My previous version of the Big Red Button featured the control board of an actual keyboard. The Beetle board, featuring the ATmega32U4 chip, and an onboard Type-A USB connector, which is very good for soldering. Don't worry, the actual chip is the same on all boards, official or not. This is an important factor, as the official Leonardo board comes only on a big bulky board, which won't fit into the box. The actual board I use is a non-official version called the Beetle, which is the tiniest of all the versions I have seen so far. This enables the programming of different Human Interface Devices (HID), such as a keyboard. Unlike the Uno, Nano and Mini boards which all use the ATmega328P chip, this one runs on the ATmega32U4, which features a built-in USB controller. It is based on an Arduino Leonardo board. It can also have multiple functions for the same button with single click, double click and long press. It can be an actual keyboard key, a combination of keys, or even a sequence of keys like a macro. What that button is, is up to the implementation. The Big Red Button is technically a keyboard with a single button.

Board press the red button