I wanted the clock to be easy to use and be flexible. The setting functions are menu-driven, you set each parameter one digit at a time (with live data validation) and you can abandon changes if you want. You can have 12 or 24-hour time. It uses a bright, legible VFD character display - you can choose even more readability with a 2x3 big-character mode.
Finally, VFDs are bright and readable, but sometimes you don't want them lighting up the room. So, you can set a schedule of when the display is bright, dim or off. You can turn the display on or off any time you want, as well.
I hope to go over key elements of the software and hardware design to help you in building a clock just like this one or to give you ideas for anything that needs menus, data validation, timekeeping and so on.
Step 1: Design Choices
Easy to use:
- menu-driven setting options
- digit-by-digit setting (who wants to go up or down to set something like longitude?)
- data validation to keep the user from inputting impossible time, date, location etc...
- buttons match the way it is used, eg. you want to look at it while setting, so the left button is really the right button
- most common functions on their own button, eg. display on/off, big/small digits, etc...
- must keep time accurately without using radio time; DS3231 RTC is accurate to 2 ppm/year or approximately +/- 1 minute per year
- no buttons visible anywhere except the back
- allow adjustment of DST start and end
- allow 12/24 time
- support display brightness schedule
I suppose you can add to "Simple" that it uses the Arduino platform. There isn't actually an Arduino board inside, though you could use one; I used a Modern Device RBBB Arduino clone and a Wicked Device RBBB shield board. I chose the RBBB because it's cheap, flexible and sports a power jack, which I needed anyway and is a pain to do properly on protoboard. I chose the Wicked Device RBBB shield as it supports the RBBB and because it gave me sufficient protoboard space to mount a 2032 coin cell holder for the DS3231 backup power and have a header for the 14-pin cable to the VFD display.