Some time in the last two and a half years, I found this HP-21 front enclosure and keypad in my university's electronics recycling bins. I picked it up, thinking I would give it some new guts. The Trinket EDC contest finally gave me the motivation to get started on it, and a constraint on the design (must use a Pro Trinket). I'm planning to use 3 SparkFun bubble displays for 12 digits, the same number as the original calculator. These and the keypad will be controlled by a SparkFun SX1509 breakout that I already had. That's a 16-port I2C port expander with a built-in LED driver and a built-in keypad matrix scanner. I also want to add a blue LED as a shift indicator, because the shift key is blue. Power will be supplied by a lipo battery.

The only things I'm not sure about are:
- How to connect to the keypad without soldering. (Links 3 & 5 on left have unclear photos of how HP did it.)
- How to drive 12 digits of common-cathode 7-segment displays with 16 pins.
Advice is welcome :)