Cash Register Transformed Into Custom Shortcut Keypad

Last spring, mechanical engineering student Bryce Dombrowski was perusing the engineering building’s free pile of parts, when he found a true gem — at least to him — an all-in-one cash register. While he had no desire to start collecting cash for goods, he was astute enough to realize its potential as a custom shortcut keypad.

After extracting the key assembly from the larger resister, he then set to work reverse engineering everything, or as he puts it, “two hours of painful probing with a multimeter and careful documentation.” When done, he had an electrical diagram that could be used to determine what key is pressed, accomplished using a double-layered switch statement running on the SparkFun Pro Micro that powers his new rig.

The Pro Micro that he employed here can emulate a keyboard out of the box, and he implemented NicoHood’s excellent HID library for its ability to use media keys. Inputs for various Greek letters and other functions helpful to an engineering student were added, along with music controls, and the already-there numpad should be helpful as well. Everything was housed in a new MDF enclosure, making it compact enough to reasonably fit on one’s desk.

Dombrowski has shared the code for his project on GitHub if you’d like to examine things further.