I’m not sure yet on how to build the front plate, but this is how I intend it to look like.
These words fall into five groups. In every group only one word is going to be lit at a given time. First, there is “Es ist” (“it is”), which will be lit all the time. Second, there are the words representing the minutes, with an accuracy of five minutes. Those words are “Fünf”, “Zehn”, “Viertel” and “Zwanzig” (“five”, “ten”, “quarter”, “twenty”). “Vor” and “Nach” (“before” and “after”) represent the relation to “Halb” (“half”), which will be it’s own group, and the hour words (“Eins”, “Zwei”, “Drei”, … , “one”, “two”, “three”, etc.), with their own group as well. Last, but not least, “Uhr” (“o’ clock”), which will only be lit on the full hour.
|B||5||3||fünf, zehn, viertel, zwanzig, uhr|
|E||12||4||eins, zwei, drei, …, zwölf|
Every word will be lit either by a single LED, or by one LED per letter, I still have to try what looks best (and is practical, white LEDs tend to use up quite an amount of juice).
Using charlieplexing significantly reduces the number of pins needed to control the plugins. The downside of charlieplexing is that in most cases only a single LED can be lit, hence the different “groups” where only one word needs to be lit. Using this configuration I only need 11 pins on the Arduino to control the LEDs, given that the 5V from the Arduino are sufficient enough.
This leaves 3 pins on the Arduino available, of which one will be used by the DCF77 radio clock module. Currently I’m considering to run this either on an Arduino Nano or even on a Lilypad, which is even cheaper but needs more care setting it up.