Raspberry Pi 7 Segment display


One of my #RaspberryPi Zeros is called PiClock, and has an 8 digit seven segment LED display. The program it runs displays the time, and sends it to two other Pis, that display it on Unicorn HD HATs. Between midnight and 8 am, it flashes the message “SLEEP” every five minutes, as well. The software library that it uses can display numbers, and most upper and lower case letters, but not all of them. I rather liked the idea of animating sequences of single segments on it, because, well you know, blinkenlights. I had a look at the software library, “7seg.py”, to see if I could get it to do that.

It turns out that the library uses a Python dictionary to look up the byte to send to the display for each of the characters it can display. Decoding the hexadecimal bytes took a few minutes, working from the code for the digits from 1 to 5.

The first bit is always a 0. The remaining seven are the seven segments, in the order abcdefg, which are laid out like this…

So, the codes for illuminating single segments are as follows…

Now to amend the library! I needed some typeable characters to put in the dictionary, ready to be used in strings in the python code. For no obvious reason, I chose a selection of brackets and the tilde character, and amended the library file. The selection of brackets didn’t work!

After trying characters until they did work, I ended up with #][£<$~ as the symbols for the segments abcdefg.

I’m only showing the amended part of the file, where the pattern to send to the display is looked up. The arrangement of the brackets and tilde for the segments is as follows…

Now I’m ready to program PiClock to do silly animations, which will be fun, and a lot easier than using the WordPress editor. Note to self: See if you can find a WYSIWYG editor for WordPress.