“Counting box” also saves calculators from small children

[Nathan]‘s son really loves numbers and counting, and one of his favorite things to do is add 1 to a calculator over and over again. Being the awesome dad that he is, [Nathan] built his son a counting box that has a 10-digit rotary switch and two arcade buttons to add and subtract.

One goal of the project was to have the counting box retain memory of the display while being powered off. The easiest way to do this is to write the display data to the ATmega’s EEPROM. This EEPROM is only rated for 100,000 write cycles (although in practice it’s much higher), so [Nathan] included a 24LC256 in a little spasm of over-engineering. All the electronics are laid out on perf board, and the case is constructed from bamboo that was laser cut by Ponoko. The quality of the case itself is fairly remarkable – we’re really impressed with the finish and the magnetic battery access door.

From experience, we know that playing with an HP-15C eventually leads to a broken calculator and having our Nintendo taken away. We’re really happy for [Nathan]‘s son, and wish we had our own counting box at his age.