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I'm looking to confirm if I need to connect the ground of 2 LED strips which have different power sources.

Power Diagram for multiple LED strips

Adafruit says

The key is to have all of the ground pins among the strips connected in common, but the +5V from each power supply should be connected only to one length of NeoPixels — those should not all be joined.

A response to the question When I connect chain of LED strips to power from both ends, do I need to make the power line disconnect in the middle? says:

For a 6m long strip, you will need to power from both ends. It is recommended that you use one power supply (10A should do depending on your LED density) and allow the ground wire to run through the entire length. The power lines in the led strips themselves are not thick enough to carry 10A over 5M without enough loss to cause dimming or color change.

However, Branden's answer in the Powering a WS2812B strip question makes it sound like I should contain the ground to each strip. (Unless I am reading his answer wrong):

So as a suggestion, make sure you power each 1m strip individually instead of tying them all together and having every LED in parallel (and the strips themselves in series). What I mean by this is, tie the +V of all the strips together to one power wire, and then tie all the grounds together. Don't just line up all the strips in a single line, end to end, and solder them. Just the data line should be like this.

I may have confused myself into a corner so I am just looking for a confirmation that the ground needs to be shared across the whole project/all LEDs and all power sources.

Edit: Title change for clarity

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  • \$\begingroup\$ I don't even know what the quote by Branden was getting at. First they say to power them individually, but then they say to tie all +V together, and tie all grounds together...Am I slow today? \$\endgroup\$ – Bort Mar 13 at 20:05
  • \$\begingroup\$ Im wondering if he was trying to describe what he meant by tying them in parallel. I got confused by that context switch as well. \$\endgroup\$ – CoJanks Mar 13 at 20:27
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You will want to tie all grounds together, including the ground of the MCU.

Remember that voltage is a potential between two points. If you have two different devices with a single common point, that will typically be "ground". That way, the potential to all other points will have the same reference (0V). Without a common ground, connections between two devices will not have a static reference.

Look at LED strip 1. If it doesn't have the same ground as MCU, then the green data wire may have "5V" from data to MCU ground, but what voltage will the strip see? It will have "5V" on data, but the ground is that of the MCU, not the strip, and thus the strip will not necessarily see 5V at all. For in intents and purposes, you could consider it "floating" which isn't really 0V or 5V.

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  • \$\begingroup\$ Perfect. To reiterate for my own understanding: The whole project will share a common/connected ground and data line, MCU included. \$\endgroup\$ – CoJanks Mar 13 at 20:32
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The grounds need to be tied. The data line is a voltage that moves high and low depending on if it's transmitting a 1 or a 0. If the ground isn't tied together the digital logic on the other strip won't be able to interpret the voltage correctly. (Because one ground might be a different voltage from the other one depending on noise, cable resistance an other factors.)

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