Unicorn Hat Diodes Running Hot at 80~100% Brightness!


Hi All,

My first post on this forum so I’m sorry if this has already been covered.

I am using my Unicorn Hat as a fish tank light - it will be going through a 24 hour cycle, through sunrise, day time, sunset and a blue moonlight. All with fading colours and intensities gradually in between.

My issue is with the fact that to get the correct brightness for happy fishies and plants I need to run the LEDs at near full whack (around 80%/0.8). I did a test run earlier at this brightness and they seemed to run pretty hot.

I obviously don’t want to damaged my lovely Unicorn Hat board or the diodes, so I was just wondering if they would be truly happy and last a good lifetime if they were run at this intensity for 12 hours a day? The other twelve hours the intensity varies between 0.8 to 0.1.

Thanks in advance, Mike.


This is an interesting use for a Unicorn HAT, and shows pretty handily just how things can be used in ways we’d never imagine.

I’m not sure how good Unicorn HAT would be for plants, since it fills out only a portion of the spectrum- red, blue, green, and isn’t really the normal kind of “white” light you’d use to light a tank. I’m not sure whether it would be sufficient to support photosynthesis, but I’d be very interested to hear how it goes.

Sciency stuff that I don’t fully understand aside, we’ve not tested the Unicorn HAT at full white at 80% brightness for any significant duration, and I don’t honestly know how long it’s likely to last. Push it much beyond that and the sheer awesome amount of current the HAT consumes has been known to de-solder the soft-start transistor right off the board. This is why we have a brightness limit in the Python library.

Apart from this case, the heat isn’t as big an issue as you might think, and I’ve definitely run Unicorn HATs hot in enclosed spaces just for giggles. Usually not at full-white, though. If you can observe it running stably for an hour, or two, then the thermal load isn’t going to vary much and it’s likely it’ll run forever.

You could use a few jump wires to separate it from the Pi for better cooling, and possibly even bung on a few heat sinks for good measure ( as you’d likely find on LED lighting solutions ), although I don’t know if the thermal coupling would be sufficient through the PCB resist layer ( the black stuff that sits on top of the copper layer ) to make any difference.

It may be possibly to duty-cycle the 12 hours daytime period slightly, for extra cooling. You could dim the LEDs slightly at random periods to simulate cloudiness and all sorts of things.

It’s been a while since I kept fish, but it’s not unheard of to keep a tank in darkness for large chunks of the day to manage algae bloom, since fish don’t need light ( this may apply to saltwater only though ). Apart from maintaining plant growth, I understand a lot of lighting choices are more for presentation?


Thanks for the awesome reply, amazing!

The plant’s I’m growing in the tank are mosses and shady plants that aren’t overly fussy about the light they receive so fingers crossed they’ll be okay.

The board itself is already away from the Pi and being externally powered, with just the data fly lead coming from the Pi. So the back (completely black) side is facing upwards, I could stick on a heat-sink with some thermal paste to aid conduction? Would the Unicorn Hat allow me to pipe a direct 5v feed from it anywhere in case I decide a small slow moving fan might help?

Good idea about varying the light levels too, it could happily idle at 0.5 brightness around midday when the natural light is high.

Here’s a link to the sunset run, but with no waiting times in the code so it runs at super speed! When it’s fully implemented it will probably be an hour and a half from full sun to deep blue.

Sunset Light Pattern

Thanks again for the reply.


So an update: Sadly, the LEDs did not enjoy their new venture - they lasted about a week on the cycle.

I had them fading between low and high brightness every ten minutes to try and keep temperatures down, as well securing a heatsink with good heat compound. But despite this, when I got home from work yesterday around half the LEDs had partially failed, and one or two had entirely died.

I think I am going to try to buy a couple more Unicorns next month and run more, but at a lower temperature?