How do touch-sensitive screens work?

With a little electrical help from the human body, touchscreens can tell where our fingers are pointing…

By Sajan Saini

Touchscreens have used a variety of techniques over the last two decades to detect the placement of a finger on a screen — ranging from mechanical, optical, and electrical sensing. Today’s capacitive electrical touchscreens have proven to be the most versatile and efficient way to sense human touch.

A capacitor is an electrical circuit that, in its simplest form, is composed of two conductive electrodes separated by an insulating gap. A direct current (DC) of electricity can’t straddle this gap, but an alternating current (AC) can induce a charge to flow from one side to the other. The surface of a touchscreen is blanketed with a grid of electrodes. Wherever our finger comes to rest, a capacitive contact is formed and the AC current generated within the device induces a corresponding current within our body — which helps span the gap and complete the circuit.

“Human beings are good conductors,” explains Neil Gershenfeld, director of MIT’s Center for Bits and Atoms, so using our fingers to close an electrical circuit makes it very easy to detect human touch with high fidelity. If a grid location on the touchscreen is to sense the AC current, “there has to be a return [electrical] path,” says Gershenfeld. For a touchscreen on a handheld device such as a smartphone, “you’re holding it with the other hand,” and this completes the electrical loop to the backside of the device, which is electrically grounded. If the touchscreen is part of an installation, such as an ATM, some part of our body is most likely in contact with an electrical ground. “It’s very hard (for our bodies) to avoid making a ground contact,” stresses Gershenfeld, which virtually guarantees that humans (or their fingers) can close an electrical loop for capacitive screens. 

If it sounds alarming to have electricity passing through your body, worry not. The AC currents in touchscreens are within levels for natural charge conduction in our bodies — and the true revolution and utility of modern touchscreens lies in the rapidity of their responses. “The unsung hero is the microcontroller,” notes Gershenfeld. Behind every electrode on a touchscreen grid lies “an embedded microcontroller that has a clockspeed of nanoseconds.” It is this fast response time that enables modern smartphones to have such smooth interaction with human touch, and it is this progress that has driven the growing appeal of touchscreens in recent years.

Capacitive sensing has led to unexpected new innovations, such as the leading sensor used in auto safety systems for cars to detect the location of their occupants, commercialized from Gershenfeld’s laboratory and based on a kind of imaging, that uses electric fields. With a little cooperation between man and machine, touch-sensitive screens have opened the doors to a host of new interactive technologies.

Thanks to Akash Laturia from Akola, Maharashtra, for this question.

Posted: June 7, 2011