Hardware
The board design was made in Eagle, has been tested and works. It can be freely modified. It can also directly be ordered on the OSHPark website:
DISCLAIMER: I am in no way affiliated with Atmel. I just put their logo on the PCB to show the circuit uses an Atmel chip, which I am a big fan of)
Software
The software was written in Arduino code, is licensed under MIT License, and can be uploaded on the microcontroller via ISP. Be sure to set the desired time in the code prior to uploading, to set the time registers on the RTC.
Functions of the code include:
- Hours, minutes and seconds
- Day and month
- Stopwatch
- Temperature reading from the real-time-clock's built-in thermometer (accurate to about 3°C)
To be able to upload the code on the watch, an Arduno IDE board file for an "ATMega328 on a breadboard" running at 8 MHz with the internal crystal, is needed. You can find it here: https://www.arduino.cc/en/Tutorial/ArduinoToBreadboardunder "Minimal Circuit (Eliminating the External Clock)"
Required Libraries
Awsome project! I'm working on my own version of such watch, but I'd like to point out minor flaw in your schematic. When you want to for example turn off first digit you pull it's cathode high but when for example segment "a" is turned off, it's state is low and this creates reverse voltage on a segment. While you are using 3.7V as a supply it's not a big issue because this HP display can stand 5V reverse voltage per segment, but when you connect it to 5V during charging this can be an issue.