This is a simple Arduino sketch that can be controlled from the TEST_ARMACOM.vr mission in ARMA 3
With this sketch, a connected Arduino's built-in LED can be turned on or off by sending it the Serial string "on" or "off". The Arduino will also send out a message when a button connected between pin 12 and ground is pressed.
You can control an Arduino running this sketch from this ARMA mission: https://github.com/FatLurch/TEST_ARMACOM.vr/tree/main
For anyone not familiar with Arduino, this sketch file must be uploaded with the Arduino IDE: https://www.arduino.cc/en/software
This article shows how to upload a sketch to an Arduino: https://www.arduino.cc/en/Tutorial/getting-started-with-ide-v2/ide-v2-uploading-a-sketch
I based this example around the popular Arduino Uno, however this should work with any Arduino that features a built in LED: https://flaviocopes.com/arduino-built-in-led/