07/02/2019 at 21:04 •
Time from the last update was spent refining and tweaking the design and making everything ready for production of first prototype.
Here are some key points of this version:
- Size of the led matrix was reduced from 7x12 to 6x12. This change will make assembly easier and will not have significant reduction to display readability(font).
- Battery was changed from 2xCR311 to single CR1220. CR1220 has bigger capacity(35mAh) compared to CR311(6mAh) and is easier to source. Only downside is lower (recommended) constant current drain of 1mAh, but considering current circuit layout it won't be needed. To support new battery, special battery compartment with removable lid was designed. Lid is held in place with small magnets, allowing for easy access.
- Component placement was packed tighter to remove components from areas of high bend angles. That meant replacing old 3.2x1.5mm RTC crystal with newer 1.6x1mm crystal(tiny!).
- Since PCB is planned to be encased in clear resin, phototransistor to support external configuration(set device settings, no reprogramming) is used. Transistor is connected directly to UART RX pin (No modulation is used for communication). Initial tests showed that communication up to 57600 baud is possible, but lower data rates are sufficient. Resin encapsulation is expected to negatively impact data rates. Photodiode would give higher data rates due to lower transient times, but is not available in smaller packages.
05/28/2019 at 22:48 •
Here are pictures showing off first concept.
3D models of PCB (top and bottom):
CR311 batteries compared to PCB:
Ring frame that holds flex PCB with holes for batteries and tactile switch.