Convert a Broken HP 48G Into a Bluetooth Emulator Keyboard

If you went to engineering school in the 1990s or early 2000s, you may have found yourself working away with one of the several calculators in the HP 48 line. While now discontinued, you can still likely find these in use today (of course using Reverse Polish Notation, or RPM), or HP 48 aficionados can do away with the physical calculator altogether, and instead run the system on a computer or smartphone-based emulator.

While the emulator looks really useful, you’ll obviously be missing out on the tactile input of its physical keys. To bridge these two worlds, hacker epostkastl decided to convert a broken HP 48G into a unique Bluetooth emulator keyboard. This current project evolved from a wired USB interface that he was able to construct a few months ago. As you’d expect, this involved figuring out the outputs from the calculator, along with a firmware USB driver for AVR microcontrollers.

With that knowledge in-hand, epostkastl went to work on the new Bluetooth version, with an Adafruit Feather nRF52 Bluefruit board leveraged to take in keyboard signals and transmit them to an awaiting smartphone. Physically connecting to the calculator keyboard was of course tricky, and there was a decent amount of wiring work, however, it looks like the second interaction may have improved a bit. Using the Bluefruit here also allows for easy use of a small LiPo battery — for phone-based calculations on the go!