Planning this out requires some simple “napkin calculations”…
A typical interior door measures about 30 inches wide by 80 inches tall (76 cm x 2 m), so we’ll aim for something close to that.
Goal: vertical LED strands to fill a 30 by 80 inch doorway, connections at top only.
Then there are technical factors to consider. A single Fadecandy board is designed to control up to 512 pixels: 8 strands of 64 pixels each, maximum. With our data connections all residing at the top of the curtain, this sets an upper limit to the number of LEDs we can hang vertically: 64.
We’d like to use flexible NeoPixel strips, they’re amazingly handy. These are sold in various pixel “densities”: 30, 60 or 144 LEDs per meter.
Given some of our constraints…the 64 pixel limit and the 2 meter door height…we can quickly determine the higher densities won’t work for this application. So…
Established: we’ll use 30 LED/meter NeoPixel strips.
Established: each vertical strip will be 2 meters long, containing 60 pixels.
This is quite convenient, it avoids lots of little fractional bits of strip. At worst a few strips will require joining two 1-meter sections. The 2m length comes close to using the full 64 pixel capability of the Fadecandy board. (This 64 pixel limit only applies because of our “single connection at the top” constraint. If you’re building something to hang on a wall, connections can be made anywhere, and much larger and more freeform layouts become possible.)
At 30 LEDs/meter, there’s about 33 millimeters spacing between pixels down the strip. To create a pleasing and proportional grid arrangement, we should try to match that spacing (or come close) on the horizontal axis. 76 cm door width (760 mm) ÷ 33 mm spacing = 23 strips.
Add one to avoid a fencepost error. 24 strips total. This is perfect! Each Fadecandy board can control up to 8 strips in parallel. 8 × 3 = 24. And the X/Y grid spacing will be very nearly uniform.
Established: there will be 24 vertical strips, 2 meters each (48 meters total), controlled by 3 Fadecandy boards. Math!
So the finished curtain will be comprised of 48 meters of NeoPixels. There’s one more number to determine, and it can’t be derived from simple math: what’s your tolerance for risk? How much, if any, spare strip should be ordered? And how much can you really budget for?
Sometimes wires get crossed, NeoPixels get fried. Occasionally a reel escapes the factory with a bad pixel or two. Defective strips can be replaced…but can you afford the downtime? Should you have some spare materials on hand, ready to swap out? Scratch monkeys?