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Introduction:
Touch technology has redefined the meaning of the term user-friendliness. This technology, once considered to be a distant dream, has touched all aspects of our life today. From mobile phones to personal computers to advanced applications in the field of scientific research, touch technology is everywhere. This is my attempt to bring you up to date with the very basics and also the new advancements in the field of touch technology, a technology that would forever change the way we look at and interact with the various gizmos at our disposal.
Touch switches
A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many lamps and wall switches that have a metal exterior as well as on public computer terminals.
There are basically two types of touch switches. They are:
• Capacitance touch switch
A capacitance switch needs only one electrode to function. The electrode can be placed behind a non-conductive panel made of for example wood, glass or plastic.
• Resistance touch switch
A resistance switch needs two electrodes to be physically in contact with something electrically conductive (for example a finger) to operate.
Capacitance touch switch
A capacitance touch switch works using body capacitance, a property of the human body that gives it great electrical characteristics. The lamp keeps charging and discharging its metal exterior to detect changes in capacitance. When a person touches it, it increases the capacitance and triggers the switch.
Finger Capacitance
At the heart of any capacitive-sensing system is a set of conductors which interact with electric fields. The tissue of the human body is filled with conductive electrolytes covered by a layer of skin, which acts as a dielectric. It is the conductive property of fingers that makes capacitive touch sensing possible.
A simple parallel plate capacitor has two conductors separated by a dielectric layer. Most of the energy in this system is concentrated directly between the plates. Some of the energy spills over into the area outside the plates, and the electric field lines associated with this effect are called fringing fields. One of the requirements for making a practical capacitive sensor is to design a set of printed circuit traces which direct fringing fields into an active sensing area accessible to a user.
Placing a finger near fringing electric fields adds conductive surface area to the capacitive system.
Resistance touch switch
A resistance touch switch works by lowering the resistance between two pieces of metal. It is thus much simpler in construction compared to the capacitance switch. Placing one or two fingers across the plates achieves a turn on or closed state. Removing the finger(s) from the metal pieces turns the device off.
One implementation of a resistance touch switch would be two darlington-paired transistors where the base of the first transistor is connected to one of the electrodes.
Also, an N-Channel, enhancement-mode, metal oxide field effect transistor can be used. Its gate can be connected to one of the electrodes and the other electrode through a resistance to a positive voltage.
Layout of the Sensor
The buttons on the top layer of a capacitance touch switch are spaced apart at an appropriate distance so that the user does not accidentally press any other switch, which would be the case if the switches are placed at distances smaller than the average size of an adult fingertip.
The ground plane is on the top layer, as shown in the figure. The sensor pad is isolated from the ground plane by a uniform gap. The size of the gap is an important design parameter. If the gap is set too small, too much field energy will go directly to ground. If set too large, control is lost over how the energy is directed through the overlay.