Desktop keyboard: Some keys are not working... Why?

cristipiticul · Jun 6, 2013

Hello!

I have been using a nice keyboard for a while. It was working just fine, before I spilled a cup of water on it. Then, some keys started acting weird, but I solved it: take out the screws on the back of the keyboard and open it... I've seen that the plastic thing was wet. So I left it until it dried and put everything back.

It worked fine for a while.

Then, suddenly, a group of keys (1 - 10 except 5 and 6, the Page Up and Home to be more exact) stopped working. Later on, the 5 and 6, -, = and left CTRL stopped working too, but I noticed that if I pressed those keys about 10 times repeatedly, they would start working again.

Now I changed my keyboard and everything is fine...

I'm still wondering: what happened? What's the connection between those keys (1,2,3,4, 7,8,9,0, Home and Page Up)? Can it be fixed?

Thank you for reading and for your time!

makkem · Jun 6, 2013

Hi
if you strip the keyboard down and clean the contacts with switch cleaner or isopropyl alcohol then there is a good chance it will work ok.

Solandri · Jun 6, 2013

A typical 101-key keyboard doesn't actually generate 101 different electrical signals. To simplify the keyboard's design, groups of keys are linked together with multiple wires, and the combination of wires that are activated tells the keyboard which key is being pressed.

The simplest way to describe it would be to imagine a 102-key keyboard with 6 rows and 17 columns. The obvious way to to wire it is a separate wire for each key. But that requires 102 wires.

A simpler way to wire it is one wire for each row (6 total), and one wire for each column (17 total). This only requires 23 wires. If the row 3 column 2 wires are activated, then you know it's the letter q. If the row 4 column 2 wires are activated, then you know it's the letter a. If the row 3 column 3 wires are activated, then you know it's the letter w. And so on.

The drawback is that depending on the wiring scheme, the keyboard can get confused if certain combinations of multiple keys are pressed. In the above example keyboard, if you pressed q and s together, that would trigger the row 3 and 4 wires, as well as the column 2 and 3 wires. Unfortunately there's no way to tell from that whether q and s are being pressed, or if it's a and w. Both light up the same combination of wires.

So the challenge is to design the keyboard using fewer wires, but still allowing all conceivable simultaneous keypresses to be read correctly. I won't get into the details of how this is done, as it's pretty complex and mathematically very similar to how error correction coding works. The wiki article is a good starting point.
http://en.wikipedia.org/wiki/Rollover_%28key%29

So the groups of keys which weren't working were probably connected by the same wire. When the water shorted that wire, all the keys connected to it stopped working.

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Desktop keyboard: Some keys are not working... Why?

cristipiticul

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makkem

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Solandri

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