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Thread: Question about creating a custom keyboard

  1. #1
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    Question Question about creating a custom keyboard

    So I went and bought a $10 keyboard and took it apart. Then I went through and figured out what key creates a short between which 2 (or more) pins.

    I plugged the PCB into the PS/2 port, and the computer sees it as a keyboard, so no problem there, but when I try to short two pins together with a wire, it does nothing.

    Am I missing something on how keyboards work? The only reasons I can think of as to why it wouldn't work is because A) I need to use a momentary switch and not a wire and/or B) the piece of wire wire (18 guage) has too high of a resistance (~1 ohm) and of the .5uA I measured not enough can get through.

    So for those of you who hacked a keyboard and wired it up yourself, whaddya' know that I don't?

  2. #2
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    Well, Im no expert but I did wire one up for my Mame machine. As I remember there should be 2 sets of pins (set1 and set2). You need to short a pin from set 1 and a pin from set 2 to creat a key stroke. What I did was download a keyboard tester (google or dogpile search) pluged my keyboard (or what was left of it) in and just started going through the pins and write everything down in a chart. BTW it only needs a short for a second, just like you would hit a key. Now remember that all the keys will be in there so if you short 2 pins and nothing is typed you may be hitting 'esc' or 'shift' or something like that. It shouldnt matter what size of wire you use, I used solid core cat5 wire (just one strand) to short them for testing.
    Steve

  3. #3
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    I did almost the same exact thing. I didn't use a keyboard tester, though. I labeled the pads on the plastic membranes with the corresponding keys, then traced the contacts and made a truth table. Then I opened up Notepad and started shorting wire pairs, verifying that my truth table was accurate. If you do it this way, you'll find that wire pairs are shared. For example,

    1 + 2 = A
    1 + 3 = B

    The 1 wire gives two different keypresses depending upon what other wire it is shorted with. This allows you to have two functional buttons with only three wires. This may not be of interest to you; it was to me as I was interested in getting as many functional buttons as possible over a single Cat-5 cable. With eight conductors, you might imagine you could get only four keys. Using the method described above, I was able to get sixteen, like this:

    1 + 5 = A
    1 + 6 = B
    1 + 7 = C
    1 + 8 = D
    2 + 5 = E
    2 + 6 = F
    2 + 7 = G
    2 + 8 = H
    3 + 5 = I
    3 + 6 = J
    3 + 7 = K
    3 + 8 = L
    4 + 5 = M
    4 + 6 = N
    4 + 7 = O
    4 + 8 = P

    Of course, this doesn't actually get you a nice, neat block of the letters A-P. On my keyboard, it included punctuation, numbers, etc. The above is just an example.

    Hope this helps.

  4. #4
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    It shouldn't be the resistence. If you OHM out your wire traces on your keyboard you will find that they can be higher than the restance on a straight peice of wire anyway.

    And it should not be because your holding the wire and shorting the pins, because this would be just like holding down a key.

    I would just check to make sure your shorting 2 pins that actually do something. With 25 of them that gives you over 600 combinations and you really only need 104. Besides, it is easy to get the little buggers mixed up.
    A 97 Dodge Ram pickup with a 14" LCD running at 1024x768. A Dell desktop PII 400, 128MB Ram, 8GB & 20GB Drives, TV tuner, Cd Burner, Garmin legend GPS, Mappoint 2002, win98, and winamp.
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  5. #5
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    Hey cybuch, Did those keyboard testers show a difference between left and right shift keys?

    I have been working on a VB program that will do some keyboard testing, but mine will not show them seperatly.

    Thanx
    A 97 Dodge Ram pickup with a 14" LCD running at 1024x768. A Dell desktop PII 400, 128MB Ram, 8GB & 20GB Drives, TV tuner, Cd Burner, Garmin legend GPS, Mappoint 2002, win98, and winamp.
    pics

  6. #6
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    You nailed it right on the nose....

    Originally posted by tation
    I would just check to make sure your shorting 2 pins that actually do something.
    and that would be the kicker. I must've had one side of the wire in between two of the traces because after reading the first reply I went back and tried again and TADA! Worked like a charm.

    Now I'm just trying to figure out what's the best way to hook the PCB up to the switches. I was thinking that I might try using a connector instead of soldering wires straight to the board so that if I need to expand what keys I'm doing later it would be a little easier.

    Any ideas?

    Micah

  7. #7
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    Yes, it showed left and right shift alt,crtl, ect.. It worked great for me
    Steve

  8. #8
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    Awesome Micahb, glad you got it working. I thought about using some sort of connector, but I have yet to find something that would work well.
    I have decided just to go ahead and solder my wires.

    Hey cybuch, What is the name of the program that you were using? I did a quick search on google and it only turned up a few sites and none of them had any downloadable programs. If I just searched for the program name I am sure I would find it.
    A 97 Dodge Ram pickup with a 14" LCD running at 1024x768. A Dell desktop PII 400, 128MB Ram, 8GB & 20GB Drives, TV tuner, Cd Burner, Garmin legend GPS, Mappoint 2002, win98, and winamp.
    pics

  9. #9
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    here is the link , its a 30 day trial version but at least its something.
    Steve

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