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In recent years, it has become increasingly difficult to imagine what our world might be like without touch-sensitive electronics. It has been a long time since phones with mechanical buttons were commonplace. Phones and many other devices have progressed and increasingly make use of touch-sensitive electronics. This revolutionary technology might seem like a complicated advancement of technology, but the reality is that it creatively uses fundamental principles of electric circuits – like resistance and capacitance –to achieve touch sensitivity functionality. Here we will attempt to explain how capacitive technology, which has emerged as the preferred implementation, works.

Before we talk about touch buttons, let's cover some basics. The technology is based on the principle of the parallel-plate capacitor. As the name implies, it is based on two separate conductive parallel metal plates acting as electrodes, with insulating material between them. When a current is applied to the plates, because of the insulating layer between them, it doesn’t flow through, but a potential difference builds up to form the charge which is hence stored in this capacitor. Now when this capacitor connects to a circuit, it will discharge, with the charge gradually passing from one plate to the other plate through the circuit.

Capacitive touch sensors are usually designed as one of two configurations.

The mutual-capacitance configuration is made by using two terminals that serve as electrodes and are preferred for touch-sensitive displays, such as your phone.

Mutual-Capacitance Measurement Graph

The self-capacitance configuration is made up of one terminal of the capacitor being grounded and is suitable for touch-sensitive buttons, sliders or wheels.

 

Self-Capacitance Measurement Graph

Our basic understanding of electric circuits tells us that a capacitor is at its simplest, just two conductors separated by an insulating material, often referred to as a dielectric. Capacitors can easily be manufactured using the conductive layers of a printed circuit board. A capacitive sensor can be modeled as a capacitor between a touch sensitive signal and ground. The important part of obtaining a signal from this device only depends upon the change in capacitance rather than the magnitude. Your finger can provide the means to cause this change!

Touch sensing relies on the change in capacitance when a finger approaches the sensor to touch it, since the body itself acts like an additional capacitor, hence providing an additive charge to the capacitor. The key here is that the amount of charge stored in the capacitor at a particular moment is not the quantity of interest—rather, the quantity of interest is the capacitance at a particular moment.

So then, why does the presence of the finger alter the capacitance? There are two reasons: The first involves the finger’s dielectric properties, and the second involves the finger’s conductive properties.

We usually think of a capacitor as having a fixed value determined by the area of two conducting plates, the distance between the plates, and the dielectric constant of the material between the plates. We certainly cannot change the physical dimensions of the capacitor just by touching it.

The finger is not located in the actual dielectric region, or the insulating space directly between the conductors, but this is not necessary to influence the dielectric characteristics because the electric field of the capacitor extends into the surrounding area.

Your finger, which is mostly composed as water serves as an efficient dielectric material and because the dielectric constant of water is much higher than air, your finger’s interaction with the electric field translates into an increase in the dielectric constant and therefore a significant increase in capacitance.

We can also take into consideration the conductivity of your finger. As mentioned above the direct conduction between the finger and the touch-sensitive button does not occur. However, this doesn’t mean that the conductivity of the finger is irrelevant. The opposite is actually true because the finger becomes the second conductive plate of an additional capacitor. For practical purposes, we can assume that this new capacitor created by the finger is in parallel with the existing PCB capacitor.

However, we can think of the human body as providing a virtual ground because it has a relatively large capacity to absorb electric charge. In any event, we don’t need to worry about the exact electrical relationship between the finger cap and the PCB cap; the important point is that the pseudo-parallel configuration of the two capacitors means that the finger will increase the overall capacitance because capacitors add in parallel.

Thus, we can see that both mechanisms governing the interaction between the finger and the capacitive touch sensor contribute to an increase in capacitance, which can be used to obtain a strong and reliable signal.

So why not make your own touch switch or purchase an off the shelve controller?

As we have learned from the example above with the human body consisting of water, any conductive material will impact the capacitance. As most sensors cannot distinguish between a human body's finger and/or water, a metal object, or other conductive mediums - a poorly designed touch sensor will eventually trigger if the change in capacitance is large enough.

Even electromagnetic interference (EMI) can eventually trigger a capacitive sensor, especially within industrial automation, supply chain, and traffic & transport where you find many environmental factors you need to take into account, ensuring reliable and safe activation.

With CAPTRON Capacitive SENSORswitches - which are equipped with state of the art technology & algorithms, taking multiple scenarios into account and allowing customers to use the benefits of capacitive technology while maintaining a safe operation.

In addition to that CAPTRON offers a wide range of capacitive touch sensors, allowing a maximum of customization e.g. switch function, connectors, colors & symbols.

Check out our latest products. You can also call us +1 (914) 619 5422 or send an email to sales@captron.us to learn more!

Benefits of Capacitive touch Switches

  • Long lifecycle with more than 100 million activation cycles possible
  • Highly ergonomic as no force is required
  • Water & Dust Prood according to IP69K (equivalent to NEMA 6P)
  • Indestructible

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