1.13: Design Example: Resistive Touchscreen
A device engineer plays a crucial role in designing user interfaces for mobile devices. One such interface is the resistive touchscreen, which fundamentally consists of two metallic layers: a flexible upper layer and a rigid lower layer, separated by a narrow gap. The high resistance between these two layers is a key characteristic of this design.
When a user touches the screen, the two layers make contact at a specific point known as the touchpoint. This contact reduces the resistance between the layers, effectively changing the electrical properties of the touchscreen at that point.
However, to accurately determine the precise location of the touchpoint, the touchscreen needs to be simplified as a one-dimensional system. The top layer, characterized by its length, resistivity, and cross-sectional area, is conceptually divided into two parts at the touchpoint. The resistances of these two sections are proportional to their lengths.
Connecting a voltage source between the two ends of the top layer transforms the circuit into a voltage divider configuration. The voltage drop at the touchpoint depends on the resistances of the two sections of the top layer.
By substituting the resistance values into the voltage divider equation and solving it, a relationship can be derived between the voltage drop at the touchpoint and its position. This relationship allows the device to calculate the exact location of the touchpoint based on the change in voltage.
This means that each touchpoint corresponds to a distinct voltage, enabling the system to accurately pinpoint the location of the user's touch. This precision is critical for the functionality of the touchscreen interface, ensuring it responds accurately to the user's input.
