Sensor Sense: Resistive Touchscreens

March 6, 2008
It’s quite common today to find touchscreens on many electronic devices.

Edited by Robert Repas

Point-of-sale terminals, high-end stereos, and video games are just a few of the typical applications.

The original touchscreen was patented in 1971, so almost all touchscreens today are patent- free. While many different technologies have been used to create touchscreens, the majority fall into one of four classifications: resistive, capacitive, surface-acoustic wave, and infrared.

The most common is the resistive touchscreen, making up approximately 70% of the touchscreens currently in use. The original resistive touchscreen used a five-wire interface that’s still employed on high-end touchscreens. Smaller and less-expensive units make do with four wires with only a slight drop in resolution accuracy.

Touchscreen construction starts with a glass substrate that has a uniform resistive coating applied to its surface. A conductive coating is also applied to one side of a hard coated polyester cover sheet that is then attached to the glass so the coatings face each other. Small transparent spacers placed between the glass and the cover sheet keep the two from touching. When pushed by a finger or other object, the cover sheet deforms forcing the two coatings into contact.

A touchscreen controller links the touchscreen to the computer. The controller first imposes an electrical voltage horizontally across the resistive coating of the glass creating a voltage drop across the surface. The conductive coating of the cover sheet taps the voltage at the contact point which is then digitized by the controller. The value identifies the position of contact along the X axis of the touchscreen.

The controller switches the applied voltage to the vertical axis, and the new value at the contact point is digitized again for the Y-axis position. Finally, the controller integrates the two voltage readings to create an X-Y plot of the touchpoint and translates the position information into a form that the computer understands. The controller repeats these steps thousands of times per second.

The typical resolution of a touchscreen is 4,096 x 4,096 points, several times greater than that of a computer monitor. However, the combination of layered materials lets pass only 75 to 80% of the light from the screen. This loss of brightness is the one main flaw of resistive touchscreens.

 

A resistive touchscreen sensor uses a resistive coating on a glass substrate. Rx represents the horizontal or X-axis resistance of the coating while Ry represents the vertical or Y-axis resistance. When pushed, the conductive cover sheet taps the Rx and Ry voltage drops at the point of contact.

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