Square Nixie Watch - Part Two
I was at a birthday party in May 2015 and my nephew suggested that I look for sealed USB connectors. I found one at Digi-Key that looked good, so I resurrected the project.
I redesigned the case in the summer of 2015 to be two half-depth aluminum shells. The rear shell holds the module, anchored by the USB connector's two screws. It has an O-ring groove design based on my ancient Casio C-70 calculator watch. The strap attachment was moved away from the rear of the watch, and the screws were changed from flat-heads at the rear surface to cap screws 1/8" away from the rear. This was done to keep the screws from irritating the wearer's arm.
The styling was changed to improve the looks. One of the joys of the round watch is viewing the insides through the large glass window. The rectangular window was resized, and the case front curvature changed, to allow the tubes to be seen fully, from nipples to legs. Oo-la-la! I also made the strap wider, from 20 to 22mm.
I wore my 2012 square watch to Burning Man in 2015. It kept working, amazingly enough, with all that playa dust everywhere. I vowed after that to get the project into production.
I made a batch of ten cases and ten modules, the NWLC version, in late 2015. The cases worked very well, with only minor repairs needed for the O-ring chamfers.
The switching power supply controller had been changed to a smaller SOT-23 part made by National, but this part ended up not being reliable. On some watches, the chip would develop a low-resistance path across the MOSFET switch, which would drain the battery in a few days. I couldn't find any obvious source of the problem, so I went back to the relatively huge LT1308B chip by rearranging a few parts.
The NWLD board works well, with no modifications needed to fit or function. I had to modify the code to use the 16LF1519 MPU, add a low-battery indicator function and handle the changed accelerometer position. I also fixed the 12/24 hour bug that had existed in the code for fourteen years.