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LCD | Types of Touch Screens

Touch screen (Touch screen) is an input control device. As the LCD is more suitable for portable devices, and the basic requirements of portable devices is small, light, energy-saving, so any keyboard type input device is not ideal. Touch screen is only a transparent sheet device, installed in front of the LCD module. When using, people only need to touch the screen, you can click on the menu, turn the page and write text at will.

Attaching the touch screen to the surface of the LCD module to form an LCD module with a touch screen has become a development trend for LCD modules. In some special LCD controller functions have been added to the touch screen control function, the LCD display control function and touch screen control function integrated in an IC. Touch screen according to its working principle can be divided into infrared, capacitive, surface acoustic wave, resistive, etc. The principles and characteristics are as follows.

Resistive touch screen

Resistive touch screen is divided into 4-wire system and 5-wire system or more wire system. Now 4-wire resistive touch screen as an example, the structure and working principle of resistive touch screen. Resistive touch screen by a piece of glass coated with a transparent conductive layer of ITO and a film coated with a transparent conductive layer of ITO relative to the combination. Relative electrodes between the insulating pad isolation, the periphery with adhesive sealing, as shown in Figure 1.

In this case, the conductive layer of the conductive glass is coated with a silver electrode at each of the left and right ends, as shown in Figure 2, x + and x- electrodes; the top and bottom ends of the thin film conductive layer are also coated with a silver electrode at each end, as shown in Figure 2, Y + and Y- electrodes. If a voltage is applied to the x or Y electrodes, a parallel and uniformly distributed electric field is generated between the x + and x- electrodes and between the Y + and Y- electrodes.

During operation, the voltage is applied to the X-axis first and then to the Y-axis according to the time-sharing principle. When the voltage is applied on X+ and X-, it will form a parallel and evenly distributed electric field between X+ and X- electrodes. And the voltage on Y electrode is 0. If you touch a point at this time and make contact between X and Y electrodes at that point, then the voltage on Y electrode will be the voltage division value of voltage on X+ and X- across the contact. From this, the X coordinate of the contact can be determined. Similarly, when a voltage is applied between Y+ and Y-, the Y coordinate can be determined by the voltage on the X electrode.

Capacitive touch screen

Capacitive touch screen with a double-sided ITO transparent conductive layer of glass as the substrate, the glass near the display side of the ITO conductive layer grounded as a shielding layer to ensure a stable working environment. The other side of the glass ITO surface covered with a very thin layer of rare earth glass layer, as the working surface outside, and in its four corners lead to four electrodes. In normal operation, a low-voltage high-frequency AC electric field is passed on the working surface. When there is a touch, through the finger and the working surface of the coupling capacitor absorbs a very small current, and this current is generated through the four corner electrodes, and its strength is proportional to the distance from the finger to the four corner electrodes. Touch screen controller detects the change of this current, you can calculate the location of the touch point.

Surface acoustic wave touch screen

Surface acoustic waves are mechanical energy waves that travel shallowly on the surface of a medium (such as glass or a rigid material such as metal), typically ultrasonic waves. A surface acoustic wave touch screen is a reinforced and treated piece of glass mounted on the surface of the display. There are 45° sparse to dense and very precisely spaced reflective stripes etched into the four perimeters of the glass. In the upper left corner of the glass screen and the lower right corner of each installed in the vertical and horizontal direction of the ultrasonic generator, in the upper right corner of the corresponding position on the installation of the vertical and horizontal direction of the two ultrasonic receiver. After the power is turned on, the ultrasonic waves emitted by the ultrasonic generator spread along the glass surface and are reflected by the surrounding reflective stripes, and finally received by the receiver; when the finger touches the glass surface, the finger absorbs part of the acoustic energy, and the controller records this anomaly and calculates the location of the touch point.

Infrared touch screen

Infrared touch screen is a frame device attached to the surface of the display, divided into digital and analog type. Digital type is in the frame up and down and left and right respectively installed on the infrared transmitting and receiving pairs of tubes, forming an infrared array. When there are objects blocking the infrared path, the controller can read its coordinates and make a judgment. The analog type only needs to be set in the corner of the frame on the scanning motor installed on an infrared transmitter tube, next to the installation of an infrared receiver tube; bottom side and the other two sides were installed on different mirrors, so that the infrared light scanned to the bottom side of the mirror reflected by the other two sides to the receiver, the touch position can be derived by triangulation. Touch screen in some applications has become a necessary part of the LCD module.

Different touchscreens have different characteristics, so you should define clear requirements for them when shopping in order to select the most suitable product.

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