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 :)
12/17/2014 at 00:51 •
I made a 3D CAD model of the rough dimensions of the case front in SketchUp, to make sure the components all fit well:
The hatched part is the boundary of the interior space; the brown boxes are exclusion zones for the screw receptacles that stick up. The gray and yellow thing is a battery; any of the reasonably capacious batteries from Adafruit and SparkFun will fit. The white thing is the SparkFun SX1509 breakout board I'm going to use to control the display (3 SparkFun bubble displays) and read the keypad.
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