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Sun, 13 Oct 2019

Maccabeam™ Part 2: the physical structure

— SjG @ 5:30 pm

Background:
Introduction
Part 1: Simulating candle-light with pseudo-random sequences

I’ve been developing the software for the Maccabeam, and have a lot to write on that subject. But before I do that, I have a few words on the physical structure of the Maccabeam. There are some basic requirements: it needs to hold the microcontroller, the power regulation, the GPS unit, the lasers, the LCD status display, and some assorted lighting. It needs to be able to stand on its own, and, ideally, it should look nice.

The software is at the point where I need to figure out some hardware-dependencies. Specifically, I’ll need to lay out NeoPixels for part of the display.

What are NeoPixels? NeoPixels are a fancy form of light-emitting diodes (LEDs). LEDs are circuits that emit a narrow wavelength of light when the right voltage is applied, meaning that LEDs are emit a single color. Tricolor LEDs were a fancy development where three separate LEDs (one each of red, green, and blue) were combined in a single device, each with its own separate input lead, so you could change the color output. Other color LEDs took this a step further, combining this three-LED device with a pulse-width modulation driver that allows discrete levels for each color, thus allowing a whole range of apparent colors. NeoPixels are a product from AdaFruit industries that add an embedded circuit so that a whole string of RGB LEDs can be individually controlled with a single serial signal.

NeoPixels are available in a large number of form factors. For the Maccabeam, I have a thin tape with NeoPixels. It can be cut at various points, and then wires soldered between segments. The thing is, the software to drive the NeoPixels thinks of them as a single string, with each one having an address 1, 2, 3, … whatever. Since my layout is not linear like that, I have to think through the placement, and then figure out what the addresses are for each desired lighting operation. I’ll write more on this subject too.

I designed a basic plan. The design makes sense on the screen, but there could be a lot of issues when converting from digital to physical. So the next step was building a prototype, discovering the errors, and making corrections.

To digress a bit, I say I designed a basic plan. This is an oversimplification. I went through many designs over the past year or so, sometimes thinking from a visual sense, somethings thinking from a practical sense (e.g., where am I going to put wires?). I even built a prototype a year ago, with a design that I abandoned. Here’s a gallery of abandoned sketches and designs.

Design detail

Here’s a design detail. The brown rectangles up top are the vials, the red rectangles the lasers, and the strips with stars are the NeoPixels. The design has a front and back plane, with layers sandwiched between to give it the third dimension.

(I’ve been designing using Serif Lab’s Affinity Designer as my CAD program. I like it beacause it’s easy to use, allows precise sizing — you can even enter things like “1/2 + 1/16 – 1/32 in” into the sizing input, and it will give you the correct size! It’s a good tool, but I’m sure there are better tools for doing 3D design and CAD output. For example, if I design everything assuming 1/8″ thick materials, and then change my mind and decide to use 1/4″ thick materials, there’s no built-in intelligence to help me adjust all the interlocking tabs.)

Back to the final design. I chose Baltic birch plywood for the physical structure because it’s relatively easy to work with, and reasonably available from craft and hardware stores. Before designing, there’s the question of dimensions. CrashSpace’s laser cutter has a 24-inch by 12-inch bed, so that determines one set of dimensions. Then the question of how thick? I chose the nominally 1/8th inch (which is theoretically 3.2mm), and designed as if it was actually that size. It turns out that neither English nor Metric dimensions are exact.

One of the things that plagues laser-cutting with plywood is that the plywood thickness and/or density are not perfectly uniform. The same amount of laser power may cut perfectly in one portion of the wood, but not in another.

So I cut out a partial prototype, and measured and checked. It was a good thing, too, since I discovered a lot of oversights and errors. This is the iterative design process! Also, to address the laser-not-quite-cutting-through problem, next time I think I will either slow down the laser, or, alternatively, speed it up and run through the entire design twice.

Laser and vials in sockets

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