Elegant and reliable solutions for touchscreens.

From industrial machines and medical devices to parking meters and vending machines:  touchscreens are increasingly used to control these applications. Capacitive keys or in 

short ‘CapKeys’ offer extra possibilities compared to conventional touch systems.

In the world of control panels, capacitive  touch systems have made an unstoppable  advance. These elegant, user-friendly and  hygienic buttons are now used in a wide  range of equipment.

Simple control panels (without screen)  often have single buttons, which are  backlit. Technically, this type of system  consists of a sensor that registers the  touch and a controller chip that links an  action to it. The surface is illuminated by  LEDs. These single capacitive buttons are  called CapKey.


The most obvious way to build such a  user interface is to mount a printed circuit  board (PCB) directly behind the control  panel. At the front of this PCB the touch  sensors are placed and at the back are the  electronic components. For the lighting, a  hole is made in the panel, over which an  LED is placed that radiates at the back.

The disadvantage of this construction is  that the homogeneity of the button lighting  leaves much to be desired. Also, a hole  has to be made in the touch sensor to let  the light through and this can negatively  influence the sensitivity.

A common solution is to mount a separate  sensor foil behind the front plate. With  SCHURTER a sensor circuit is printed  on the foil by means of screen printing.  To prevent the touch sensors on the foil  from blocking the light, they are made of a  transparent conductive material based on  Indium Tin Oxide (ITO).

A spacer is placed between the foil and  the PCB. The larger distance between  the PCB and the foil ensures a more  homogeneous illumination of the keys.  Optionally a diffuser can be used to further  even out the light.

The contact between the sensor and the  PCB is usually made by allowing the foil to  pass through further and then connecting  it to the PCB in a connector. If this is not  possible, spring contacts are mounted  on the PCB and contact is made with a  contact point on the foil, through holes in  the spacer.

Depending on the field of application, the  CapKey needs certain protections against  unintentional touching, the so-called false  touches. For example, contact with (salt)  water, palm of the hand, rain or dew. Such  protection is often achieved by means  of a guard system around the CapKey.  This requires a somewhat more complex  sensor.

The CapKey and the accompanying  guard can be refined thanks to computer  simulations. Finite Element Method  simulations are done in the electrostatic  domain. This way, “what-if” scenarios  are tried out very quickly, on the basis of  which the design can be optimized.

Touch controller 

The applied touch controllers can be  divided into two groups:

  • The standard touch controllers, which communicate with a host controller via a communication bus (usually I2C). After start-up, the host controller  configures the touch controller once.

These configuration settings are used to determine which CapKey  are active, what the sensitivity is and  which filter settings are used.

  • The microcontrollers are able to read out the CapKey by means of software (-libraries). Some controllers have special hardware on board for this purpose, but this is not always necessary.

Depending on the application, the electrical  engineer will make a choice from the  different possibilities. The standard touch  controllers are useful if the host controller  can no longer do this work or if the already  selected controller is unsuitable for touch  measurement. The microcontroller is  preferred if (RGB) LEDs also need to be  controlled or if other functionality needs to  be integrated.


The capacitive touch interface is often not  the core business of most companies and   therefore this beautiful technology remains  underexposed in new products. SCHURTER  has the knowledge and experience to deve- lop a suitable sensor design with associated  electronics for every application for a reliable,  effective and eye-catching operation.