Capacitor and Resistor with Neopixels

Hi All,
Another newbie question…

1- I have neopixel stirps that come pre-wired with 3 pin (female) connectors, I have similar 3 pin (male) connectors connected to my development board, the length of the connecting wire in total would be around 7-8 inches. Now the adafruit best practices guide says to add a 300-500ohm resistor near the pixel end, but my question is that can I add that to my dev board given the length of the connecting wire is not too long? or is it really too long for that resistor to be effective? I just don’t want to re-solder the connections to the strip after adding the resistor.

2- I will have 1 meter strip either 30 or 60 pixels depending on the use case, can I use 1000 uf capacitor with both these strips or do I need to change the capacitor for 30 pixels to a lower value?

Additional Info: The controller will be ATTIny85 and the power source will be common for both ATTiny and the strip (either a 3 cell AAA power pack, or 5v USB)

Help please. Thanks.

The guide is quite clear. “The resistor should be at the end of the wire closest to the NeoPixel(s), not the microcontroller.”
The resistor resists currents coming from the cable to the Neopixel. Wires act like antenna and signals captured by the wire will be filtered by the resistors. If the resistor is on the other side of the wire it cannot do its job. The output at the Arduino end does not need protection from “antenna signals” because the output is driving the voltage on the wire.

You can use the 1000uF for both strips. As the guide says: “The capacitor buffers sudden changes in the current drawn by the strip.” A larger buffer is better in that case.

8 inches = 20 cm.
1 meter ledstrip with 60 Neopixels could require 3.6A.

It will work without the resistor and without the capacitor. They are required to make it safe and reliable (and therefor the resistor has to be at the end of the cable).

If you use a power pack with a USB output that can deliver only 500mA instead of 3.6A, then you have to lower the brightness.
If you set the overall brightness to 30, is that still bright enough for you ?

60 leds would be an extreme case, normally I am hoping to get the required effects working with 30 LEDs, and 30 brightness would be a little too low but I am quite happy with ~64. If I am not terribly wrong, I guess 30 LEDs with 64 would be roughly the same as 60 LEDs with 30 brightness right? and I think it turns out to be about 4 hours run time? I still need to test this though.

Okay got it, so I guess I will have to remove the heat shrink and re-solder the wires! I don’t remember exactly but I think I read it somewhere that if you use the power from LED strip to power the arduino (those two extra + and - cables coming out of the strip along with regular +, data and - cables) then I don’t need to use the resistor? does that make sense or I am imagining things terribly wrong? If that is true, I might just power the ATTiny through that and avoid re-soldering.

Again, sorry about referring to something else I read somewhere, but people have suggested using smaller (such as 100 uF) capacitors for smaller strips. I just went out to buy the 1000 UF one and its HUGE, I don’t think I can fit this in my design. Would 100 uF (25v, thats the closest I could find to 5v) work for my 30-60 LEDs?
Thanks.

In an ideal world (e.g., wire have no resistance, power supplies are infinitely fast, no noise …), you would not need the buffer capacitor and filter resistors. The questions is how far from ideal is your system. Every system is different and what you read where general guidelines that worked in a lot of cases.

Setup your system in a way that it is close to your final system, but you can still do some modifications. Then try and see what happens.

Let’s look at the buffer capacitor. If your power supply is reacting slow to a sudden increase in current e.g., switching on LEDs, the buffer capacitor can help by providing the necessary current for a short amount of time while the power supply increases the current flow. The same is true for the wire. If the power wire is long or/and thin it will have a negative impact on the ability of your system to react to sudden current changes. Short and thick wires have less of an impact and will reduce the requirements for the capacitor.

If your system is stable for long enough you are done. If you later find the system is not stable under certain conditions (temperature …) you can get back modify it.

My first pixel has just started to do its own thing! the colors don’t match with the rest of the strip now, I guess thats my answer right there, I do need protection. Thanks for your feedback.