I purchased an Ugee Graphics Tablet for my little brother last Christmas. The tablet worked great, however the pen broke. After examining the pen, I found that the pressure sensor's wire snapped. I figured this would be a great opportunity to work on my soldering skills and chose to fix it.
You will need:
- Soldering Iron and solder
- Soldering Skills
- Scotch tape
- Copy paper
Step 1: How Load Sensors Work
Feel free to skip this, it is just a FYI section.
In a load sensor, a wire is looped around a material (called a substrate). As force is applied to the substrate, the wire is stretched.
As the wire stretches, the cross-sectional area (A) is reduced and the length (L) is increased. This increases the resistance (R), as we can see with the equation R=p*L/A (where p is the resistivity of the wire's material. For example, copper has a fairly low resistivity).
As the resistance of the wire increases, the current decreases (as we can see with the equation V=IR). This information can be processed by an ADC (Analog to Digital Converter) and therefore we can figure out how much pressure is being applied.
If the wire is snapped, there is infinite resistance and no current. Therefore, the sensor is useless unless it is repaired.
Pressure sensing pens for graphics tablets use these sensors. Wii Fit boards also use these types of sensors.
Step 2: Securing the Wire
Before getting a soldering iron near your project, you will want to put a paper between the snapped wire and the rest of the pressure sensor wire. Don't skip this step, you may end up getting solder on the rest of the pressure sensor and that is the beginning of a really bad day.
To prepare, I cut a small piece of paper and folded it in half. Then I wrapped it around the rest of the pressure sensor and secured it with scotch tape.
Afterwards, I ended up taping the snapped wire to the paper so that the ends of the wire are touching or very close to one another. Leave enough room so that you don't end up getting solder on the tape.
Scotch tape is ideal for this because it doesn't stick that well. Don't use strong tape, as that will make it hard to remove the paper later.
Do this nicely, or soldering is going to suck. In the picture above, you can see that there is some solder on the wire already. That was from my first attempt, when I wasn't so careful with the taping.
I also taped the entire pen to a piece of paper. This made it a lot easier to solder.
Step 3: Solder
This is not an Instructable on how to solder. If you are not good at soldering, I recommend practicing beforehand. Soldering this is quick, but it requires a light touch. You do not want to burn through the paper or get tons of solder on the sensor.
The wire for a pen's pressure sensor is really thin, so I found that it worked best to put the solder on the soldering iron rather than add it as your are holding the soldering iron to the project. You should always have solder on the iron anyway, so in this case you would add a small amount more. These are extremely small wires, they do not require a lot of solder.
The picture shows the wire right after I soldered it. I'm not the best at soldering, but it seemed to turn out okay. As you can see, the paper was slightly burned but not burned through.
Step 4: Remove Paper
After the pen has cooled, gently remove the tape and paper. Afterwards, you will be able to see the finished pen. It should look something like above. However, if you are better at soldering than I am, it will probably look a lot better.
Step 5: Testing
After fixing the pen, I put it back together and tested it out on the computer. I was very pleased to find that not only did it write, but the pressure sensing ability was working as expected.