I got the urge to design a thin, rechargeable Nixie watch in 2011. I collaborated with
Melinda Maerker who had useful ideas about the
shape and styling. Thus, the case design with rounded sides and symmetrical tube layout.
The design criteria were:
2. As thin as possible
3. Simple, clean styling
5. Easier to set time
I first thought that it would use two small batteries, one on each side of the tubes. Then I realized that the buttons and internal switches take up a good bit of space on the right side, so I switched to one battery on the left side.
I also thought at first that a flex board would be the best way to make it extra-compact. I eventually concluded that this was not necessary, as the flex board didn't actually make the design any smaller if I made the tubes sit in cutouts in the rear board. The components behind the tubes were made smaller, from 1206 size down to 0805, to handle the reduced space available with the tubes sitting in cutouts.
I chose the micro USB connector for charging, as it's ubiquitous. However, I was not able to find a sealed micro USB connector for sale. I left the area around the connector open on the case. This ended up being a bad idea. Sweat from my arm got into the watch case via the opening, and caused trouble with the circuit board. This would clearly have to be changed for a viable product.
The pushbuttons had to be activated through the case, because the cover was no longer easily removable. They sit at a slight angle relative to the switch PC board, so their inside ends are rounded.
The front window was made rectangular to fit the general shape of the watch. The first version used a window just large enough to reveal the digits in the tubes. The fit ended up being just ever so.
The circuitry had to change to accommodate the rechargeable battery, as its output voltage when full is 4.2V, which is higher than the accelerometer can withstand. A 3.3V regulator was added to solve this problem. A lithium-ion charge controller was also added. The USB port only provides 5V and not data.
I made some 3D prints of the case design using my new Ultimaker. It seemed to be a good size to wear. So I paid to have a batch of ten cases made at the same local machine shop that makes the round Nixie watch cases. The back is stainless, with pockets to accommodate the pins projecting from the circuit board. However, there was no recess for the power vias, so one of the cases developed a power short circuit at that point.
I wore one of these NWLB watches for a couple months. It worked OK. My brother also wore one, but his failed after a while. My son also tried one, but it also had a problem. I got distracted by robots and let it languish for a few years.