Display Technology
How to quickly implement reliable touchscreens
By Rolf Horn, applications engineer at DigiKey A
touchscreen is increasingly preferred over a keyboard and mouse as a human- machine interface (HMI) for programming, configuring,
and controlling consumer, enterprise, and industrial systems. Touchscreens are intuitive, fast, and have a single integrated interface that replaces a combination of input devices. They also offer greater convenience for people with physical challenges and can be made quite compact.
The wide variety of applications for touchscreens means they must be rugged, operable by a bare or gloved finger, and cost-effective. Resistive touchscreens meet these requirements, but designers need to be able to get to market quickly with off-the-shelf solutions comprising a touchscreen matched to an appropriate controller. They must also understand the differences between 4 and 5-wire resistive touchscreen interfaces. This article briefly describes resistive touchscreens. It then introduces example touchscreens and controllers from NKK Switches and shows how to design with them.
How resistive touchscreens work Resistive touchscreens are standalone components that overlay a flat panel display. In conjunction with a controller, a touchscreen lets users interact with displayed symbols by touching specific areas. A touchscreen can detect the precise position of a finger or stylus touch. Application software then determines what further screen actions should occur based on that position.
Resistive touchscreens are suited to various consumer, retail, enterprise, industrial, and medical applications because they are inexpensive, rugged, and can be operated with a bare finger, a gloved hand, or a stylus. The technology uses a deformable plastic film that is coated on its back side with a conductive layer such
46 September 2024
as indium tin oxide (ITO). The back of the touchscreen is formed from a glass or acrylic panel, which includes an ITO layer on its front face.
Non-conductive spacer dots separate the plastic film from the glass or acrylic backing panel. When the plastic film is pressed with a finger or stylus with a force of one or two newtons (N), the film contacts the backing panel, effectively closing a switch in the local pressure area. The controller board, with an attached four or five-wire connector, can determine the location of the closed switch, and the software responds accordingly (Figure 1). Resistive touchscreens are popular when cost, ruggedness, and operation with a gloved hand or non-conductive stylus are essential.
Components in Electronics
Figure 2: 4-wire resistive touchscreens use two edge electrodes on the bottom plate and two on the top plate. The pairs run perpendicular to each other and allow the XY position of a touch to be determined. (Image source: NKK Switches)
Figure 1: Resistive touchscreens work by using a touch to press together two conductive surfaces. (Image source: NKK Switches)
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