Scientific RPN Calculator (with ATTINY85)

[Maximilian Hohmann]

Hello!

Incredible what can be got from am ATTINY85 if one is patient enough to fine-tune the code :-)

Yesterday my package with display/keyboard modules arrived from China. Of course I instantly wanted to try out to run your calculator. But without success, my Macintosh still runs OS X 10.6.8 and the latest supported Arduino IDE is 1.6.0 which does not yet have the put() and get() methods of the EEPROM class yet.

So I decided to do something different instead and join two such boards to get a 32-key 16 digit calculator.

Looking at the boards one can see that there are very few traces close to the border which will be destroyed by cutting away 5 millimeters from either side.
Left side of the board:


Right side of the board:


So I went in the garage digging for my jigsaw (which actually required the most time in this project) and cut away pieces of two boards - my jigsaw skills are a bit rusty I'm afraid:


In the last step I soldered the boards together to join them into one mechanical unit (using the original connector pins for strength), replaced the sawn-off PCB traces with wires and wired the connection lines across the boards. They only require an individual strobe signal each, Vcc, GND, data and clock can be shared - so a total of four Arduino pins will be required minimally:


And this shows the "calculator" with everything else needed for it's operation. LiPo battery, battery holder, switch, voltage booster and an Arduino nano board. I am lazy and want my USB connector, therefore I did not use an Arduino mini or similar. We are still below 10 Euros for the combined cost of all these components. Incredible... The GPS module in the lower right corner is an option for later.


And here is an image for size comparison: The dimensions of this calculator are almost identical to an HP-15C. Only that I will be able to read the display at night :-)


And now some programming is left to be done.


EDIT: For some reason the shared data and clock signals did not work with my boards, so I had to split them and now require six Arduino pins.