RPN Scientific Firmware for the Spikenzielabs Calculator Kit

[jklsadf]

The tactile switches it comes with are decent, with positive tactile feedback, they just require a lot of force to press. The atmega328 in the DIP-28 package (used to be used in the arduino) has the same number of pins, but a different pinout including power pins, so it wouldn't be a drop in replacement. There are other people who have done fully custom calculators with an AVR microcontroller, including the original kit calculator this thread was started for, ARC, and also SCARY.

SCARY fits an impressive amount of functionality into only 8KB of flash. I've considered using similar trig algorithms (which valentin albillo also used for an HP 12C implementation earlier, not sure if he actually came with the algorithm or not, but I don't know of any earlier examples) because of flash size, although I think the ones used in the HP 35 are potentially more accurate.

There are also people who have done much more advanced fully custom calculators with ARM microcontrollers. The diyleyuan kit is definitely the easiest to get though, and widely available.