I bought a 100 pack of WS2812S SMDs last year (product link) and wired most of them together for a string of Christmas lights for a tree. It was more work, but way cheaper (at the time) than buying a strip.
I had a few left over that I kept stored away until tonight when I decided to work up a little circuit to enhance my wife's Halloween costume. Problem is, I can't get them to light. Initially I had 3 soldered together, making sure that the voltage and data were going in the right direction, but when I plugged them into my Arduino Uno with a very basic NeoPixels based sketch, none would light. Figuring I had a bad one, I desoldered them all and tried, one by one, on a breadboard. Still no joy.
I tested and confirmed with a multimeter that I was getting the required voltage from one LED to another and reading voltage on the DIN/DOUT pins, but never any light. If I put the Christmas strand from last year on the same pins it works great.
Is there anyway to tell if these are just bad LEDs, or have I missed a step somewhere? I figured if the multimeter showed no voltage on one or more of the pads, that would be an obvious sign to trash it, but since all three show voltage, both when wired together and separately, I can't figure a reason they shouldn't light. Is there more I can do to test?