Resistive touch screen

Resistive touch screen

A resistive touch screen is a sensor that converts the physical position of the touch point (X, Y) in a rectangular area into voltages representing X and Y coordinates. Many LCD modules use resistive touch screens. This screen can use four, five, seven or eight wires to generate screen bias voltage and read back the voltage at the touch point. The resistive touch screen is basically a structure of thin film and glass. The adjacent sides of the thin film and the glass are coated with ITO (Indium Tin Oxides) coating. ITO has good conductivity and transparency. . When touch operation, the ITO of the lower layer of the film will contact the ITO of the upper layer of the glass, and the corresponding electrical signal will be transmitted through the sensor, and then sent to the processor through the conversion circuit, which is converted into the X and Y values on the screen through calculation to complete the point. The selected action is displayed on the screen.

The touch screen consists of two transparent layers stacked up and down. The four-wire and eight-wire touch screens are composed of two layers of transparent resistive materials with the same surface resistance. The five-wire and seven-wire touch screens are composed of a resistive layer and a conductive layer. Use an elastic material to separate the two layers. When the pressure on the surface of the touch screen (such as pressing with a pen tip or finger) is large enough, contact will occur between the top layer and the bottom layer. All resistive touch screens use the voltage divider principle to generate voltages representing X and Y coordinates. The voltage divider is realized by connecting two resistors in series. The upper resistor (R1) is connected to the positive reference voltage (VREF), and the lower resistor (R2) is grounded. The voltage measurement at the junction of the two resistors is proportional to the resistance of the lower resistor. In order to measure a coordinate in a specific direction on a resistive touch screen, a resistive layer needs to be biased: connect one side of it to VREF and the other side to ground. At the same time, connect the unbiased layer to the high impedance input of an ADC. When the pressure on the touch screen is large enough to make contact between the two layers, the resistive surface is separated into two resistors. Their resistance is proportional to the distance from the touch point to the bias edge. The resistance between the touch point and the ground edge is equivalent to the lower resistance in the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance from the touch point to the ground edge.

The four-wire touch screen contains two resistive layers. One layer has a vertical bus at the left and right edges of the screen, and the other layer has a horizontal bus at the bottom and top of the screen. In order to measure in the X-axis direction, the left bus is biased to 0V, and the right bus is biased. Set to VREF. Connect the top or bottom bus to the ADC, and a measurement can be made when the top and bottom layers are in contact. The structure of the four-wire resistive touch screen is: two layers of turbine, uniformly conductive ITO layers are covered on a glass or acrylic substrate, which are used as X electrodes and Y electrodes respectively, and they are separated and insulated by uniformly arranged transparent grid points. The lower ITO is attached to the glass substrate, and the upper ITO is attached to the PET film. The positive and negative ends of the X electrode and the Y electrode are led out from both ends by the "conductive strips" (the black strips in the figure), and the positions of the X electrode and Y electrode conductive strips are perpendicular to each other. The terminals X-, X+, Y-, Y+ have four lines, which is the origin of the name of the four-wire resistive touch screen. When an object touches the surface of the touch screen and applies a certain pressure, the upper ITO conductive layer deforms and comes into contact with the lower ITO. This structure can be equivalent to a corresponding circuit