Getting back to the product design, I now have constraints to work with and the major component of the product (WS2812) determined. Now comes the fun part! Now is when I take out plain old pencil and paper and start sketching ideas. This is by far – for me -- the fastest way to iterate designs, bar none. Using pencil allows me to quickly sketch and modify ideas. It requires zero electrical power and can be done anywhere an idea strikes you.
I can quickly discard ideas as impractical and move on. Once my ideas are solidified I move them to the computer.
So what did my first sketches look like? My first inclination was to try to minimize the amount of soldering required to build the kit. Maybe even eliminate it. The only way I could think of to eliminate it and still have a three dimensional object was to use connectors.
Lots of connectors.
I would need a small, thin circuit board for the LEDs. These would be mounted horizontally and stacked vertically. In order to stack them, I'd need small, thin boards on each end of the horizontal LED boards. These vertical boards would have to mount into some sort of base board that handled connections to the outside world, LED control, and power distribution. It looked like this:
On one end of each horizontal was a connector for power, ground and serial data input. On the other end of the horizontal was a connector for power, ground and serial data output. I could polarize these connectors such that they could only be plugged into the verticals one way -- it would eliminate the possibility of plugging the horizontals in backwards which could potentially destroy the LEDs.
Each vertical would then have the appropriate mating connectors as well as connectors to plug into the main board.
A little more thought and I realized that with a connectorized solution, I could actually make the cube expandable, or perhaps even allow non-square shapes. For example, stacking two vertical boards would allow an 8x8x16 cube to be built. I just had to ensure that the connectors and power planes were capable of handling more current than just an 8x8x8 cube.
Now for the type of connector. Many a designer will curse at the mention of connectors, myself included. They can be one of the most unpleasant aspects of a design to contend with. Frequently the perfect connector doesn’t exist, or it is unobtanium, or uber-expensive. Ugh. Connectors suck.
In an attempt to keep it simple, stupid (KISS), my first thought was pin headers. There are many suppliers so second sourcing should be easy. Headers come in both genders and in a variety of pin pitches so that it should be fairly easy to find what I needed. They are ubiquitous.
A horizontal board needed to handle current for eight LEDs and potentially more if multiple horizontal boards are connected together. At 60 mA per smart LED, times 8 LEDs, this works out to .48 A per horizontal. Each vertical therefore needs to handle 8 (or more) horizontal boards, so the verticals need to handle at least 8 x .48 = 3.84 A.
Referring back to the sketch, each horizontal input end would need +5V power at 0.48A, a ground connection and a serial data input signal = 3 pin connector. The opposite polarity of that connector would be needed to plug the horizontal into the vertical board.
One vertical cube assembly would be comprised of 8 horizontal boards plus 2 vertical boards. Each horizontal would require one 3-pin male header and one 3-pin female header. Each vertical would require 4 male headers and 4 female headers for the horizontal boards. The verticals would also need a connector to attach to the main board.
The total number of connectors for a vertical assembly works out to be 16 male + 16 female of the 3-pin headers. Don't forget the 2 male and 2 female headers connecting the verticals to the base.
A x8 cube requires 8 vertical assemblies, so the full cube connector totals are 128 male + 128 female of the 3 pin headers...
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