Prime Time Clock
David Johnson-Davies’ LED matrix digital clock goes green when it shows a prime number.
Just when you think you’ve seen every variation on the DIY electronic clock possible, someone comes up with yet another clever take on the concept. The Prime Time clock, by David Johnson-Davies, shows the current hours and minutes on two side-by-side 8x8 bi-color LED matrix displays. These displays illuminate pixels in either red or green, or as a combination of the two colors as yellow.
The prime-ary cool feature of this clock, however, is that the digits change color when time is prime… which rhymes. Normally, the two displays show two red digits each, with a yellow colon to separate the hours and minutes in 12-hour mode. When the hours and minutes of the display show a prime number (e.g. 11:29, or 1129, divisible only by 1 and itself) the digits turn from red to green.
While DJD knows what this means, it could be an interesting conversation piece when other observers question why the clock has changed colors. I doubt I could figure it out.
The code features several display options, and if you’re not crazy about its prime function, you can change it to instead show the hours in red and minutes in green. 24-hour mode is also available, and numbers can be squared off or rounded.
The device runs on a 32-pin AVR128DA32 microcontroller, using charlieplexing instead of an external driver to control the dual display’s 256 individual LEDs. A single interface button sets the time, and a 3-pin power/programming header socket is available on the other side of the board. It’s a nice compact design which should be lots of fun to observe!
