Upcycling a Palm Portable Keyboard Into a Bluetooth-Enabled Accessory
Bluetooth HID adapter for the Palm Portable Keyboard based on Adafruit's nRF52 Bluefruit LE.
Mobile phone game developer, Xinming Chen, also known as pymo, stepped back a couple of decades for a mobile device hack. This Palm Portable Keyboard (PPK) Bluetooth adapter uses an Adafruit nRF52 Bluefruit LE to upcycle the vintage fold-up keyboard into a usable accessory for modern mobile devices.
"DIY project that turns the classic Palm Portable Keyboard into a Bluetooth keyboard. It can be paired with any laptop, phone or tablet that has Bluetooth, giving us a very portable and comfortable type-on-the-go option." — pymo
The Palm Portable Keyboard folds itself into a convenient carrying case when not in use. When unfolded, however, the user had a keyboard about the same size as contemporary laptops. Since these keyboards generally pre-date Bluetooth, there is not much utility for them today — unless you are still using a Palm V!
Chen's PPK BT Adapter plugs into the keyboard where a Palm III or Palm V would normally go. The 3D-printed enclosure cleverly uses wires to adapt to the proprietary connector. Once connected, the Adafruit's nRF52840-powered Feather communicates with (and powers) the keyboard matrix and pairs to mobile devices. Unfortunately, the keyboard's IO signals are not directly compatible with the nRF52840's GPIO pins. Fortunately, a simple transistor and voltage divider-based logic converter circuit do the necessary shifting.
This PPK adapter is similar to another project which used an ESP32. However, the nRF52840 is more energy-efficient, and since it is a Feather form factor, it has a built-in LiPo battery charging circuit. On the other hand, the Adafruit nRF52 Bluefruit LE is slightly more expensive than an ESP32.
The Arduino IDE can program the Feather. However, Chen provides ready-to-go firmware if you prefer just to plug and play. The existing firmware controls an LED to indicate the LiPo battery's charging state, enables pairing with a Bluetooth-enabled mobile device, and the necessary code to interface to the Palm Pilot Keyboard.
For those who want to build a Palm Portable Keyboard Adapter, this GitHub repo includes clearly written build instructions, extensive pictures of the construction steps, 3D-printable enclosure pieces, and Arduino IDE compatible firmware files