HP-21 calculator restoration/Arduino-fication

Description

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 :)

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CAD sketch of how the components will fit
PointyOintment • 12/17/2014 at 00:51 • 0 comments

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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Discussions

rjtescher wrote 09/11/2015 at 23:25

Oh, I'm sorry, you mentioned it was only a shell with keypad, but the MoHPC members will probably want to hear from you anyway.

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rjtescher wrote 09/11/2015 at 23:03

hpmuseum.org has a forum

They might be able to help you with how to hook up to the existing display and keyboard (once they get over the initial shock of some one butchering a beloved HP calc!) Or they may be able to help you resurrect it instead. I used to follow the forum daily, some pretty bright and helpful people there.

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Claus Buchholz wrote 03/15/2015 at 13:34

I think HP used a decoder external to the CPU to select which digit lights up at a time. So that's 4 bits to select and 8 bits for segments (including decimal) for 12 total, not including keyboard scanning. But that's a custom circuit so not practical. How many DIOs on the Trinket?

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PointyOintment wrote 05/14/2015 at 04:51

20 according to this: https://learn.adafruit.com/introducing-pro-trinket/pinouts

I was thinking of using an SX1509, but I don't know if it could be driven fast enough for display multiplexing. I'll have to try it when I get around to working on this again.

Are you sure? yes | no

HP-21 calculator restoration/Arduino-fication

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CAD sketch of how the components will fit

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