Each of the reels has a DIY binary encoder that uses IR transmit/receive pairs to generate a binary word. These four binary words are compared to a binary clock module using comparators.

The key of the build is the encoder wheel and pegs, which act as the “converter” between decimal and binary and actually performs the switching.

In this Design Idea , an old technique of diode-based 16-to-4 encoding is combined with a new design, to scan a record number of buttons – 74 – with only four I/O pins! The diode encoder: Background ...