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OpenRPNCalc is a scientific calculator based on STM32 microcontroller. Its source code, schematics and 3D-printed case design files are released under open license.

The hardware is inspired by the SwissMicros DM42 calculator (which itself mimics the famous HP-42), but designed from scratch. Firmware-wise, however, there is no intention to simulate programmable HP series. Currently, the calulator features:

- Reverse Polish notation with 4-element stack.
- Double-precision arithmetics.
- “Standard” scientific calculator functions (trigonometric, logarighms, exponentiation, square root and power).
- Error function (erf) and its inverse (erfinv), Gamma and log(Gamma) functions, number of combinations and permutations.
- Fixed, scientific (SCI) and engineering (ENG) display modes (including SI prefixes in ENG mode), variable 3-10 digits precision.
- Calculations with uncertainties using error-propagation formulas (UNCERT mode). Something that I’ve never seen in any of the hardware calculators, and very rarely is present in the software ones.
- Formulas from relativistic kinematics (center-of-mass two-body decay momentum, conversion between angle and pseudorapidity, beta and gamma factors).
- Low power consumption (40-50 uA in standby mode with LCD display on).

Hardware Overview

The calculator is based on low-power 32-bit ARM microcontroller STM32L476 running at 8 MHz. The display is Sharp memory LCD module LS027B7DH01 (400x240 pixel monochrome). Keyboard uses light-touch tactile switches Panasonic EVQQ2B01W (50g actuation force). All electronics runs off the 3V lithium battery (CR2032) that should be sufficient to provide power for around one year of operation.

The case and keyboard is made of four 3D printed parts (top and bottom parts of the case, keypad and switch spacer). The two parts of the case are held together by simple snap fit joints, no screws are needed.”

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