Problems with partial neopixel string

I use a lot of neopixels so I have a working set of code etc. I buy the Neopixel (WS2812B) strings of neopixels rather than the ‘flat’ strips which don’t flex to suit my needs. Some of my projects do not need the full 100 neopixels so I cut off the excess for other uses. I clearly mark which end is the input end (the end I made the cut). However, when I connect a connector to the leftover strip it will not work. Yes, I have the protective capacitor and resistor in place. If I connect a full strip of 100 it works. Connect a leftover strip of 20 and nothing. I do adjust the code to indicate I am using 100 or 20 as required.

I can find nothing to say a neopixel string is programmed so each neopixel has a set ID number to it. I have ample power as the 100 string works. If I connect the leftover to the end of 100 to make it 120, nothing lights in the add-on. I have plenty of experience in electronics so I have checked connections, looked for any shorts, etc. I have about 5 offcuts and they all refuse to work.

What is it that I might be missing?

Thanks in advance.

Hi Lindsay, Welcome to the forum!

This is an interesting problem! First off, if you could share images of how you’re hooking up the strings, we might spot something, so see if you can grab some pics. There are a few more things to consider:

  • Neopixels have a “data in” and “data out” pin, so you can’t connect the strip “backwards” and have it work. Sounds like you’ve got this covered but I thought I’d mention it.
  • You’re right about IDs, I believe how the protocol works is that the first LED takes the first data packet it recieves and passes the rest on to the next LED, reconditioning the signal in the process. Make sure there aren’t any very long runs (50cm+) of wire without an LED in the mix, as the signal could get degraded without any LEDs to recondition.

Keen to see how you’ve got everything hooked up, I hope we can help you towards a solution!

Hello Josh,

Thanks for getting back to me. I have attached some pictures of my wiring with explanations on each one.

As for my power supply, connection and code in the Pico, and the same main feed cable, it all works plugged into a new set of lights. I unplug the new string and plug in my short string and nothing works. Due care is taken to turn off all power in the process. I have 5 cutoff stings with approx 15 LEDs on each. None work as a short string.

Plenty of current available as I can light up a 100 LED string with this power supply.

I know I am sending out data on a 3.3V pin on the Pico but it works okay with the new string. I cannot see why the same 3.3V data would not work with the short string.

I have tried everything and yet nothing wants to work. Hopefully you might see something.

It totally has me puzzled.


Hi Lindsay
Refer last pic. The Data wire (green) does not appear to be connected to the centre wire in that photo. The white wire (neg or Gnd) is connected to the centre.
I think you might need to re check.
Cheers Bob

Thanks Robert but it is just a bad photo with the wires not separated properly. The three wires are connected properly.


Hi Lindsay
Yes when I blow it up and look closely you are right. I don’t know what else can be suggested except get yourself access to an oscilloscope and see how far you control signals (and DC power for that matter) are getting down the string. That instrument (oscilloscope) has a hell of a lot of uses believe me. Don’t know how anybody gets on without one.
Cheers Bob

Thanks for you help. I don’t think I will invest in an oscilloscope as the partial string is not really worth it. At my age, I doubt whether I would make much use of it. Once again, thanks for responding.


Hi Lindsay
Yes an oscilloscope ay be a bit of overkill. OK if you have one I suppose.
There used to be a small simple tool around which I have not heard of for years called a logic probe. From memory it was simply an OpAmp or other high impedance drive with a LED for indication and some AA cells. It was simple and at least gave a visual indication of digital activity. Could be quite handy in cases like this where a more expensive thing like an oscilloscope is not warranted.

Just had a quick look for logic probe. Jaycar have a kit, KD6100. Lots on Ebay. So it would appear I just haven’t been looking.
Cheers Bob