WS2811 Strips design help

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mikemike

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Hi,

I am attempting to put together a wearable LED strip design, and have some questions.

I have WS2811 strips, 60leds/m, cut into 27 led strips. I would like to have 24 of these strips, giving a grid of 27,24 (x,y) leds. I am using a Teensy 3.0.

I am grouping them into 3 strips for power, and am using a 12v battery pack for each group, using a UBEC step down convertor to 5v. Here is a diagram of the basic layout:

led_strips.jpg

Shown are 6 strips, in 2 groups of 3. I'll multiply this out for all 24 strips. The entire run will share the same data line (green line in the diagram.)

My question is about power and ground (maybe). When testing this setup, I get some weird results. I am assuming that all grounds should be connected to a common line shared by the Teensy 3.0's GND. When I wire this up to test it, the results are sporadic. I am using alligator clips and various lengths of wire, which I think might be part of the problem. The lights on the second set of 3 strips do not work properly sometimes. When I first wired it up, all 6 worked perfectly. After playing with the wiring some, trying to get the lengths of wire correct for my setup, things broke. Now depending on how the grounds are connected, I get it to work partially, sometimes not at all, or brief moments of working just right. I am using a breadboard right now for the Teensy as well.

Anyway, I need some guidance on how this should properly be wired up. I'm pretty sure the soldering on the WS2811 pads is ok, as things do work sometimes, without touching them, depending on how the power ground wires are interconnected. I think my problem may be just this mess of wires getting all the grounds connected. Before I move ahead and do any more permanent soldering I want to make sure I know what I should be doing.

I hope this isn't too confusing, my brain is sluggish after so much messing around with this thing. Should all the battery packs, and the Teensy, share a common GND? Does wire length/gauge matter? I am using 20AWG stranded wire for most of the wiring, nothing over 1 meter in length right now. That may increase to about 1.5-2 meters, at the most, for some runs between battery pack and strip. Do I need to isolate/filter the power in any way? I test 5v at the end of each strip with the multimeter, and they are only running at 25% brightness and never full white. So the power should be sufficient.

Thanks in advance for any guidance!
 
All 3 strips in each group should have +5V and GND wires running directly to the power supply. Do not connect the power in a serpentine fashion like the data. Use at least #22 wire. For only 27 LEDs, the current in each wire should be at most about 1.3 amps, so you probably don't need #18 wire. But in serpentine, the currents add, so you get about 5 amps and the LEDs at the end get really poor quality power. You really want good direct power connections for every strip.

Every group of 3 needs a ground wire connected to the Teensy. The ideal way is if the ground wire runs together with the data wire, as a pair twisted together or as the shell of a coax cable, or at least located as close as feasible if it's a separate wire.

Even if you use twisted pair or coax, you still might run into the signal quality issues mentioned on the OctoWS2811 page. The 2 solutions are 100 to 220 ohm series resistors and a 74HCT245 (or similar) 3 to 5 volt buffer chip. Using both of those provides the most robust signals.
 
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Thanks! When I run them in groups of 3 individually, they work flawlessly, with the serpentine wiring. It is when I connect a second set of 3 that things get a bit weird (no shared power between them). I am making sure to keep the power at 25% or less, and have moving color patterns so they will never reach full power. I will try breaking out the power and see if that helps.

I tried using a series resistor of 220 ohm suggested on the OctoWS2811 page, but that made the brightness fluctuate. Without the resistor it is rock steady (on 3 only.) With 2 sets of 3, the second set flickers and doesn't want to work right.

When connecting the strips to the shared ground with the Teensy, should the power supply ground branch out to the Teensy ground before the strip or have a separate line run from the end of each strip to the Teensy? Does this matter? Alternatively, should I wire all the grounds at the end of each strip to the next strip for one long ground line?

I am going to solder the Teensy to a permanent board eventually, but right now its on a breadboard. So I have jumpers coming off and alligator clips connecting the jumper wires to the wires on the strips. Those seem flaky to me, could that be causing problems?

Thank you for your help! I know this is elementary stuff, but I'm a programmer, hardware confuses me a bit.
 
mikemike said:
when I connect a second set of 3 that things get a bit weird (no shared power between them).
Click to expand...

This is very likely a signal quality problem from crosstalk.

Maybe try 100 ohm resistors? Twisted pair or coax cable for the signals ought to help.

Using a 74HC245 buffer may be necessary. 5V signals have a lot more noise margin.
 
That board looks really cool.

I may look at getting the buffer chip to drive this. The 3.3v control signal is only between the Teensy and the strip, right? I was under the impression that the strip forwards the signal onwards, so the connections between strips would be 5v control signals generated by the WS2811 chips.

I'll also try your suggestion about the twisted pair wiring for the control wires. It does seem to be dirty data. It also reacts when I touch the control wire with my finger. It seems to stabilize it a bit.
 
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