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I need to power my camera peripheral with an external power supply, since apparently I can't power both the camera and the camera's relay switch with my 5V out pin (My relays cut out and continually reset themselves if I try to run my 5V to both the camera relay switch and the relay board VCC.)

So I'm powering the camera with a 5V battery pack and my arduino with my PC, but I still need to communicate with it via UART, so I need the camera ground to be the same as my arduino ground. Can I connect the two? Is this a bad idea? Because I'm doing it now and it seems to work. I just don't want to cause a problem later on down the road, whether I'm connected to my computer or on battery power.Any advice would be greatly appreciated.

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A battery's ground is floating. That means that it's "happy" to take on any absolute value by simply connecting it to another ground - it just stops floating. This is why it's easy to connect a battery-powered phone directly to a computer to get the photos off or whatever.

The only time you shouldn't connect two grounds is if neither are floating. A good example would be the grounds of two different houses, which are physically connected to the Earth (as in the planet). It'd be difficult to budge the whole Earth's ground level!

But shouldn't they be the same? Unfortunately not. You could measure a distinct voltage difference (sometimes in the hundreds of volts) between two house earths due to the differences in their makeup.

One small note: if you do connect two grounds, only connect them in one spot. If you have two other ground pins a small distance away from the connected ones, there may be a voltage difference between them: now the two are explicitly relative to each other, so neither can be considered floating relative to each other. The pair as a system - if neither are earthed at least - is still floating.

These so-called "ground loops' can induce current where none is desired.

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  • Outstanding answer. I guess I never realized this happens every time I plug a phone or a camera into a computer! – LegitimateWorkUser Jun 30 '16 at 16:36

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