In an attempt to master Eagle and surface-mount soldering, I decided to design a simple LED wristwatch with as small a footprint as I could manage. As you can see in this schematic (board files here), the watch runs on an ATMega328P.
I chose this chip because it’s the same chip in an Arduino Uno, and there are many tutorials online for how to upload code to it via an Arduino ISP. I also decided to use a real time clock with a built-in crystal to keep time. This RTC is pretty expensive ($8 on mouser?), but keeps time extremely accurately. On V2, I might use a cheaper RTC or forgo the RTC and add a 16Mhz crystal instead. At the present, the Arduino is designed to use its own internal 8Hz oscillator as a clock.
I meticulously reviewed the schematics and printed 3 boards for around $10 from OSHPark.
Nonetheless, I found after soldering 90% of the components that the schematic is broken! There’s a whole row of LED’s that are connected to nothing, and there’s no easy way to get at the clock pin. I tried bootloading the thing with an Arduino, but haven’t had success yet. News to come with V2.