A touch pad is a pointing device like mouse. It is very much popular among laptops and notebook computers. A touch pad is a small flat square or rectangular surface on which the user slides his finger to move the cursor on the display. While moving on screen the finger touch can select objects and run programs in the same manner as the mouse click does.
In acoustical touch panel, high frequency sound waves are generated in the horizontal and vertical directions across the glass plate. Touching the screen causes part of the wave to be reflected from the finger to the emitter. The screen position at the point of the contact is calculated form the measurement of the time interval between the transmission of each wave and its reflection to the emitter.
The touch screens are also similar to the touch pad. The touch screen systems or monitors are very popular for the interactive displays in museums and in automatic teller machines. They could not achieve much popularity in business world because users have to hold hands in mid air to touch the screen which becomes tiring very quickly. A touch screen is a transparent plate that is fits over the CRT screen. When the plate is touched the contact position is recorded by optical, electrical or acoustical methods. An orthogonal set of horizontal and vertical guides contain a light source and an opposite horizontal and vertical guides contain the detector. The pointing finger obstructs the light source the detector on both horizontal and vertical guide signals to the computer interface to record the coordinates of the pointing finger. The light is generally in infrared region so as not to be visible to the human eye.
The working of touchpad is depending upon the principal of coupling capacitance. It uses a two layer grid of electrodes to hold an electrical charge. Small vertical electrodes are used in upper layer of the pad and small horizontal electrodes are used in the lower layer of the grid. The grid is connected to an IC which is used to detect any changes in the capacitance of the pad. It constantly monitors the capacitance between each horizontal electrode and corresponding vertical electrode and when the user places his finger over any pair of electrodes it works as a conductor and alters the capacitance of that pair of electrode. This change is immediately detected by the chip and the data is sent to the computer in the same fashion as it is done in case of mouse, when the finger moves on the pad the affected electrode pairs are converted in corresponding X-Y placements by the chip and the data is transmitted to the computer. A touch pad has no mechanical or moving part so it does not need preventive maintenance. In most of the systems touch pads are integrated into keyboard or notebook itself but an external touch pad can also be attached to a computer via a PS/2 port.
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