Cheap WS2812 LEDs: Test Fixture

Given that I no longer trust any of the knockoff Neopixels, I wired the remaining PCB panel into a single hellish test fixture:

WS2812 4x7 LED test fixture - wiring
WS2812 4×7 LED test fixture – wiring

The 22 AWG wires deliver +5 V and Common, with good old-school Wire-Wrap wire passing to the four LEDs betweem them. The data daisy chain snakes through the entire array.

It seems only fitting to use a knockoff Arduino Nano as the controller:

WS2812 4x7 LED test fixture - front
WS2812 4×7 LED test fixture – front

The code descends from an early version of the vacuum tube lights, gutted of all the randomizing and fancy features. It updates the LEDs every 20 ms and, with only 100 points per cycle, the colors tick along fast enough reassure you (well, me) that the thing is doing something: the pattern takes about 20 seconds from one end of the string to the other.

At full throttle the whole array draws 1.68 A = 60 mA × 28 with all LEDs at full white, which happens only during the initial lamp test and browns out the supply (literally: the blue LEDs fade out first and produce an amber glow). The cheap 5 V 500 mA power supply definitely can’t power the entire array at full brightness.

The power supply current waveform looks fairly choppy, with peaks at the 400 Hz PWM frequency:

WS2812 4x7 array - 200 mA VCC
WS2812 4×7 array – 200 mA VCC

With the Tek current probe set at 200 mA/div, the upper trace shows 290 mA RMS. That’s at MaxPWM = 127, which reduces the average current but doesn’t affect the peaks. At full brightness the average current should be around 600 mA, a tad more than the supply can provide, but maybe it’ll survive; the bottom trace shows a nice average, but the minimum hits 4.6 V during peak current.

Assuming that perversity will be conserved as usual, none of the LEDs will fail for as long as I’m willing to let them cook.

The Arduino source code as a GitHub Gist:

10 thoughts on “Cheap WS2812 LEDs: Test Fixture

  1. That wiring is visually attractive, but it makes me think they should have integrated it into the PCB, so they’re all connected before they’re snapped apart. And I like the “conservation of perversity” phrase – I usually go with “the general cussedness of things”.

    1. I was mildly surprised that the panel didn’t include test wiring, too. I can’t imagine unit-testing each LED on the assembled panels would be cost-effective; maybe they just trust that nothing can go gnorw.

  2. none of the LEDs will fail for as long as I’m willing to let them cook
    If it comes to that, you can always dial more voltage on the (different) PSU… quick look around Adafruit website didn’t produce Absolute Maximums but around 10V should do the trick in any case :)

    1. The adafruit datasheet is badly formatted (top of page 3, header at the bottom of page 2).

      Click to access WS2812.pdf

      The absolute max is in there. Vcc (abs-max) is 6~7V for the supply and Vdd (abs-max) is also 6~7V for the LEDs.

      We used to specify a single voltage for absolute max–can’t say I’m fond of a range. Where I worked (usually in bipolar facilities, though we were doing MOS before the dot-com V1.0 collapse), if you stayed below the abs-max supply, the part would survive, but Thar be Dragons if you go above it. I also get nervous about running afoul of power dissipation if running above the nominal Vxx maxima (5.5V here). Usually not so bad in a room temp ambient, but the magic smoke can be let out when you go above the max for a while.

      1. Oh, I couldn’t of been bothered to go look for the datasheet. I did say a “quick” look though :)

    2. Well, as it turns out, I was wrong: the first LED failed after a few power-on days.

      No need for extreme measures: they’ll fail all by themselves!

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