Hi I'm Alex and welcome to "Super Make Something Basics."
Today we're fixing a broken keyboard.
To fix your keyboard, you must first break your keyboard.
For the last two years or so, I've owned a Logitech K400 bluetooth keyboard with an integrated
track pad.
The K400 is the perfect keyboard for setting up a Raspberry Pi, since it combines both
a keyboard and mouse, while supplying its own power off of two AA batteries.
As an added bonus, the wireless functionality also makes it great for surfing the internet
from my couch using an old computer connected to my TV.
Unfortunately, the keyboard recently had an unintentional encounter with a beverage, which
caused the "o" key to stop working.
As it turns out, a lot of websites have an "O" somewhere in their URL, which makes browsing
the internet or doing anything else useful on the computer very difficult.
While I could have bought a replacement online for about $25 dollars, I felt a bit guilty
about throwing away an otherwise perfectly functional keyboard.
As such, I figured that since it was already broken anyway, I might as well crack it open
and try to repair it, which, in the worst case, would at least allow me to learn more
about how this piece of tech works.
So how does a keyboard work?
The vast majority of keyboards in use today work by mechanically completing a circuit
when a key is pressed.
This triggers a voltage change, which is sensed by an integrated circuit inside the keyboard,
that then generates a key command and sends it to a computer.
A variety of different mechanical actuation technologies exist, from heavy duty mechanical
switches to one-piece rubber domes that sit beneath each key, but all of these types of
keyboards make use of a specially designed set of grided circuits known as a "key matrix."
Since keyboards have a lot of buttons, it is impractical to have a dedicated sense line
for each key.
As a result, the key matrix is designed in such a way that button presses result in a
specific combination of two integrated circuit pins changing voltage at the same time, each
of which maps to a specific key.
This greatly cuts down on the number of required sensing lines, which makes a keyboard's integrated
circuits less complicated and ultimately lowers its production cost.
I began by flipping my keyboard upside down and carefully removing its rubber stand-offs
and sticker located in the center of the backplate.
I next removed the battery cover and used a screwdriver to undo the 10 tiny Phillips
head screws holding everything together.
I then ran a flat head screw driver along the keyboard's center seam to separate its
top plate and backplate.
Once done, I flipped the keyboard over, and carefully folded over the top plate, making
sure not to damage the small ribbon cable that connects the keyboard's trackpad to its
main circuit board.
I next carefully lifted the latch holding the ribbon cable in place, and gently pulled
it out of the trackpad connector.
This separated the keyboard into two parts.
With the top plate off, I first removed the rubber membrane located in the bottom half
of the keyboard.
This exposed the key matrix circuitry, which is composed of a top sheet, an insulating
middle sheet, and a bottom sheet.
The next step was to carefully slide the top sheet out of the connector located on the
keyboard PCB, which allowed me to remove it from the keyboard.
I then also removed the insulating middle sheet and put it aside.
To find out where the keyboard traces were interrupted, I used a multimeter set to the
continuity mode.
In this mode, the multimeter sends a small 1mA current through its positive voltage probe,
and displays the voltage drop between the two probes on its output screen.
Using Ohm's law, which states that the current flowing between two points is directly proportional
to the voltage across the two points, it is possible for the multimeter to calculate the
associated resistance between the two probes.
If the resistance between the two probes is less than 70 Ohms, a very small value, the
multimeter beeps to indicate electrical conductivity between the two points.
In cases where a circuit is interrupted, like in a broken trace on a keyboard, the electrical
resistance between the two points is infinite.
Therefore, it is possible to identify where the keyboard's circuitry needs to be fixed
by seeing where the multimeter does not produce a beep.
I placed the multimeter's common probe at the end point of each trace on the keymatrix
sheet, and walked the positive voltage probe along each trace, listening for a beep.
One thing to note is that resistance is also proportional to the length of a trace -- the
longer the trace, the higher the resistance, meaning that a non-beeping multimeter did
not necessarily indicate a broken trace.
As such, I needed to sometimes reposition the multimeter's common probe while I tested
the traces, to verify that the electrical circuit was actually interrupted.
Once I had found all of the broken traces, it was time to repair the keyboard using conductive
paint.
For this, I squeezed a small amount of paint onto a piece of paper, and then used a toothpick
to draw a small amount of it over the broken traces on the keyboard.
I then let everything dry for 3 hours.
Once the paint was set, I replaced the keyboard's insulating middle sheet and top sheet, reinserted
the top sheet into the keyboard PCB's connector, replaced the rubber membrane, and reinserted
the ribbon into the keyboard's trackpad PCB.
I then replaced all of the screws and reinserted the battery cover.
Finally, I tested the keyboard by plugging the bluetooth adapter into my computer's USB
port, sliding on the keyboard's power switch, and trying to head to my favorite website.
With a few minutes of work, the keyboard was as good as new and able to type the letter
O to my heart's content, ready for unlimited couch surfing and continued use in my Raspberry
Pi Projects.
While not strictly necessary to repair broken traces, I found that conductive paint was
the easiest and longest lasting way to repair the keyboard.
Since the sheets that make up the key matrix are plastic, it was not possible to solder
anything to the circuits because they would have melted.
I also found that the electrical resistance of the adhesive on the back of copper tape
was too high, and that taping aluminum foil over the traces also did not work, because
they would lose contact over time.
I also read that pencil graphite can be used to repair circuit board traces, but I didn't
have any luck with this method.
However that is not to say that conductive paint is the only way to repair a keyboard.
If you know of another way to repair circuit board traces or other have another cool DIY
trick that you would like to share, please post them in the comments below!
Also, if you enjoyed this video and found it useful, please let me know by giving the
video a thumbs up and sharing it with your friends.
Thanks for watching and good luck with your keyboard repair, but in the meantime, let's
go
Super Make Something!