I have changed the battery from a coin cell to a 24mm.sq x 3mm thick LiPo cell from an iPod Shuffle Gen 2. This reduces the height a bit and gives a bit more run-time. They are also cheap and readily available. I had to steal the gnd connection from the usb connector on the other side of the board as the battery gnd pad is right in the middle of the board underneath the new battery. Justin plans to add some new pads on the PCB for this purpose.
To help with the mechanical design Justin sent me some ‘.step’ files exported from his PCB software (Altium I think). These import nicely into DesignSpark Mechanical which is a great free 3D CAD package from RS Components. I have tried various free CAD packages including Sketchup, OpenSCAD and FreeCAD and this seems like the best to me.
The design had to be circular to fit in with the ‘radius’ name. I designed the buttons to be all part of the same assembly. The idea is that this makes them feel more solid and also provides a support for the display board which is currently only attached the bus connector. I am planning to get the plastic parts printed via an external service. This will probably be in nylon using an Laser Sintering process; I don’t think basic FDM will be accurate enough to create such small parts and the nylon should allow the button spring mechanism to flex. I haven’t used this before, so I don’t know exactly what to expect in terms of tolerances of finishes so we will see how it turns out.
There is a removable plug to cover a recess needed to get the body of a micro USB connector sufficiently close to the PCB to connect. Hopefully this will fit snugly. The base is designed to be removable for servicing. I’ve made the tolerance pretty tight so hopefully with a bit of sandpaper it will make a nice friction fit.
The sharp memory display has a larger border on the bottom edge to accommodate some electronics and the flex ribbon that goes to the connector. To accommodate this within a circular housing would have meant making the diameter greater than I was happy with or adapting the shape to be non-circular. I chose a third option which was is to cut off the bottom corners of the glass. After close inspection there didn’t appear to be any vital electronics in this section and the display still function correctly after the trim. J
To cover the screen I am planning on purchasing a 40mm 1mm thick replacement watch glass. There seems to be a thriving small market for watch makers and repairers. It seems that you can even get a sapphire screen if you want to pay a bit extra.
To cover the electronics I plan to create either a sticker or face with the appropriate square cut out for the screen. I haven’t yet decided on the best way to achieve this neatly.
I have designed it for an 18mm wide watch strap so this should be a trivial part of the process.
To generate the renders I first had to export the model to Sketchup. Annoyingly I actually I had to do this one component at a time and manually align them afterwards as DS Mech didn’t seem able to export the whole model. I then used a Kerythea plugin to export the model to Kerkythea for rendering. Kerky is a very powerful and relatively easy to use free rendering software. I swapped out the basic sketchup materials for some better Kerkythea materials, setup some lighting etc and hit render (actually I went though an awful lot of iterations trying to get it to look right).