A touch screen is an input control device. Since the liquid crystal display is more suitable for portable equipment, and the basic requirements of portable equipment are small size, light weight, and energy saving, any keyboard-type input device is not ideal. The touch screen is just a transparent sheet device mounted in front of the LCD module. When in use, people can click menus, turn pages and write text at will by simply touching the screen.

Attaching the touch screen to the surface of the liquid crystal display module to form a liquid crystal display module with a touch screen has become a development trend of the liquid crystal display module. The control function of the touch screen has been added to the functions of some special-purpose liquid crystal display controllers, and the liquid crystal display control function and the touch screen control function are integrated into one IC. According to its working principle, touch screen can be divided into infrared type, capacitive type, surface acoustic wave type, resistive type, etc. The principle and characteristics are as follows:

1. Resistive touch screen

Resistive touch screen is divided into 4-wire system and 5-wire system or more. Now take the 4-wire resistive touch screen as an example to illustrate the structure and working principle of the resistive touch screen. The resistive touch screen is composed of a piece of glass coated with ITO transparent conductive layer and a piece of film coated with ITO transparent conductive layer. The opposite electrodes are separated by insulating spacers, and the periphery is sealed with glue.

2. Capacitive touch screen

The capacitive touch screen uses a glass with a double-sided ITO transparent conductive layer as the substrate, and the ITO conductive layer on the side of the glass close to the display screen is grounded as a shielding layer to ensure the stability of the working environment. The ITO surface on the other side of the glass is covered with a very thin rare earth glass layer, which acts as the working surface and leads out four electrodes at its four corners. During normal operation, a low-voltage high-frequency alternating electric field is applied to the working surface. When there is a touch, a small current is absorbed by the coupling capacitance between the finger and the work surface, and this current is generated by the four corner electrodes, and its strength is proportional to the distance between the finger and the four corner electrodes. The touch screen controller detects the change of this current and can calculate the position of the touch point.

3. Surface acoustic wave touch screen

Surface acoustic waves are mechanical energy waves that propagate shallowly on the surface of a medium (such as rigid materials such as glass or metal), usually ultrasonic waves. A surface acoustic wave touchscreen is a piece of tempered and treated glass that is mounted on the surface of the display. Four perimeters of the glass are etched with 45° reflective fringes ranging from sparse to dense and very finely spaced. The upper left corner and the lower right corner of the glass screen are respectively equipped with ultrasonic generators in vertical and horizontal directions, and two ultrasonic receivers in vertical and horizontal directions are installed in the corresponding positions in the upper right corner. After the power is turned on, the ultrasonic wave emitted by the ultrasonic generator propagates along the glass surface and is reflected by the surrounding reflection stripes, and finally received by the receiver; when the finger touches the glass surface, the finger absorbs a part of the sound wave energy, and the controller records this abnormality and calculates The location of the touch point.

4. Infrared touch screen

The infrared touch screen is a frame device attached to the surface of the display screen, which is divided into digital and analog. The digital type is to install infrared transmitting and receiving pairs of tubes on the upper and lower and left and right sides of the frame to form an infrared array. When an object blocks the infrared path, the controller can read its coordinate position and make a judgment. The analog type only needs to install an infrared emitting tube on the scanning motor set at the corner of the frame, and install an infrared receiving tube next to it; different reflectors are installed on the bottom side and the other two sides respectively, so that the infrared rays scanned on the bottom side are reflected by the other two sides. The mirror is reflected to the receiver, and the touch position can be obtained by triangulation. The touch screen has become an essential part of the liquid crystal display module in some applications.

Different touch screens have different characteristics, so you should determine clear requirements for them when purchasing, so as to choose the most suitable product.