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 lines 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.

Touch screen principle

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 a 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. 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. To measure in the Y-axis direction, the top bus is biased to VREF and the bottom bus is biased to 0V. Connect the ADC input terminal to the left bus or the right bus, and the voltage can be measured when the top layer is in contact with the bottom layer.

The structure of the four-wire resistive touch screen is covered with two layers of turbine, uniformly conductive ITO layers 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 respectively drawn from the two ends by the "conducting strips", and the positions of the X electrode and the Y electrode 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.