name in line 125. Even
on a mac, your com port will probably have a different name 
as it is set by your USB to serial driver.

Bob Gates
7 Jan 2011