FidoLight RGBW Spotlight: The Beginning

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

For various art-lighting projects, I’ve been in search of a high-power RGBW (or maybe just RGB) spot/flood light fixture that is water/UV resistant and ideally has a SK6812 (RGBW) compatible serial interface. I’ve seen (but not tried) this 10W addressable RGB fixture, but it’s a bit large for most of my applications. I bought this 3W addressable RGB fixture, but the lens creates horrible color fringing. Neither of these provides a W channel, and the composite white on the 3W fixture is very poor.

So…I’m thinking about designing and building my own fixture, and I was hoping I could get some feedback and ideas from you folks. The general idea is to steal some technology from the flashlight world (so I’ll cross-post this to r/flashlight). Here are the components and specs I’m thinking about:

• Cree XML 3W RGBW star module (and there are other less-expensive options)
• 1″ diameter machined aluminum housing/heatsink
• Carclo 20mm TIR lens (various beam angles)
• The driver electronics would be mounted in a separate, very small waterproof enclosure with a 1 – 3 foot cable to the LED housing. This will allow the “light fixture” part to be very small and unobtrusive, with the electronics enclosure mounted out of sight. The interconnect cable is likely to be standard Cat6 patch cable (8 conductors) possibly as small as 28-30 AWG.
• The driver electronics will be a custom surface-mount PCB containing the following:
  • 4-channel constant-current driver based on the AL8861 (x4) LED buck driver, running at 12V, driving up to 350mA per channel
  • Something to implement the 32-bit RGBW SK6812 interface, both input and daisy chain output to other fixtures. See more on this below.
  • An RS-422 receiver and driver to convert the single-wire SK6812 interface to a differential signal, to enable long cable runs.
  • A linear 3V or 5V voltage regulator to provide the logic supply voltage

It looks like the serial interface protocol will be the hardest part. If only there were an RGBW version of the WS2811 chip, or a non-integrated version of the SK6812RGBW modules. But if this has to be a clean-sheet emulation of the protocol, I can think of two options:

• A small pin count FPGA, maybe something like the MachX02
• A small, low-power MCU chip, maybe the ATtiny85. This seems like an easier path, assuming it’s fast enough to run the 800Kbps interface (both upstream and downstream)

For the cabling from a central controller to one or more “strings” of these addressable fixtures, I’m thinking about using weatherproof direct-burial Cat6 cable, 23-24AWG. One pair would be used for the serial interface, with the other 6 wires used to distribute +12V and GND. I’d like to support at least 8A per string with a 30′ cable run. A 30′ cable with 3 x 24AWG wire has a resistance of about 0.26Ω, so the voltage drop would be about 2v (in each of the 12v/GND runs). That leaves a worst-case 8V for driving the LEDs, which should be fine. I’d use screw terminals instead of RJ45 connectors, eliminating that potential weak point.

Back to the optics for a second: I’ve found very little information about how well TIR (total internal reflection) lenses work with multi-color LED modules. It’s possible that color fringing, caused by the different light paths from the adjacent LEDs (in the same package), might be objectionable. Has anyone tried this?

Stay tuned for future updates!