How to use shielded cable
Add stability to your project with shielded cable
Unlike regular cables, where a wire is covered by a plastic or insulating material, cable shielding uses an extra layer of metal shield surrounding the conductive wire or wires inside. As the name implies, shielded cables are used to minimise interference and reduce the sensitivity of the cables. In an unshielded cable, for example, a crocodile clip, the wire is not protected by anything other than by an insulative plastic jacket. Capacitive sensing can work through this plastic, so touching the cable can be incorrectly interpreted as a touch event.
In a shielded cable, the outer shield creates a barrier that separates the electric field surrounding the central conductor from anything outside. Ethernet cable uses shielded cable to protect the cable from electromagnetic interference and electrical noise. Capacitive sensing works by detecting changes in the electric field surrounding an electrode, so by creating a barrier around the central conductor inside the cable, we have no electric field beyond that barrier. If we have no electric field, we have no capacitive sensing ability, so touching the cable will not be detected by our system.
In this tutorial, we explain how to solder shielded cable to the Touch Board, so to use this cable you need to be able to solder. The Interactive Wall Kit uses shielded cable via an AUX port as a plug and play connection.
Step 1 Choose which shielded cable to use
There are two different types of cable shield used within shielded cable – foil shielding / foil shield and braided shielding / braided shield. In this picture, the left-hand cable uses foil and the right-hand cable uses braid. For our purposes, they both protect equally well and either should work well for your project.
Within the shielded cable there is at least one insulated wire – for example, the foil cable on the right has three. Depending on how many electrode connections you want to make to the Touch Board, each of these conductors needs to be soldered or attached to the Touch Board.
If you look closely, you can also see an uninsulated conductor sticking out of each cable. They are marked with a white arrow. This is the drain connection to the shield, which allows for grounding the foil or the braid. Not every shielded cable has a drain wire, so make sure to get one that does have one.
Generally, shielded cables with a thicker inner conductor, with a resistance of around 30 milliohms per metre, perform better.
Step 2 Why does shielded cable help?
The most important step is to ground the drain wire of the shielded cable. Solder the drain wire to the ground pane of the Proto Shield. Insert the conductor into the screw terminal and screw it in place.
On the other end of the cable, you need to attach the conductor wherever you want it to connect to, for example, a crocodile clip. You don’t need to solder the drain wire at this end of the cable.
Step 3 Next steps
Using shielded cable isn’t without its downsides. The cable’s shield adds capacitance to the cable, which decreases sensitivity and diminishes the signal. The longer the cable, the worse this effect gets. If you’re trying to detect proximity at the end of a run of shielded cable it’s going to be very difficult. As a general rule of thumb, three to five metres is about the most shielded cable that you’re likely to be able to use for touch applications.
Use the Grapher to visualise the sensor, click here to find out how to set it up. You can then also change the sensitivity of the board to max out the performance with shielded cable. But remember that decreasing the thresholds causes signal interference. The key to building a successful setup with these tips is experimentation and remembering that everything is a compromise. If you’re looking for a more robust method, we’d recommend using the Interactive Wall Kit.