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Fixing a Small Gotcha With Adafruit's NeoPixels

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1/1/2014 08:00 AM EST
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A slight problem may have taken out one of Max Maxfield's Adafruit NeoPixel rings. Happily, there is a solution.

But what about my rotary potentiometers? There are five of these in the upper portion of the control panel. As you can see in the image below, I've decided that each will have 16 associated LEDs.

Fortunately, Adafruit supplies NeoPixel rings with 16 elements. These are perfect for my project, as illustrated below. Adafruit also offers 12-NeoPixel and 24-NeoPixel rings.

In addition to power and ground connections, each NeoPixel ring has a data-in connection and a data-out connection, thereby allowing you to connect a bunch of them together in series. All of this brings us to the crux of my problem. This morning, I brought one of my rings into work to show to my chum Ivan. When I proudly turned everything on, however, the result wasn't what I had expected. Instead of a glorious rainbow pattern rolling around the ring, all we saw was one lonely NeoPixel giving a very half-hearted attempt to light up.

Ivan immediately bounced over to his office and returned with a multimeter and oscilloscope. It took only a few seconds to discover that we were getting power to the ring, and the programming signal was reaching the ring's data-in pin, but there was no programming signal at the data-out pin. Hang on. I've been thinking about this as a long shift register, but it just struck me that this might not be a good analogy. Maybe we shouldn't see anything at the data-out pin unless we modify the program to behave as though there were more NeoPixels in the chain. (Hmmm... I'll have to go back and experiment further.)

It may well be that I accidently blew out the input stage to the first NeoPixel in the chain by electrostatic discharge (ESD). However, Ivan also pointed out that we were seeing a lot of overshoot and undershoot on the programming signal, as seen in the oscilloscope screenshot below.

In order to address this, we added a potentiometer into the circuit just before the signal line was presented to the NeoPixel ring. We started off with the potentiometer set to 0Ω. Then we gradually increased the resistance until the overshoot and undershoot were completely removed from the signal, as shown below. Increasing the value further started to impact the slope of the signal's edges.

The ideal value for this particular configuration was 230Ω, which we subsequently approximated by strapping two 470Ω 1/4-watt resistors in parallel. The required resistor value will no doubt depend on the characteristics of the rest of the wiring harness, so I will be repeating this procedure for future iterations of the design.

As I say, it may be that this overshoot and undershoot is not a problem, and that it was my own stupid fault that my NeoPixel ring was toasted due to my failing to take ESD precautions. On the other hand, I feel much happier knowing that we are now working with a clean programming signal. But now I must away to order a replacement NeoPixel ring. In the meantime, please feel free to post any questions or comments below.

Related posts:

— Max Maxfield, Editor of All Things Fun & Interesting Circle me on Google+

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Max The Magnificent
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Re: Keep testing your NeoPixel ring, use the next pixel
Max The Magnificent   1/8/2014 9:50:24 AM
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@n7ray: I'll second that.  Have fixed two rings that I thought were toast by replacing the first LED in each.

Cool Beans -- I will have a bash at swapping my first pixel out -- also I'm going to take much better anti-static protection when handling the little scamps from here-on-in. 

 

n7ray
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Re: Keep testing your NeoPixel ring, use the next pixel
n7ray   1/7/2014 8:49:18 PM
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I'll second that.  Have fixed two rings that I thought were toast by replacing the first LED in each.  

BTW: If you want an easy way to drive hundreds of them, check out this project:

 

(The PSOC 4 boards are only $25)

http://www.element14.com/community/message/89756/l/psoc-4-pioneer-kit-community-project100-psoc-4-times-square-led-billboard#89756

Max The Magnificent
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Re: Keep testing your NeoPixel ring, use the next pixel
Max The Magnificent   1/6/2014 12:15:54 PM
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@sdtrent: You should not trash your NeoPixel ring just yet. Try powering the ring and sending your data into the data-in line on the second pixel, or third, etc. There is a chance that the first pixel is damaged and not flowing the signal data to the next pixel in the chain.

That's actually a really good idea -- and, in fact, if that works, then it probably wouldn't be too difficult to simply replace the first pixel... thanks for the suggestion.

sdtrent
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Keep testing your NeoPixel ring, use the next pixel
sdtrent   1/3/2014 6:12:11 PM
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You should not trash your NeoPixel ring just yet. Try powering the ring and sending your data into the data-in line on the second pixel, or third, etc. There is a chance that the first pixel is damaged and not flowing the signal data to the next pixel in the chain. Other than overvoltage, I would be surprised if you managed to kill all of the pixels.

Also, part of the perceived update speed is that the pixel LED data is updated for the desired pixels before SHOW is used to update the output currents of the whole strip at one time. This allows a flicker free update because you are not seeing the changes in each pixel each time the individual pixel's data is updated.

Shane

George Hahn
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Re: Addressing
George Hahn   1/3/2014 4:17:18 PM
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@Marty MacGyver

You can control them with a BeagleBone Black though. Look into the pair of programmable realtime units on the SoC.

Max The Magnificent
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Re: Addressing
Max The Magnificent   1/3/2014 1:15:46 PM
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@Marty: I've pondered putting a small FPGA board between an Arduino (or RPi, or something else) and using it as a high-speed dedicated controller for my WS2812-based ring, matrix and strip.

Thanks for the great information -- very interesting -- and if you do use an FPGA to do this, I woudl LOVE for you to write the project up and to post it for others to peruse and ponder here on EE Times.

Marty MacGyver
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Re: Addressing
Marty MacGyver   1/3/2014 1:12:22 PM
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I was messing around with these LEDs long before they were called "NeoPixels". First, the LED units are the WS2812 (or WS2812b, which is better suited to the higher current paths large strips and arrays demand). That unit consists of a 5050 SMT RGB LED combined with a WS2811 controller.

The WS2811 protocol is detailed in the datasheets as well as numerous sites (I found http://hackaday.com/2012/12/07/driving-a-ws2811-rgb-led-pixel/ to be helpful when I was getting started with it). Conceptually, it works like this: at any point after a 50 us quiet period (data line held low) a given led is ready for input. The first WS2811 in the chain reads 24 bits (8 bits each for red, green, and blue), and after that it just forwards the rest of the bits downstream. The next WS2811 does the same, and so forth, until you stop sending data (obviously extra data beyond the last LED goes nowhere).

You can search for the WS2811 datasheet and see the exact timings involved (there's both a low and a high speed mode).

Keep in mind that the AVR at 16 MHz doesn't allow a lot of overhead for data processing. I managed at one point to marry the strip with a controller... but there wasn't a lot of room to do anything else:

http://www.falatic.com/index.php/72/github-long-time-user-first-time-contributor

I've pondered putting a small FPGA board between an Arduino (or RPi, or something else) and using it as a high-speed dedicated controller for my WS2812-based ring, matrix and strip.

PS: it's virtually impossible to directly control these with a Raspberry Pi... which is another reason I'm considering the FPGA idea as I revisit these projects.

 

Max The Magnificent
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Re: Coax connector
Max The Magnificent   1/3/2014 11:20:31 AM
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@bobdvb: Those coax power connectors are quite readily available...

I agree -- the thing was that I'd never looked for one before, so when I realized I could do with one and there was one on the Adafruit site I thought "Wow -- that's a good idea!"

bobdvb
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Duane Benson
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Re: Product placement?
Duane Benson   1/2/2014 7:06:03 PM
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Adafruit sells products of their own design and products of someone else's design. What I really like about them is the amount of value that add to whatever it is they're selling. The libraries and tutorials they provide are pretty incredible in terms of quality and breadth. They also give a lot of support to what used to be a re-emerging hobby community, but is now also a re-emerging small start-up business community.

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