Progress continues... I made some experimental buttons (above) to test how they’d work and feel.  I made a prototype with these buttons (shown here with a back piece), and it felt surprisingly well.  But, they had to go... I changed some basic assumptions about how I used the space inside the case, and they didn’t fit anymore.

The new PC Boards came in yesterday, courtesy of Advanced Circuits / 4PCB (which is close to me, in Denver, CO).  Here are the basic changes:

1. 1.The new board had to be about 20% smaller to make the case symmetrical.  This was the Spread Sheet disaster I made earlier.
   

2. 2.The buttons where actually designed to fit with the case and the board - I made some changes that should make them work better. The buttons should feel better, be more rugged, and last longer... well, that’s the plan.
   

3. 3.I added a battery level detect circuit because I want to know when the battery is going to die.  Originally I thought that the microcontroller could do this without any additional components -- it’ll work fine when the battery is running (3 - 3.7 volts), but it exceeds the limits of the chip if you charge the battery (4.25 volts).  This seems like a mistake on SiLab’s part ... it would be nice if the easily accommodated rechargeable lithium batteries.
   

4. 4.I improved the ground plane for better noise - this isolates the 12v power supply from the sensitive accelerometer.
   

5. 5.I also switched to a digital accelerometer.  It costs slightly more, and there will be a slight power reduction, but I’m hoping that I’ll get less noise in the system.  The accelerometer also some some nice features like a tap-to-wake detector.
   

6. 6.I also fixed some capacitors that didn’t fit - I hadn’t left enough room for them.  Tantalum are usually used for these bigger capacitors (4.7-10uF), but, to my surprise, some new ceramic capacitors have come on the market that are actually smaller (and also substantially thinner).
   

6. 6.I used a thinner board - 0.031 inches instead of the usual 0.062.  This means a thinner watch and/or thicker batteries.  My previous board vendor, while they do an excellent job, would have charged 3x the price 4PCB did for this simple change!
   

But the big change was how I placed the parts. In the previous design, I put all the chips to one side and used the remaining space for the battery.  Because I had to shrink the board and add more functionality, this wouldn’t work for the new design.  So, besides the thinner PC Board, I made sure every component was thinner.  The only components I couldn’t get thin - the switches and one power supply - remained on the edge.

Relative sizes are correct; Battery shown in red overlay.

I made a wax prototype (which is quicker to machine than plastic) to make sure everything fits with the new circuit board.  Here it is:

While this looks very similar to my previous cases, I’ve totally redrawn it with a real CAD package.  I can recommend Alibre Design - it has a lot of powerful features, and a limited-use version is available for free (windows only, though).  They have good video tutorials, too. This opens the door to more complex designs (but I think I’ll keep it pretty close to this shape), and the ability to have other people produce the design from drawings.

At the lower right, you can see the new button switch.  This should have a better feel and be more durable.  At the top right is the new key cap -- this replaces the squarish ones I made shown at the top of this post.  It’s in backwards, but that’s good enough to verify the clearance.  It’s an off-the-shelf part with a nice textured finish, and here’s what it will look like:

(call me weird... I think that’s a good looking button!)

Next up: designing the back, making a plastic case, making a jig to accurately locate the holes for the buttons (my milling machine can only do top and bottom; it can’t do sides), and then testing/building up the circuit board. I’ve got software to take advantage of the buttons somewhat, but that’ll have to improve, too.  Wow, still lots of work to do!