While newer Arduinos and Arduino compatibles (including the Hackaday.io Trinket Pro. Superliminal Advertising!) either have a chip capable of USB or rely on a V-USB implementation, the old fogies o… ...

The Arduino Uno board detects the tattoo artist’s gestures via a distance sensor, and signals the machine with an on or off output. Watch this video on YouTube. Source: AH: WTech.