Have you ever been in a situation, whether in a conference room, a private office, or a room in your own home, where you can't tell if someone is inside (or if the light is on) from the outside ...

Optiboot (an older version) is installed by default on the Arduino Uno and (as of 2018) official Arduino Nano boards. It can be installed on all older mega8, 168 or 328 based Arduinos. As of 2019, ...