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Touch screens: resistive & capacitive – working principle

Touch screens are quite common these days. In this post, we will discuss the physics behind the working principles of different touch screens. Specifically speaking, we will cover here the working principles of Resistive Touch Screens and Capacitive Touch Screens.

Resistive Touch Screens

One of the most common touch screens we experience is found at a bank ATM. We have to press firmly or push on the screen for our selection. These are called resistive touch screens, and they literally “resist” our touch. Here we push to bend two electrical-carrying layers together at the selection point. The electrical current changes at the point of contact indicating the point of screen location (Figure 1). The ATM software processes the location and activates the icon program associated with it. We may feel the screen bend slightly when applying pressure.

Figure 1: Resistive Touch Screen

Capacitive Touch Screens

Most of us encounter a touch screen, such as on a smartphone or tablet, where just by simply touching an icon we can immediately access an application. This saves a lot of time from moving a computer pointer on a monitor screen with a mouse and pressing Enter. Now, let’s see how exactly these touch screens work.

For a common type of screen, if you looked inside, you would see that the screen basically consists of two layers of glass coated with a transparent conductor. When conducting electricity, the screen behaves like two parallel electrical conductors with an electric field between them. (This arrangement is called a capacitor, and hence is a capacitive touch screen.)

When the screen is touched by our finger (an electrical conductor), a tiny electrical charge is transferred to the finger and the electric field of the screen is altered at that location (Fig. 2).

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The software processes the location and initiates the programmed action. It must be an electrical conductor to activate the screen. A normal gloved finger or pencil eraser wouldn’t work. Capacitive screens can be touched in more than one place at a time, as in the case of a two-finger zoom to enlarge text.

Figure 2: Capacitive Touch Screen
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