NeoPixels are great when they work, but I’ve managed to create many circuits that didn’t.
They failed in every imaginable way — NeoPixels that wouldn’t turn on, NeoPixels that flickered, NeoPixels stuck on a color (blue seems to be my NeoPixels’ favorite) and — the most baffling of all — NeoPixels that worked when powered from one USB port, but not when powered from a different USB port.
Despite the wide range of symptoms, almost every one of these failures had the same underlying cause: powering NeoPixels with 5V, but connecting their data line to a 3.3V microcontroller.
The root of the problem here is that — according to the NeoPixel data sheets — the NeoPixel data line voltage (in logic-high state) has to be at least 0.7× the NeoPixel power voltage.
In other words, the power voltage has to be at most 1.43× of the data line voltage. Therefore, if the data line is at 3.3V, then the highest voltage you can put on the power line and still have a reliable NeoPixels is 1.43 × 3.3V = 4.71V.
As a result, if your NeoPixel power is close to 4.7V, the NeoPixels will be unreliable in interpreting its data line, resulting in flicker and random color changes.
Similarly, if your NeoPixel power is substantially higher than 4.7V, then the NeoPixels will not see any data on the data line, which will lead to it being stuck on black (for most NeoPixels) or blue (for some older NeoPixels).
Most notably, powering from USB (which provides 5V) does not work if your microcontroller logic is at 3.3V.
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