My take on the single key keyboard:

main1 main2

Minimal Atmel SAMD21 circuit (the Arduino Zero Chip) plus a 3.3v voltage regulator, a Micro USB port and a mosfet for the LED.

Can be flashed with the CircuitPython firmware or the Arduino bootloader to act as an Arduino Zero.

PCB Schematics Schematics

Schematics, Eagle-files and firmware is on github.

The idea is blatantly copied from Glen Akins very nice Single Key USB Keyboard (his documentation is great).