There’s an old joke... An inventor carrying two huge suitcases gets off a train. Another guy notices that he’s wearing a really nice watch and asks about it. The inventor describes it, showing how it works as a cell phone, along with the tiny screen that shows television from anywhere in the world, and even the emergency beacon that could summon help if lost in the wilderness. He proudly proclaims that this prototype watch is most technologically advanced in the world. The other guy is amazed and insists on buying it there on the spot. The inventor is reluctant at first, but they agree on a price. The buyer takes the watch and starts walking away, when the inventor lifts his two heavy suitcases and calls out to him: “wait, don’t you want the batteries, too?!”

 

So, the moral of the story is that a cool watch is practically useless if it isn’t practical. I want to make sure that my watch is a decent size and that the battery lasts long enough.

 

The picture above is the back of the OLED display. While all of its parts fit within the 3.8mm-thick steel case, there is a bit of extra room available inside to make the watch thinner.

 

The gray bar at the right (in the middle of the green flex) is the controller chip. This connects to the 96 column wires and the 64 row wires that make up the display. It contains a tiny memory to remember what is displayed on the screen, and uses this to drive the display. This display, like a TV, is constantly redrawn. Talking to this chip only requires a few wires -- reducing wires is important because I don’t think I could reliably solder 200 wires in such a little space.  With all these microscopic connections to the green flex board underneath, this chip must be treated with care to avoid breakage.

 

The gold colored flex circuit on the left contains support components and is bonded to the green flex.  These components are mainly capacitors used to help stabilize the chip’s power supply. These capacitors also form the thickest part of the display -- it’s a shame that they are in the middle because they get in the way of other components I want to put in the watch. Don’t worry though.. with a little solder and a tiny wire, I can relocate them.

 

 

Fitting the battery vertically would make the best use of space:

 

 

... but that blocks the display’s connector. The connector isn’t really that big, but it’s in the way. I could twist the connector 10 degrees to the right & it will clear, but these connectors are really susceptible to working loose if slightly twisted. Another possibility would be to widen the watch and put the battery more to the left... but that would make the watch bigger.

 

So, instead, I’ll mount the battery horizontally:

It wastes a little space on the left, but the connector will fold neatly over the battery. At the top right corner of that picture, you can see the processor I plan to use. Even though this is must be small enough to fit on my wrist, there should be lots of room.

 

 

The other dimension to this is thickness. My goal is to be roughly the same thickness as my mechanical watch -- 10.5mm.  Once I put the electronics in a case, it’ll be pretty close:

 

 

... if that doesn’t work, then I’ve got another watch that is even thicker!