PWM Current Output for LEDs - Question

My team is looking at running WS2811b LEDs this year. To eliminate the hassle of running a co-processor we are looking at the option of running them off of the RoboRio this year. We have a few questions below:

  1. What is the current output of the PWM port on the RoboRio?
  2. How many LEDs can be driven before a brown out?
  3. Would we run into issues running the power and ground to the VRM and the Digital Input to the RoboRio?
  1. 2.2A at 6V (Connecting the WS2812 LEDs to power source in the Robot)
  2. I’m not sure. I’ve heard numbers of 1A/meter but clearly that depends on the number of LEDs/meter.
  3. That’s fine, but you probably want to split the ground signal so you connect both ground and signal to the Rio.

I am thinking of going with option 3. Why do you think a split ground would help? Is the ground plane on the RoboRio isolated from the ground plane on the VRM?

It’s a common ground (the two grounds are tied together at the PDP), but the signaling is quite fast (over 1MHz), so signal integrity might be a concern. Running a ground wire along with the signal wire means that the return loop for the signal follows the same path as the signal itself.

That makes sense. Is it legal to run two wires into the VRM 5V/2A ground? One going to the LEDs and one going to the RoboRio?

There are no rules that prohibit that :).

ws2812b current draw will depend entirely on what color and brightness is being shown. each “led” is actually 3 diodes and a teeny tiny driver ic. So you can imagine that a led showing RGB(5,5,5) will consume much less power than one showing RGB(255,255,255) for instance.

all that being said; according to adafruit’s data on the neopixel (their name brand for ws2812), the current draw at max brightness is up to about 60mA per “pixel” i.e. per led. Though that would be at max brightness white, so… think of that as your theoretical max, since it is quite rare that you’d light a string of all max white, i’d imagine.

also, ideally don’t forget a 300-500 ohm resistor on your data in line, and a 1000uF cap on your power source. it’ll work without them but you greatly increase your chances of damage to the strip (plus itll help your leds deal with any momentary brownouts)

Quick LED note, I would strongly suggest using something like: https://www.amazon.com/gp/product/B07V7YY5X6, rather than using the VRM (especially the same VRM that your radio is using).

As a point of reference, the “Clyde” animation on our Power-Up robot had 340 NeoPixels between the two sides of the robot which drew a total current of 2 amps.

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I will definetly order that item from amazon. That way we can get more than 2 amps out. More than likely we will only be using the LEDs as a progress bar showing how many balls are currently in the robot. We will be using a break beam sensor at the intake and shooter to count the balls.

I believe the maximum number of LEDs the roboRIO can handle is 5460. I’m not sure about current draw for this number but I’m pretty sure that it’s the most it can handle from a controls standpoint.