Although you can buy your own calculator very cheaply you can learn a lot by building your own. Kale_3d has developed an Arduino based calculator that uses a keypad and an LCD display to your calculations.
The project is very well documented, including instructions on how to build the case, full code examples and a wiring diagram. This is a great project to learn about electronics assembly and coding and learn how to develop something more than a blinking LED! If you are keen to get started building your own calculator checkout the following link.
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