This entire project is based on an erricson keyboard, the ones used for typing messages on your cellphone. You can probably get them on ebay for $3. It has all alphanumeric keys, left and right arrows, a shift and backspace key and 7 special keys. In it’s original form, it sends sequences of digits to imitate the use or a numeric keyboard to enter text on a cell phone. Possible to decode but rather messy.
The best part is that the original MCU they have on is pin compatible with some AVR’s, so the author had no trouble replacing it with an AVR.
AVR Chatboard Made From Erricson Cellphone Keyboard: [Link]