The touch screen comprises two transparent layers stacked one above the other. The four-wire and eight-wire touch screens consist of two layers of transparent resistive materials with the same surface resistance. The five-wire and seven-wire touch screens consist of a resistive layer and a conductive layer, usually Use a flexible 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 between the top layer and the bottom layer occurs. All resistive touch screens use the voltage divider principle to generate voltages that represent the X and Y coordinates. As shown in Figure 3, the voltage divider is implemented 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 two resistor connection points is proportional to the resistance of the next 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 to VREF and the other 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 divided into two resistors. Their resistance is proportional to the distance from the touch point to the offset edge. The resistance between the touch point and the ground is equivalent to the lower resistor in the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance from the touch point to the ground.