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2 January 2011
Author: Giorgos Lazaridis
How Touch Buttons work

Touch sensors WITHOUT direct metal contact

Finally we come to the point. Because it is cool to touch a metallic surface (sensor) and turn on your PC for example, but it is absolutely awesome to touch the wooden surface of your modded PC case, or the dark plexiglas with the icon and blue backlit... Isn't it?

Until now, we've talked about touch sensors with metallic touch pads. The first method we explained was the resistance. We cannot do much with this method. Then we talked about the AC Hum sensors, which is not good for switching applications. Finally, we talked extensively about the capacitance touch sensors. Now let's take a closer look to this method. No matter which capacitance sense method is used, they are all based on the fact that the human body and skin can hold charge and act as capacitors. When the sensor is touched, the human body adds a parallel capacitance to an existing capacitor somewhere on the circuit.

This happens if the sensor is metallic. But what if the sensor is covered with a plastic layer, or a piece of wood? How can we transfer the body capacitance to the sensor through this layer? As a matter of fact, we do not need to do absolutely nothing. And that is the awesome part with the capacitance sensors. The key is the dielectric constant. All insulating materials have this property. The dielectric constant, is the ability of a material to store electrical energy. Some materials have bigger dielectric constant than others and can perform larger capacitors. You can find a table with some materials and its DC in this link: wikipedia.

Check out now this drawing:

This is a very simple touch sensor plate. Actually, it is a simple PCB with a circle in the middle. The diameter can begin from a 4-5 millimeters and can grow bigger. There are 3 regions: the grounding layer, which is a copper layer connected to the ground of the power supply, the sensor which is the copper circle in the middle, and an insulating layer, which is an electrically insulated region between the grounding layer and the sensor, with a material with small dielectric constant (simple air for example). Look what happens, when the grounding layer is connected to the ground and the sensor to the input of a capacitance sensor circuit:

Here you see a cross-section of the above sensor. The grounding layer and the sensor layer can be see, as well as the insulating layer in between. The blue lines shows the electric field energy which packs in this sensor. And because the only dielectric material that it has is thin air, it has an extremely low capacitance (the constant Îµ for air is 1).

Now look what happens, if the sensor is covered with a material that has high dielectric constant, like for example glass, that has Îµ from 3.7 to 10:

The energy lines are not buried under the thick layer of glass. Instead, they flow within it much easier than air! This is something that someone does not expect to experience, unless he knows it. This property is what makes the covered capacitance switches functional.

When the finger touches the glass, the energy lines goes through the finger, which adds an extra capacitance and finally changes the overall capacitance of the measuring capacitor in the circuit.

So? What should i change in a capacitance switch, so that i can cover it and still work?

In a word: only the circuit sensitivity.

And what if i increase the sensitivity TOO MUCH?

Then you have just make yourself a capacitance proximity sensor! It will be able to sense the human presence from a distance, without even touching it. Do not expect of course something like 100 meters range. It works for very short ranges. To increase the range, you can use larger sensors with bigger area.

What materials can i use as a cover?

Almost any insulator will do, but prefer those with larger dielectric constant if you want to have thick layers. If you ask me the maximum thickness, i cannot answer it. There are several parameters to determine this. I can tell you though, that i have try with success 8mm Plexi Glass, 16 mm MDF wood and 16 mm chipboard. The sensor that i used was a rectangular 2-sided PCB with 1cm sides. The front side was the sensor and the back side was the grounding layer. This is also another technique that you can use if you want to make a single button on a single PCB. If you feel like putting more sensors on the same PCB, then make sure that there are insulating layers between the sensors. Here is an example:

The sensors are marked with the letter "S". The white regions are insulating layers, and all the rest is a grounding layer. This specific design has all the wiring at the bottom side of the PCB with vias. Instead, you can reduce or completely remove the grounding layer, and place the traces on the same layer.

Here is another interesting technique. Suppose that you want to cover the keypad with a thick plexiglas. Then you need to increase the sensitivity of the buttons. But this may result in false button presses, if the operator presses a few millimeters displaced from the correct button. You can effectively direct the energy lines to the point that you want, by simply removing material from the cover, above the regions that you do not want the energy lines to go. Look for example this cross section:

What you see, is a cross section of a sensor with 3 buttons on the top side of the PCB, and a grounding layer on the bottom. Above the buttons there is a thick plexiglas cover. To prevent the energy lines to go outside of the region of each buttons, the material between them is removed. This leaves an area with air, and as we said before, the air has very low Îµ. So, the flux will not be expanded sideways too much to cause problems when pressing buttons.

Microchip has done a very good research in capacitance sensors. I strongly recommend you take a look the mTouch(R) pages. Also, do not forget to take a look at this application note, and more specifically in pages 11 through 13, in which they explain how to make paired-keys capacitance touch sensors, how to implement key matrices with capacitance sensors and how to make linear slides.

You can go even deeper by viewing the webminars that Microchip has:

• Introduction to mTouchï¿½ Capacitive Touch Sensing (Link)
• Capacitive mTouchï¿½ Sensing Solutions: Design Guidelines (Link)