LEDs dim, wrong colour, or the board keeps resetting
Last updated · SimHub 9.11.22
Addressable LEDs that dim as more of them light, turn white or yellow instead of the colour asked for, light only partway along the strip, or reset the Arduino mid-effect are almost always one problem: not enough current. USB power is fine up to roughly 500 mA, which is a small number of LEDs at full brightness, and beyond that you need an external 5 V supply. When you add one, the wiring rule is specific and getting it wrong is the way to damage something: connect the supply's ground to the Arduino ground and to the components, and its 5 V to the components only — SimHub's documentation says plainly "Don't connect the 5v to the arduino 5V". The Arduino keeps its USB power.
Four different-looking symptoms, one cause. Recognising that saves you from rewriting a sketch to fix a power problem.
Four symptoms, one cause
| What you see | What is happening |
|---|---|
| Colours dim as more LEDs light | The supply is sagging under load. Each LED takes current and the total has exceeded what the source can give. |
| Colours drift towards white or yellow | Voltage droop along the strip. The far end is receiving less than 5 V and the colour mix falls apart. |
| Only the first section lights | The same droop, further along. The data is arriving; the power is not. |
| The Arduino resets or reconnects mid-effect | The board is browning out — the LEDs are pulling the rail down far enough to reset it. |
The quick confirmation. Turn the brightness right down in SimHub. If every symptom disappears, it is power, and no amount of rewiring the data line will help. This costs nothing and rules out half the possibilities.
Adding a supply, correctly
The threshold is roughly 500 mA. Above that, an external 5 V supply is recommended — and a full-brightness white pixel is on the order of 60 mA, so it does not take many.
"Don't connect the 5v to the arduino 5V" That is the documentation's own wording. With an external supply: its ground goes to the Arduino ground and to the components, and its 5 V goes to the components only. The Arduino continues to run from USB. Feeding an external supply into the Arduino's 5 V pin puts it in contention with the USB rail, which is the wiring mistake that costs hardware rather than time.
- Size the supply for the worst case, not the typical one — every LED white at full brightness, which is what a startup test does.
- Take the ground to the Arduino as well as to the strip. A shared ground is what makes the data signal meaningful; without it, the strip has no reference and behaves randomly.
- On a long strip, inject power at both ends, or at intervals. This is the fix for droop specifically and it is more effective than a bigger supply.
- Cap the brightness in SimHub. Most rig LEDs are far too bright at 100% anyway, and it cuts current roughly proportionally.
When it is not power
If the brightness test above changes nothing, look elsewhere:
- Nothing lights at all — data pin, sketch, or the board itself. Start from whether SimHub sees the board.
- Colours are consistently wrong in the same way — red where you asked for green, for instance — is a colour order mismatch, not a power fault. Strips vary between RGB and GRB ordering.
- Random flicker unrelated to load — usually the data line: too long, unshielded, or run alongside a power cable.
- Faults that respond to being touched — a solder joint. Poor joints cause most of the disconnects and display faults in these builds.
The distinction that matters is load-dependent versus not. A fault that gets worse as more LEDs light is power; a fault that is the same at one LED and at fifty is not.
Common questions
Why do my LEDs go dim when more of them light up?
Not enough current. USB power covers roughly 500 mA and a full-brightness white pixel draws on the order of 60 mA, so it does not take many. Add an external 5 V supply, or cap the brightness in SimHub.
How do I wire an external power supply to an Arduino LED build?
Ground to the Arduino ground and to the components; 5 V to the components only. SimHub's documentation states plainly "Don't connect the 5v to the arduino 5V" — the Arduino keeps running from USB.
Why does my Arduino reset when the LEDs come on?
The LEDs are pulling the rail down far enough to brown the board out. It is the same underlying problem as the dimming, at a level that reaches the microcontroller. External supply, or lower brightness.
Only the first part of my strip lights. Is it broken?
Usually voltage droop rather than a fault — the data is arriving at the far end but the power is not. Inject power at both ends of the strip, or at intervals along it, which works better than simply fitting a larger supply.
My colours are wrong — red where green should be.
That is a colour order mismatch rather than a power fault: strips vary between RGB and GRB ordering. The tell is that it is consistent and does not change with how many LEDs are lit.
How many LEDs can I run from USB?
Enough to stay under about 500 mA in the worst case. Since a white pixel at full brightness is around 60 mA, that is not many at full output — though capping brightness, which most rigs want anyway, raises the count considerably.
Do I need to connect the ground if I use a separate supply?
Yes, always. A shared ground is what gives the data signal a reference. Without it the strip behaves unpredictably even when the power itself is fine, and it is the most commonly skipped wire in these builds.
Sources
Related guides
SimHub is developed independently by Wotever and is not affiliated with OESimRacing. This page was checked against SimHub 9.11.22 on 4 August 2026; where the interface has moved on since, the version stamp tells you how far. More on what SimHub is and what it costs.