About 75% of the keyboards have FPC cables that end with exposed metal traces on one side and a plastic backing on the other side. The two bins of laptop keyboards shown above are from Re-PC, a local recycling store. The keyboard controller drives each row low, one at a time while reading the columns (with a pull up resistor) to see if any switch is pushed. Laptop keyboards use a flexible printed circuit (FPC) that connects all the key switches in an array of rows and columns as shown above. I will include download buttons for many of the files in the Instructable but all the files are located at my GitHub repository.

The touchpad code described in step 22 can be added to the keyboard routine to create a composite USB device. After every key has been pressed, the results can be transferred to a row-column matrix and used by the TMK keyboard controller software or a home-brew Teensyduino routine. The Teensy will report over USB, the two pin numbers that are connected when you press a key. Instead of taking the keyboard apart or using an ohm meter, this Instructable will load the Teensy with an automated continuity tester. An Instructable from alpinedelta disassembles the keyboard in order for the connections to each switch to be visually traced back to the connector. I did this when I converted a Sony Vaio into a Raspberry Pi laptop. One approach, (that I never want to do again) is to exhaustively check every connector pin combination with an ohm meter while holding down each key. Whatever software you decide to use, it will require a key matrix that maps out how your keyboard is wired.

If you would rather write your own keyboard software using Arduino, the Teensyduino functions give you total USB control. The TMK code is a bit of an overkill if you just want a simple USB keyboard but it will certainly provide all the features you could ever need. Teensies are often used by the mechanical keyboard enthusiasts at Geekhack and Deskthority and the TMK software is the most popular controller code. I use a Teensy microcontroller mounted on a connector board to take over this function. A typical laptop keyboard relies on the motherboard for the keyboard scanning and touchpad interface. I created this guide and video to hopefully make it easier for people to re-purpose an old laptop. This Instructable will provide a step by step procedure for building a USB laptop keyboard and touchpad controller.

Use external keyboard with laptop