Led Power supply question.

HI.

I have an Arduino mega controling a led grid of 512 5050 leds. (Adafruit Neotrellis boards)

I have a 5v 10 amp Power supply connected to 12 points on the grid 6 positive 6 ground ( it says ground on the board) The grid not connected to arduino.

And arduino usb connected and just SDA and SCL pins connected, no volt o ground

There seems to be aproblem with the ground connection.

With that set up nothing happens, bt if a plug a cable from the board to ground on the arduino then it works....

Educate me please.

Seems like the SDA and SCL pins that you connected to the led grid require a common ground. Basically, I2C communication requires a common ground between all of the connected modules because the current passed between the communication lines needs to get back to the source-the board that sent it in the first place. By adding a common ground between the Arduino and the led grid, you essentially completed the circuit allowing I2C communication between the boards. You can read more about this in PerryBebbington's post.

Thank you.

ALL grounds should be connected together.