I have a handful of Raspberry Pi 3B systems and one of them is doing something odd.

It is working fine but the red power light is blinking.

It is linked by serial to an Arduino Nano (to read voltages and temperature and humidity, and to program via the Arduino IDE), and the Nano has a red light that is flashing in unison.

All of these systems are programmed and wired very similarly. Almost identically.

It isn't causing any problem that I can detect.

But my question is: Why is it flashing like this? Can it be stopped?

--UPDATE: Based on comments and the wisdom of the community it was determined that the light flashes (or may refuse to come on) for only one reason: low voltage. Turned out the power cable was the problem. It was one that had a power switch in it. Swapping it out to a straight-through cable without a switch ended up solving the problem.

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    The red power LED flashing indicates under voltage – Steve Robillard Apr 17 '17 at 3:34
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    Lack of voltage. Most of the times, it's due to using a cheap Micro USB adapter. – Zachary Vincze Apr 17 '17 at 3:54
  • You were right. When I changed the power cable it fixed it. I see the moderators suggested this one: raspberrypi.stackexchange.com/questions/28034/… and they came to the same conclusion. – SDsolar Apr 17 '17 at 7:24