Altared Space: LED physical layer

In a previous post we took a look at how we built the structure for Altared Space. Here we’re going to go into a little bit more detail about the physical layer for the LEDs.

The LEDs were driven by an Arduino Mega controller. If you don’t know anything about Arduino, it’s a great platform for beginners to get their feet wet with basic C programming skills as well as electronics projects in general. The Mega was chosen for several reasons:

1. It has a lot of memory for this type of controller.
2. It runs at a faster clock speed than other controllers in this class
3. There were no significant space constraints for the controller.

The Mega also has a massive amount of I/O pins, but in this case we only ended up using 12.

Power needs were met with a Meanwell passively cooled 12v power supply. This power supply is extremely robust: it is sealed against the elements (typical use cases include powering interactive billboards) and has no moving parts (useful in the desert since everything gets completely choked up with dust). It also packs enough wattage to run a ton of LEDs. The one flaw? It outputs at 12v, and all of our electronics ran at 5v. This is actually a benefit, as you will see later.

The LEDs were WS2811 controlled in the “Christmas” package. These are in strands rather than the more common WS2812 strips (WS2811 controller with a 5050 LED package), with large diffuse bulbs. I do love me some strips, but they are not particularly robust, and the LED density means that you scale a ton of bulbs really quickly, increasing your processing and power needs by a huge amount in an installation of this type. Standard JST connectors were soldered for each strip, and hookups using three conductor 22ga copper wire were used to link them to power and data.

The last piece of the physical layer was several 12v to 5v buck converters. You can grab these on Amazon or Mouser for relatively little money. They passively convert a range of input voltages to an output of 5v.

Materials for the physical layer:

1. 1x large enclosure box
2. 4x smaller enclosures
3. 4x 30amp, 8 post terminal blocks
4. 12x 10amp, 12 post terminal blocks
5. Meanwell 12v 360w power supply (passive)
6. Arduino mega
7. About 100 yards of 22ga hookup wire (three conductor)
8. about 20 yards of 18ga hookup wire (2 conductor)
9. 3 post JST connectors for days
10. 5x passive voltage converters (12v/24v to 5v)

In the main enclosure was the Ardiuno, power supply, a voltage converter to drive the Arduino, the 30 amp terminal blocks to distribute power to the hubs, and two of the smaller terminal blocks to distribute the signal. Each output pin of the Arduino was wired to a post on the terminal block. If you take a look at the project design, you will see that there are four naves, 3 with 6 ribs, and one with 12. The first 6 ribs of the long nave, and the three shorter naves, all used the same signal from the Arduino. So we just split the signal from the first 6 pins 4 ways with the terminal block. This greatly reduced the processing load on the Arduino by not needing to output the same signal on an extra 18 pins. However, care must be taken when doing this, as you can reach a point where the voltage drop of the signal is no longer enough to drive the LEDs. I’ve had success doing this 4 ways with no more than about 20 feet of wire – more wire or more splits and you will start to get erratic behavior in your LEDs.

Power and signal were distributed from the main enclosure to the hubs via 18ga two conductor wire for the power and 22ga three conductor wire for the signal. So for the three shorter naves, there were 2 signal cables and one power cable, and the long nave had a power cable and 4 signal cables. In each of the hubs was a voltage converter and several terminal blocks. Power, ground, and signal for each rib were separated out with the 3 post JSTs. From there, it was just a matter of connecting each LED strand with a bit of 22ga 3 conductor with a JST on each end.

Obviously this description is going to be a little hard to visualize without diagrams and pictures, but sadly, I didn’t really take any pictures of the setup, and my circuit drawing skills are essentially non-existent. But if you have any questions, I shall do my best to answer them in the comments.

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