Yesterday I stopped making knots with the wires: I soldered them to the LED strip on one end, and put pins on the other.
The issues I had to control the last LEDs of the strip were actually due to the APA102 datasheet being wrong, in addition to being poorly translated from Chinese into English. The “end frame” of the SPI message it described was indeed required, to supply more clock signals than the length of the payload. But not enough if you have a hundred LEDs, as explained on this very informative blog. The data signal being delayed by half of clock cycle by each LED, the length of the end frame should be proportional to the number of LEDs.
Then there’s the brightness setting, 5 bits on each LED frame, which everyone agrees to set to 0b11111 and forget about.
We should soon have a 3D printed triangle that fits the LEDs (see my friends’ posts). The LEDs shall then be rearranged: we’ll cut the ribbon into smaller ones and re-solder them. Then we’ll look at the signal coming out on the end of the ribbon and see if we can put another one in series.
To build the 19 other faces, we might need a few more LED strips. We did some measurements, and one 5V LED strip consumes a bit more more than 1A. Until now our test card could supply enough power when using a reasonable brightness, but with more than 15 times as many LEDs, we’re looking at big batteries for bouLED to be autonomous.