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continuity in NeoPixel circuit
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continuity in NeoPixel circuit

by wylbur on Wed Aug 07, 2013 5:14 pm

hi Adafruit,

I've wired 8 NeoPixels together, and they light up sometimes and not others. I'm debugging. Can you tell me: if they don't have power, should I find continuity between + and ground? between + and input? Between - and input? Etc. Basically what should I see when I'm testing continuity, and what should I not see? thx -- wylbur.

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Re: continuity in NeoPixel circuit

by adafruit_support_rick on Thu Aug 08, 2013 9:31 am

I don't have an internal schematic, but NeoPixels are not simply LEDs - they have an integrated control chip - so there won't be any direct current paths between any of the pins.

If you're just wiring them to power, then they will light up (or not light up) in random states. You have to supply them with data to make them work.

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Re: continuity in NeoPixel circuit

by wylbur on Thu Aug 08, 2013 11:17 am

When I touch a continuity tester to each of the 4 pads on an unconnected NeoPixel, it chirps (indicating continuity) between every pair of pads. Thus there was a direct current path between all of them. I'm confused: can you clarify? Thanks.

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Re: continuity in NeoPixel circuit

by adafruit_support_rick on Thu Aug 08, 2013 11:35 am

That's not right. There should be no continuity between any of the pads. There's either something wrong with your tester, or your Neo-Pixel has been shorted out and reduced to conductive slag, or you've got some sort of conductive substance applied to the NeoPixel which is shorting all the pads together.

Can you post a picture of both sides of this NeoPixel?

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Re: continuity in NeoPixel circuit

by wylbur on Thu Aug 08, 2013 8:15 pm

hi Adafruit, I've posted a video on my blog of a NeoPixel fresh out of the bag, running, and then showing that there is continuity between + -, + input, + output, in fact between every pair of pads except input and output.

http://wylbursinnergeek.net/2013/08/08/ ... ploration/

Let me know what you think -- wylbur.

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Re: continuity in NeoPixel circuit

by adafruit_support_rick on Fri Aug 09, 2013 8:44 am

I've done the very same thing here, and I get absolutely nothing between any of the pads.

I don't know what to tell you. Is your tabletop conductive?

If there really was continuity between any of those pads, the thing simply would not work. It's impossible. How could it possibly work with a dead short between + and -? How would the data and clock signals toggle if there were a short between them and + or -? They'd be stuck high or stuck low.

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Re: continuity in NeoPixel circuit

by wylbur on Fri Aug 09, 2013 11:15 am

I definitely hear you that this should NOT happen, that's why I'm posting this question.

I just checked all 4 NeoPixels from a fresh bag. I put each pixel on a paper notepad which I think is not conductive (the meter shows no continuity when I touch the surface). All 4 NeoPixels had the same continuity pattern as I show in the video.

Here's what really worries me here. First, on Wed I was trying to debug a circuit and burned my finger on a VERY hot pixel. I figured I'd screwed up and shorted something, but I couldn't find the short because everything seemed to connect to everything else. This led me to test a single pixel, and now we're here. If somehow I'm shorting the pixels, I can't use them on a wearable -- they'll burn out, of course, but more importantly, they'll burn me.

Any other thoughts you could share, e.g., tests I could do, would be very helpful -- wylbur.

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Re: continuity in NeoPixel circuit

by adafruit_support_rick on Fri Aug 09, 2013 11:35 am

What kind of meter is that you're using? If it's the kind electricians use to ring out large circuits, then it may be putting out enough juice to overcome whatever circuits are between those pads.

Why don't you just try measuring the resistance between the pads instead of ringing them?

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Re: continuity in NeoPixel circuit

by wylbur on Fri Aug 09, 2013 11:50 am

the meter is a Amprobe 37xr-a:

http://www.amprobe.com/amprobe/usen/Multimeters/Compact-Multimeters-/37XR-A.htm?PID=73036&utm_source=google&utm_medium=cpc&utm_campaign=LT-10-Awareness&utm_adgroup=LT-10-AwarenessDynamic&utm_kw=_cat:amprobe.com&&mkwid=s&pcrid=37625286329&mt=b&utm_domain=&gclid=COHEkpfa8LgCFS9dQgodfHgA7w

I tested the resistance between all the pairs of pads on all 4 NeoPixels.

On all 4 pads, there's 84-88 KΩ between + and -.

On 2 pads, there's 15MΩ between - and input & output.

On all the other pad combinations, there was no reading. More info seems helpful. I'm not sure what we've learned, though. More thoughts? -- wylbur.

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Re: continuity in NeoPixel circuit

by adafruit_support_rick on Fri Aug 09, 2013 12:31 pm

What we've learned is that there is no continuity between the pads. Resistances of 85K ohms and 15M ohms do not constitute continuity. Continuity is < 1ohm.

So, sanity is restored ! :D

What is it you were trying to do, again?

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Re: continuity in NeoPixel circuit

by wylbur on Fri Aug 09, 2013 12:59 pm

I was trying to figure out where there might have been a short in my NeoPixel wearable project. Some LEDs lit up when testing, others didn't, and something (maybe an LED, maybe a wire) got crazy hot. I'll use resistance testing to keep looking, and I'm glad we're sane, that seems better than insane. In most contexts.

Thanks! -- wylbur.

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Re: continuity in NeoPixel circuit

by wylbur on Sat Aug 10, 2013 10:49 am

Per adafruit_support_rick's note in the other thread, I retested with an Adafruit multimeter.

I have a MAS830 that I got from Adafruit last year. It chirps on only one pad of one pixel (the one linking + -). That pixel does work, using the same test I tried in the video. Per our discussion in the other thread

Also I note that the MAS830 display shows a series of numbers as it's testing for continuity. When there's no connection at all (like touching the leads to a pad of paper), the display shows a 1.

For all 4 pixels, there's some set of numbers that flash on the MAS830 when I touch the leads to any combination of pads. Thus as before, there is some connectivity between the pads, though with high resistance, which adafruit_support_rick said is not continuity.

I'll try again this afternoon to see if I can get it sewn without shorts. I'll pull out all the bus wiring I was trying, maybe that's the problem.

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