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Topic: "Reverse engineering" a laptop keyboard (Read 3173 times) previous topic - next topic


First, let me apologize if my questions are "too newbie".

I have a couple of laptop keyboards that would make neat input devices.  I've read quite a bit about people converting them into PS2 keyboards etc.  I know it's not an easy task because laptop keyboards are usually very custom.  I ordered a multimeter (I'm a noob) and haven't gotten started yet but I don't know exactly how to start mapping the keys.  First, I don't even know if it's a USB or PS2 bus.  There is limited documentation available for the hardware itself so I was just going to do it the hardway and check for resistance (or something) as I plugged away at the keys.  I don't even really want complete success.  I just thought it would be fun to serial.read() off a spare keyboard.  So there's my use-case/goal.

This keyboard has a (25pin+5pin=30 pins) ribbon cable that I assume goes to a controller.  Someone on the web has successfully wired their laptop keyboard to an old ps2 controller (I don't have one) but I'm wondering if the arduino could take the place of the controller, that's neither here or there.  First, I want to see something happen with serial.read() when I'm mashing keys.

So.  How to power this thing?  I have no idea what the power specs are.  I'm guessing it's close to what regular keyboards are but I could be wrong.  3v @ 0.8mAh is what I was going to start with.  Sure, I can get a resistor to drop the power off the arduino power to this but what if it's too much, what if it's too little?  Can I use a potentiometer to adjust the voltage on the fly?

I guess my bigger question is, how do you start "reverse engineering" something that is unknown?


Laptop keyboards are usually just a big array or grid of switches.  The PS/2 interface is a four-wire thing (5V, Ground and two signal wires) that's at a higher level than the switches.  A normal PC keyboard has the switches and LEDs, plus a microcontroller that converts switch closures into data packets on the PS/2 interface (or USB if it's a new keyboard).

You should be able to trace out the wiring of the grid of switches with the multimeter.

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