Adafruit’s got a handy breakout board for the DS1307 RTC available. This chip isn’t nearly as accurate as the DS3231 used in the Chronodot but it’s quite a bit cheaper. The breakout makes this easy to breadboard or plug into an Arduino and has everything you need; clock crystal, a backup battery, filtering capacitor, and pull-up resistors. Our favorite part is that Adafruit designs are open source so you can etch the board yourself if you checkout the files from their git repository. This will make a great addition to our prototyping hardware collection.
Incidentally, we were surprised to see the choice of 2.2k resistors for the I2C bus pull-ups. We were under the impression that 4.7k was a standard value here. We’d love to hear your thoughts on this in the comments.
[via Dangerous Prototypes]