[Augusto Marinucci] While I loved the aesthetics of the classic Cartier Tank series dress watches, I wanted something a little more high-tech with plenty of runtime on a single battery. E-Ink displays are often used in such applications, but finding one that fits into custom case designs can be difficult. If ordering something ready-made is not easy, the solution is to make it from scratch.
This article doesn't have much information on the E-Ink side, which is a bit disappointing. However, from the information we can gather, the shape of the segment (in this case based on the famous Apollo DSKY) is formed in the top copper of a 4-layer PCB, using filled and capped vias. It is connected so that it cannot be seen from below.
The donor E-Ink display is cut to size with scissors (no more!) and glued around the edges to make the common electrode connection. The display PCB is connected to the control PCB at the rear using low-profile board-to-board connectors. This board hosts a PIC16 micro and an RV-3028-C7 RTC that measures time while consuming only 45 nA.
The 5 volts are delivered through a MAX1722 low-power boost converter powered by a CR1616 cell via a logic-controllable load switch. Getting this right is important in low power designs like this. If you make a mistake here, your run time could be significantly reduced. It's easy to put too much stress on a small button battery and cause the cells to die prematurely.
The case appears to be printed with a translucent resin, with the PCB stack sealed inside with a UV-cured resin poured inside. It's not immediately clear if the back panel can be removed to access the battery and programming ports. There's something like a screw hole, so maybe that's possible. Or maybe it's on the back of the PCB mounting post. Who can say?
If DIY hardware is too much of a hassle, you also have the option of hacking new firmware into your existing watch, or going to the middle man and making something out of the junk E-ink tags that come along from time to time.
thanks to [JohnU] For tips!