A Teensy Adapter Board Brings an HP 200LX Keyboard Back From the Dead as a USB Input Device
A dead palmtop from the 1990s donates its keyboard to a new Raspberry Pi-powered project, with a little careful reverse engineering.
Pseudonymous maker "Sunshine701c," hereafter simply "Sunshine," has designed a Teensy-powered adapter which lets you connect the keyboard of a vintage Hewlett-Packard 200LX palmtop to a modern PC as a USB device — as part of a project to upcycle a deceased example of the device.
"I'm working on a project at the moment, basically sticking a Raspberry Pi into the case of an old [HP] 200LX palmtop computer that broke on me," Sunshine explains. "The start of this project is to adapt the old keyboard so that it can be used as a standard USB keyboard. I used an old Teensy 2.0 microcontroller and kbfirmware.com (very old I know) to [do] this, along with a custom interposer PCB to adapt the very weird pad spacing on the keyboard connector to the Teensy."
Released in 1994, the HP 200LX was a palmtop PC built around an Intel 80186-compatible processor running at 7.91MHz and supporting up to 4MB of RAM with storage expansion available via a PCMCIA slot at the side of the device. The lowest-performance current Raspberry Pi model, meanwhile, is the original Raspberry Pi Zero, which boasts a 1GHz CPU and 512MB of RAM.
Inserting a Raspberry Pi into an HP 200LX, then, would be something of an upgrade — though Sunshine was a request. "I beg, PLEASE DON'T tear apart a perfectly working 200LX for a project like this," the maker writes. "I tore mine apart because the LCD and [motherboard] were both busted. Unless you're in the same boat, with a broken keyboard AND LCD, consider using a different small form factor keyboard, it'll probably be easier to implement and use, I'm only using this one to make use of old technology that would otherwise be thrown out."
To get the keyboard talking to current systems, Sunshine had to probe each trace on the flexible circuit to figure out which key corresponded to which pin on the unusual connector. With that map in mind, the maker built an interposer board which sits between the keyboard connector and a Teensy 2.0 microcontroller board — which, in turn, listens for key presses and sends them over USB to a modern host machine.
More information on the project is available on Sunshine's Reddit post, while source code and KiCad project files have been published on GitHub under the reciprocal GNU General Public License 3.