You are reading a single comment by @ltj and its replies.
Click here to read the full conversation.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@ltj
Both supplies, USB + led, has it's own ground. These must be tied together for the circuit to function. The pin strip is connected to the led supply ground, so the yellow wire simply connects the ground of the Espruino to the ground of the led supply.
...and don't hesitate to keep asking questions here :)