Arduino Nano + EIGHTY ONE seven-segment displays = Mega Sudoku

It all started when I discovered some really cheap (10 cents per digit!) 7-segment LEDs from China.
Thanks to everyone's help it has become a keypad controlled electronic Sudoku using just one Arduino Nano, some shift registers, and EIGHTY ONE seven-segment digits!

Here is a playlist if you want to watch it from beginning to end.

Here is the final episode where I showed off the features: