- LilyGo TTGO T-OI PLUS ESP32-C3 board (no battery holder version, $5.98+shipping)
- 22 AWG single core wire for making the connector.
- or, a custom made PCB if you are making for Palm M500 keyboards. You can fabricate it yourself, or buy it from my Tindie store.
- Some thin, flexible wire for the switch connection. I use 30 AWG PVDF insulated single core wire.
- 802040 Li-Po battery 700mAh (40mm x 20mm x 8mm in dimensions).
- One travel limit switch, model No. KFC-V-204F or MS-V-204F. Please use this exact model, because the enclosure is specifically designed around it.
- 3D printed enclosure (stl models are in the 3d_print/ folder).
- 2 screws with the length around 5mm and the outer diameter around 2mm. You can also use other size if you are willing to modify the enclosure (The thread density does not matter because it self-taps into the hole)
- Double-sided tape, and super glue.
ESP32-C3's hardware serial port has built-in invertion function, and its GPIO has built-in pullup/pulldown, so it does not need any standalone components like the version 1 does.
Some notes about the schematic:
- cy384 explained the keyboard's pinout and protocol in his project page very well. And here is the official Hardware and Electronics Reference for the keyboard.
- We directly use GPIO as the "VCC" and "GND" of the keyboard, this way we can easily reboot the keyboard. The keyboard uses less than 2.5mA, so it's OK.
- I used a travel limit switch, and embeded it inside the socket which connects to the keyboard, so that it will be automatically on when the adapter is plugged in.
See the stl files in 3d_print/ folder. It comes in 4 files: the upper cap, the middle frame, a battery spacer and two board clips. The board clips need to be super-glued to the middle frame during the assembly. The battery spacer is to prevent the battery to be overheated by the ESP32-C3 chip (just a pre-caution, the board only get slightly warm during normal usage).
I used Tinkercad to create these models. If you are interested in modifying the 3D models, you can visit this link: https://www.tinkercad.com/things/iDDUoFQ8ppK
Desolder the 4-pin connector and the power switch.
(optional) Desolder the resistor next to the red LED, solder a 2K ohm resistor between the [8] pad and the positive pin of the red LED. This would re-purpose the red LED to be used by the GPIO8, instead of always on.
Solder two 5 cm lenth thin wires to the pad where the switch was, and the other ends of wires to the travel limit switch.
If you are making adapters for Palm III or V, or Handspring Visor:
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Cut the single core wire into five (for Palm III or V) or three (for Handspring Visor) 3.2 cm sections, leave 6mm of the insulation on them.
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Palm III or V: solder the wires to pin [4] [5] [6] [7] [10].
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Handspring Visor: solder the wires to pin [4] [6] [10].
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If you are making adapters for Palm M500:
- Cut 5 segments of thin wires, solder them to the custom PCB, then solder the wires to pin [4] [5] [6] [7] [10]
Solder the positive of the Li-Po battery to pad [+], the negative to the [GND] pad.
Use a double-sided tape to glue the battery spacer to the back side of the board, then use a double-sided tape to glue the battery to the battery spacer.
Put some super glue and install the switch inside the middle frame, glue it in-place.
Insert the wires into the holes of the middle frame, make sure you are inserting them into the correct hole by cross-check with the schematic.
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Palm III or V: from left to right, pin [4] [5] [6] [7] [10] goes to hole 2, 3, 4, 7, 10
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Handspring: from left to right, pin [4] [6] [10] goes to hole 1, 5, 8
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Palm M500: insert the PCB into the middle frame.
Glue the two board clips to the middle frame, the board clips hold the board like this:
Trim the wires to suitable length, then bend the wires and fold them inside. This is what contacts the keyboard connector.
Put the cap over the middle frame. Make sure it is the correct orientation (the USB type-C port should align with the opening on the cap).
Screw the 2 screws, and you are done!
I find this tutorial very good for teaching how to use the Arduino IDE to program the ESP32-C3 board. Other than the steps in the tutorial, you also need to install the library NimBLE-Arduino, refer to the "Installation" section in NimBLE-Arduino's README.
Once you know how to program the board, open up version2_esp32c3.ino, plug in the board to the PC using a USB type-C cable, and hit upload, wait for the programming to be done. Then you can test it on the keyboard!
If you are compiling for Handspring Visor keyboard, uncomment the line #define HANDSPRING in the file version2_esp32c3.ino. If you are compiling for Palm III, V or M500, keep it commented out.
Notes:
- The micro switch needs to be at the "on" position for the board to accept programming, this means you need to plug in the adapter to the keyboard before trying to program.
- Avoid using a USB hub for programming the board, because it may not provide enough current. Use a USB port that is directly on the PC.
- I've heard complaints about this LILYGO T-OI-PLUS's USB-RS232 chip driver is not very stable under macOS. So Windows is preferred.
- Green + Red LED constant on: The adapter is handshaking with the keyboard.
- If it is stuck at this state for more than 3 seconds, it means the adapter is not communicating correctly with the keyboard. Please consider the following possibilities:
- The adapter is not seated properly, please re-seat the adapter, or check the contacts visually.
- The keyboard may be broken and not responding to the handshake.
- The adapter may be low on battery, try to charge it for a while.
- If it is stuck at this state for more than 3 seconds, it means the adapter is not communicating correctly with the keyboard. Please consider the following possibilities:
- Green LED fast blink: Bluetooth pairing mode.
- Green LED slow blink: Paired to the host device, normal usage.
- Blue LED on: charging battery.
- Blue LED off: charge complete.
- Red LED one blink: battery is below 20%.
- Red LED dual blink: battery is below 15%.
- Red LED triple blink: battery is below 10%.
- Please recharge as soon as Red LED starts blinking (<20%), to maintain the battery health.
- It can only connect to one device at a time. It automatically connects to the last device it is paired to (if that device is in range). If you want to connect it to another device, you can manually disconnect it in the last connected device's operating system.
- The board will discharge the battery to a very low state, so please charge the battery as soon as you see the red LED blinks. Otherwise the battery health may be affected.