This week, I read some Beetle documentation, wrangled WordPress into displaying weekly reports properly, and made some more design decisions for the layout and materials/manufacturing.
First, I looked into finding polyester cord and cord locks. The reason I chose polyester is because I believe it would be less likely to fray other materials, and is also quite silky so it feels nice. In terms of size, I went with 3mm because it should be compatible with the cord lock size I chose (it has a small size and a simple mechanism), and seems decently thick enough to be strong.
- Amazon.com: INSPIRELLE 3mm Black Satin Cord Rattail Silk Cord Chinese Knot Thread for Jewelry Making, 50 Yards Spool
- Adjustable Cord Lock – Round Ball Style – Single Hole End Toggle for D – SpecialistID.com
Not only will the cord be used for adjustability, but I think it’s a decent way of protecting wires that connect the batteries as well (wrap/knot the thread around the wires, friendship bracelet style). Since this could be done last, it would be easier than putting tubing around the wires because that would need to be done before the soldering (unless you want to cut and reseal the tube, which is painful).
To attach the cord to the bracelet, I came up with using “stoppers” that fit the holes on the ends of the plastic tube, with a hole in the middle for the string to go through. Basically, it looks like a metal disk washer. The stoppers could be glued into place to help seal off the components from outside damage like dust/water, and a knot on the end/some glue can also prevent the cord from slipping through the hole.
Unfortunately, plastic tubing to protect the PCB was a lot more painful to find, which I will get into later.
My initial plan was to CAD model some designs so we can have a more concrete idea of what the bracelet will look like. Unfortunately, that requires some designs first, and drawing on paper is much faster, so see those sketches in the team report. The idea with those sketches was to aesthetically space out the lights such that there wouldn’t be a black hole where the microcontroller is, although it might be a bit hard to tell since they’re not drawn to scale. We’re also not sure what number of lights will be sufficient to not look empty, but also balance power usage. The reason the microcontroller is in the middle instead of the edge is to minimize the max length needed for the traces. Although if there are sufficient benefits to putting the microcontroller on the end, perhaps we will consider doing that instead.
Another annoyance is that the Beetle doesn’t have all the desired dimensions (the 25mm length doesn’t include all of the USB port, and no height listed), so we will just have to guesstimate in the CAD and PCB. At first, it seemed like the height of the PCB + components would only be about 5mm, which is great because many plastic tubes can be that size. Unfortunately, since tubes are circular, we also need to consider our width dimension, which is going to be about 30mm when accounting for some buffer room because of the length of the Beetle. We can’t rotate the Beetle to make the width “smaller”, because then the USB port would be hard to access for the user (perpendicular to the PCB board length is easier to reach, instead of being along the PCB length because of the angle the wire would have to bend to avoid other components). Finding a 25mm tube was a lot harder, and it’s also a bit of a waste of space in the height dimension. Bendable, oval tubes are also quite hard to find. So we have some options that we are going to experiment with and hopefully at least one of these prototypes will work:
- Buy a smaller diameter plastic tube, and use heat and weights to smush into an oval shape. McMaster-Carr
- Buy those plastic sheets that are used to cover tables, and cut out the correct shape, use hot glue to wrap into a tube shape. Amazon.com: NECAUX Custom Multisize 1.5mm Thick Clear PVC Table Cover Protector – 14 x 14 Inch Waterproof Crystal Soft Plastic Square Tabletop Protective Pad for End Table/Night Stand/Side Table, 2pcs : Home & Kitchen
Since we initially thought we didn’t need a PCB board and now we do, we will be designing that concurrently while waiting for the orders of the other parts to come in, so I don’t think we have encountered any “blocking” actions and can be relatively on schedule. If the other parts come by next week we can unit test/sanity check some things before we put it onto a PCB. I’m not sure if it would be ambitious to have a PCB ordered by spring break? That way we can use spring break to wait for the shipment.
For next week, I plan to:
- Design some CAD models of bracelet layout (they are going to be simplified boxes with guesstimated dimensions if I can’t get pre-made CAD of the objects)
- Try PCB prototypes if CAD goes well
- Read more Beetle documentation
- If the beetle comes in I can start poking it with hello world and other sanity checks
- Think of more ideas for materials/prototyping/edge cases for how it’s built?