FidoLight RGBW Spotlight: Update #1

[This blog entry was originally posted to Reddit in April 2021]

This is an update to my original post describing my plans to design/build an addressable LED spotlight based on a 3W Cree XM-L color module. There are three key challenges I’ve been grappling with:

  1. The lights require a weatherproof enclosure that is very small and unobtrusive
  2. The design should support a variety of optics to achieve different beam angles, with an “acceptable” level of color mixing (i.e. studio/theater quality not required)
  3. The interface must support long cable runs, and provide 12v power distribution (~250mA/fixture)

Enclosure

I’m not giving up yet, but I haven’t found a source for an enclosure that meets all of my requirements. My best option so far is to “harvest” just the enclosures from these inexpensive landscape fixtures. The mechanical quality was better than expected, and they were easy to disassemble and rework. I did a quick test with a different LED (3W RGB) and it worked pretty well. The thermal path from the LED substrate to the housing isn’t great, but I think it will be good enough for a 3W LED. It’s possible that I can also cram the electronics into the housing, but I’ll save this for a next-gen design. For the first cut, I plan to house the electronics (more below) in this weatherproof junction box.

Optics

I learned (with help from others) that most small/inexpensive TIR (total internal reflection) lenses aren’t designed for use with RGB or RGBW LEDs. They just create horrible artifacts since they aren’t designed to mix light from the different discrete LEDs within the module. The enclosure shown above includes a simple reflector that serves to reduce the beam angle somewhat, with an acceptable level of color fringing at the beam edge. This will probably be fine for many of my applications, but I’ll continue to experiment with lenses like this Khatod color-mixing lens to achieve narrower beams.

Interface

I briefly thought about using an 32-bit SK6812 RGBW interface, but now I’ve decided to use a “smart” multi-drop differential RS-422 interface. Instead of sending a constant stream of RGBW data, each fixture will accept higher-level “effect” commands (e.g. “Fade-In”) using the HSV color space. The commands will be decoded by an ATMega328P microcontroller chip, and the PWM outputs from that will drive a 4-channel constant-current buck LED driver circuit.

Cabling

Long-term UV resistance is a requirement in some of my applications (outdoor art installations), so I’ve decided to use CMX-rated 24AWG solid copper Cat5e cable for both the long multi-drop runs from the central controller to each spotlight, as well as from the electronics box to the LED enclosure. The multi-drop main cable will use 3 conductors each for +12V and Ground, and the remaining two for the RS-422 differential signals.

Name?

I need a project code name. I’m thinking fidoLight or just fido for my “spot” lights. Hehe. Better ideas?

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