LED Strips - Revisted

Hello, you lovely vex-forum dwellers!
I’d like to revisit a topic which has been touched on countless times in the past.
Following the rules, we know that LED Strips are allowed as a non-functional decoration on the robot in competition usage.
We have our finished Turning Point robot and are currently in the process of testing it before our comp in a few weeks so we wanna give our robot a massive “wow” factor.
So we want to make some sort of underglow.
So my question to you- vex forum, are you aware of any rule changes that would affect the usage of LED Strips? How would we power these off of the Cortex? and do you have any LED Strips you have used in the past or would recommend in accordance with the Vex Rules?

As far as I’m aware, powering them off the cortex is not competition legal.

False. Powering them off the Cortex is the only competition-legal way to power non-functional decorations.

The main question is whether you want software-controllable LEDs or software-independent LEDs.

In both cases, power generally comes from the outer connections of the 3-wire ports. Sensor ports have 5 V; motor ports have battery voltage (7-9 V).

If using software-independent LEDs, all you need is power.

If wanting software control, there is a whole rabbit hole you can jump down regarding how to control LED strips with the Cortex.

Last year, legally they HAVE to be powered of the cortex. No external batteries allowed. I have not looked at the rules regarding this again, but for a preseason event, I would doubt that anyone will care.

That being said, we used 5V LEDs, and just used the 5V rails on the sensor side of the cortex. We were using addressable LEDs, so we had 2 extra wires that were digital out’s used to control the data and clock inputs of the strips.

We had a team last season have them on their robot last season. They were able to pass inspection.
Oddly enough it isn’t mentioned in rulebook.

It’s been mentioned a few times in Q&A. And Q&A is the official extension of the rulebook. Legal as many have described above. And as @8232X points out, you can control the addressable ones with some IO pins. One Q&A from last year asked about building an external circuit to simplify bit-banged control of them; that got the official okey-dokey as well.

We’ve successfully used LED strips in our past season. They were a great addition to our robot - hopefully I can provide a bit of info. I’ll leave you to figure out most of the software aspect as it was an interesting experience for me to learn through (look up bit-banging).

We found these to be the best strips for vex purposes - the APA102’s are not timing intensive, but instead communicate through SPI which is important for the limitations of the cortex.

Make sure not to go crazy with length or count - we went with 30 LEDs total, usually running on a brightness level of around 22/30. You don’t want to go over the current limit on the cortex (you can calculate this with the sensors you’re using included to be safe).

There’s a fairly limited level of soldering involved - you can search previous threads and on the discord to get all the information you need. We spliced 2 3-wire cables to the 4 connection points, then (since we ran LED underlighting on either side of our robot) connected two 15-LED strips with 4-wire cables, which makes them easily removable. You can cut the strips between any of the LEDs. 30 LEDs/m (or the 36/m) gives you around one LED per inch, which was very ideal.

Be careful with your wiring and cortex power, but otherwise it was a great experience that I would highly recommend.

Personally, I think powering them off the cortex is a waste of ports and battery power, because you are not allowed to power them on during a match. When I got my LEDs, I bought some with a USB port at one end, and I plugged them into a portable battery I put in the robot. During a match, Id just remove the portable battery. I just had to be sure I removed the battery, or else I’d be breaking alot of rules.

While you have a point, the power draw is pretty small for LEDs. And if you’re not using the ports for something else, it doesn’t matter much.

However, your solution is very clean.

Yeah, I was under the impression it was game legal (assuming your opponents are not using colour or line sensors)
I recall seeing s robot having a purple under glow when watching the NZ Nationals stream last year-
As for a static LEDs did you guys recommend any?

I think it’s legal regardless of whether or not the other team is using the vision sensor. At least, there is nothing in the manual that prevents it.

If you match it up to colors that could mess with an opposing robot’s vision sensor, I would think it would cease to be non-functional. There is something in the manual preventing that.

There was a mention that the LEDs were fairly low current… which is mostly true. If using the 5V, addressable LEDs, however, remember that each of the three (R,G and B) diodes draws about 20mA at full brightness… meaning that a single RGB LED can draw up to 60mA. So ten LEDs… at full brightness… is 600mA. Not too much… but 30 LEDs might start to become non-trivial, unless they were programmed such that they were not all full brightness all the time.

Have fun!


I think you’re right there. It would be illegal if it was intended to interfere with the opponents, as that escapes the definition of a non-functional decoration. However, I think an underglow-type LED string that does not shine up or towards another robot would still pass inspection; any interference with a light sensor would be incidental and probably very rare. The bigger issue would be determining the intent of the decoration if its placement could cause more trouble (brighter LEDs, concentrated cluster of them, etc.). That would be up to the refs, at that point.

You cant have lights that correspond with your auton color as well. (Red/Blue). I’ve heard of some teams complaining that The lights act as a distraction.

Correct, because then the lights are acting as an indicator, something functional instead of nonfunctional decoration.

I think that, as long as the lights aren’t red, blue, green, or yellow, this is legal. If they are those colors they could confuse vision sensors

Exactly. Choosing any of those colors brings it close to becoming a functional decoration, so if you can stay away from those you’ll never have an issue.

Eh, not really. Again, even if you use other colors, you’ll have an issue if you use the lights to indicate things about your robot’s state such as if it is aimed in just the right direction or which autonomous mode it’s in.

I would still stay away from those colors so you don’t get a good 5 min argument with your Head Ref