Neopixel LED rings: LED flashes when using show

In my main loop I have:

  strip.clear();
  strip.show();

When strip.show() is called a random LED blinks very quickly. I have tried using the FastLED libray and that does the some thing with the equivalent show command.

I am using a 12 LED ring with a 1K resistor on the data line.

The power to the LED ring is off a separate 5v source and has a 1000uF capacitor across the 5 volt input and ground.

Just to be sure, you did connect the ground of the separate 5V supply to the Arduino ground?

No, it's connected to a separate power supply i.e. the Arduino is powered through the computer USB and the LED ring from a USB charger plugged into a powerboard. The computer and USB charger are on the same circuit.

What @koraks is saying is that you have to connect the ground of the strip to the GND of the Arduino.

If you don't, the data signal from the Arduino does not have a return path to the Arduino and hence no current can flow and your setup is not working.

Ah, I see. Is it fine to connect the ground from the LED power and the Arduino power?

Not only fine. It's necessary if you want the whole thing to work - and to survive.

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