Designing a Simple Touch Sensor.

With the development of touch sensors new horizon of sophisticated devices evolved with reduced mechanical parts. This was advantageous in many ways because the weight, size, space requirement and ultimately cost reduced. But before putting this concept into an application, you should be first familiar with the printed design considerations. This post will help you in designing a Simple Capacitor Touch Sensor.

A simple capacitor stores energy between its two conductor plates which are separated by dielectric. But some of energy manages to spill over outside the plate area. This electric field lines are called fringing fields. The concept is to direct this fringing  field into an active sensing area accessible to user.

So the question arises how can this be done? This can be achieved by simply placing a finger near the fringing field which will add conductivity to the surface area of the capacitive system. "Wondering How?" 

The tissue of human body is filled with conductive electrolytes, covered by a layer of skin 'a lossy dielectric.' The conductive property of finger makes capacitive touch sensing possible. the additional charge storage capacity added by the finger is known as finger capacitance.

The fig shows a simple capacitive sensor button. The button diameter is kept as the diameter of the human finger ie; 10mm. The sensor pad is isolated from the ground plane by a uniform gap. The size of the gap is important because if the gap is too small, too much field energy will go directly to ground. But if it is set too large then, it becomes difficult to direct the energy through the overlay. The gap of 5mm works well for 10mm diameter button.

Whenever a finger is present on a capacitor the capacitance is increased. This concept can be put into a number of applications. The figure below shows a charge transfer touch sensor QT113 which requires a common capacitor to function. 

So you can interface the touch sensor with it and design your own system.