RPN (Reverse Polish Notation) scientific calculators are used by entering the operators following the operands. That means that parentheses aren’t needed, which makes them easier to use. Importantly, that also makes the order of operations easier to process, which is why RPN scientific calculators were most popular in the ’70s and ’80s, particularly for models produced by Hewlett-Packard. If you’re interested in building your own RPN scientific calculator, GitHubber ZOOXO has a great open source design.
This RPN scientific calculator, called SCOTT, is the successor to ZOOXO’s previous design ScArY. That earlier project was based on a commercial development board and only had a 7 segment display. For this new model, ZOOXO decided to build the circuit from scratch, which allows for the use of a more versatile SSD1306 OLED display. The other major components are a Microchip ATtiny85 microcontroller and a 16 key one-wire keypad. Of the five regular I/O pins on the ATtiny85, only three are being used: two for the display’s I2C connection, and one for the keypad.
The calculator is powered by a single CR2032 lithium coin cell battery, which is good for 20 hours of use with the display at full brightness, and 200 hours with the calculator in sleep mode. Once assembled, ZOOXO’s code offers a number of features, including: standard RPN scientific calculator functions, power settings, a function menu, user-definable keys to store constants or functions, and a type recorder to store and playback three user-defined sequences of keypresses. If you want to experiment with RPN scientific calculators, SCOTT is an inexpensive way to do so.