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The art of capacitive touch sensing

In past, we have heard many stories about wizards and witches. How with some weird action things disappear or a man is turned into a rabbit or people are knocked off with just a mere touch of hand. They are believed to have magical powers and stuff like that. Many of us said this is just a myth.  Now after hundreds of decades all these myths are coming to life. Still confused? Think of all the new and innovative technologies and try to relate with it. Ex. Microsoft Kinect, Lucid touch, 3D, smartphones, tablet PCs etc.
One such technology Is what we are gonna discuss about. This is called “CAPACITIVE TOUCH”. As the name suggest it is made up of a capacitor and your skin. This is one technology which is gaining massive importance and is used in almost every gadget available in the market. Such technologies mark the start of the next big phase in the tech world.
Have you ever noticed how useful and significant the electrons flowing in your body are? produced by tiny ions inside your body is stored inside a capacitor, with the help of energy differences, the appropriate function carried out depending upon the value of that difference. Our body consists of conductor and electrolytes in the form of tissues. It is this conductive action of our body that makes touch sensing a reality.
Generally a capacitive sensor is designed with a glass overlay of 3 mm or less. Obviously the  sensing action is more and more difficult if the thickness is increased. So it is kept at minimum. Why glass? Cheap, easily available and transparent so we can see underlying components.
As we all know, a simple capacitor consist of two electrodes containing a dielectric in between. Energy is consisted inside those electrodes. Some of the energy is spilled out eventually, electric field lines associated with it are called fringing fields. Body tissue acts as conductor which interacts with electric field. As we place our finger on the sensor some extra conductive area is formed. The additional charge storage capacity added by the finger is known as finger capacitance, CF. The capacitance of the sensor before touching is denoted as CP, which stands for parasitic capacitance.

PCB Layout of the Sensor
Figure shows a PCB (printed circuit board) showing the implementation of a capacitive sensor button.
  It is 10mm in diameter which is also the average size of our fingertip. Now let us assemble a PCB with two such buttons with their centers spaced 20mm apart. The size of the gap formed between two buttons plays an important role in designing. If it is set too small, too much energy will go directly to ground. If set too large, control over energy is lost.
Now comes the role of dielectric constant. For air it is 1 and for a normal glass it is around 8.  The dielectric constant, ε influences how closely the electric field energy can pack into sensor pad. This glass sheet is mounted over PCB with an adhesive. A precision analog comparator is used which gives the output is fed into a pulse width moderator as shown in figure. The extra capacitance from touch increases this count and hence it is sensed. 


This is just a basic idea of how this technology works. It is long lasting as compared to mechanical switches. Because of huge popularity, it comes at much cheaper and affordable prices. A touch phone with tons of features now comes for as little as 3000 bucks. Thus they are cost-effective and with the growing development its performance has increased many folds. They are already considered as replacement for mechanical switches and keypad type phones.
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