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I have a set of LED strip lights that I am attempting to control with an Arduino. When I power the first few LEDs from my Arduino 5V out pin (or 3.3V out pin for that matter) it works exactly as I would expect, I can set the colour and switch on and off.

However, I'm clearly going to need an external power supply to handle the dozens of LEDs on the strip, so I've got a 5V, 3A DC switching power supply plugged into an AC outlet. I would expect this to work equally well in comparison to the Arduino V out pin, but when I switch to the power supply my lights begin to behave erratically. Random LEDs all along the strip will flash intermittently, and the specific LEDs I'm turning on with the Arduino will flash different colours than I've set them to.

To highlight the issue I've uploaded a video to YouTube: https://youtu.be/6k9c2WWS2_4 In it you can see the lights working as expected when connected to the Aruduino, with the ground connected to the power supply's ground. As soon as I switch the VCC to the power supply things go off the rails.

I'm sure there's some fundamental difference in the nature of the Arduino pin and the power supply, but an hour of Googling hasn't cleared things up for me.

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    \$\begingroup\$ This type of question needs a schematic because there are a lot of ways that someone can figure out how to hook this up. (Youtube isn't a substitute, they can remove videos at random) \$\endgroup\$ – pipe Jan 8 at 6:10
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    \$\begingroup\$ I am not sure of what you are doing, since I think you move only one wire from the arduino to the breadboard. Do they have a common gnd? Draw a schematic. \$\endgroup\$ – thece Jan 8 at 6:26
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This is a typical SMPS common mode noise creating a differential noise voltage on the data.

These issues require an understanding of mutual coupling, cable shielding and ground network noise. Many things can fix it.

I suggest you use your fingers to connect the DC 0V to some local earth grounds. If that works, then 1nF to 10nF will replace your fingers and body.

Other solutions such as CM chokes with Pi filters to Earth ground shielded cable for all noisy for LED and uC power and signals.

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