A New Type of Touch Sensor

A new story of mine is up at Technology Review. It’s about a novel touch sensor that’s quite versatile—good for wires, fabrics, rubber, paper, and hard flat surfaces like walls, floors, and coffee tables. The researchers, Patrick Baudisch (Hasso-Plattner Institute) and Raphael Wimmer (University of Munich) presented the touch sensor project at the User Interface Software and Technology conference last month. The paper is here.

How it Works

Transatlantic cable. Credit: The Geneva Cholock Collection, Clarenville Heritage Society

The technology that underpins the sensor is called time domain reflectometry, or TDR. It’s a decades-old approach for finding problems in telecommunication cables. TDR works like this: an electrical pulse is sent down the cable and when it encounters a change in impedance or a break in the line, part of that pulse is reflected back. The time it takes to detect the reflection is used to find the precise location of the problem. It’s basically like radar in a wire.

Instead of looking for a break in a telecommunication line, however, Wimmer and Baudisch use TDR to determine the location of a touch along a set of wires. When a person’s finger comes close to or touches the wires, it changes the capacitance between these wires, reflecting part of the electrical pulse.  The video below shows some examples of how TDR can be used as a touch sensor.

Toward the end of the video you see a waveform on an oscilloscope that changes in response to the position of a person’s finger along a wire. So, how do you convert a waveform to precise touch data? Right now, Wimmer uses a webcam and open-source software to digitize the trace. The software also converts the wiggles into usable data so that the touch input has some meaning.

A Designer’s Dream?

Wimmer is enthusiastic about the potential for TDR touch sensors in the do-it-yourself community. “What really excited me…is that this allows you to quickly prototype touch sensitivity on surfaces,” he says. “Usually as a designer, you can’t easily build a touch screen and attach it to your object.” But with TDR, a designer can use wires, conductive ink, or copper tape to quickly and easily add a touch sensor to any form factor.

Of course, the she would need a reflectometer that can generate and detect picosecond pulses. Wimmer found his on eBay for about $300. The designer would also need cables to connect the reflectometer to the touchable wires, and a computer to run Wimmer’s software for interpreting the input.

While the hardware setup is a little clunky right now, Wimmer hopes that the TDR touch sensor catches on with the DIY community. He’s started a public wiki with information about the project and is currently trying to develop low-cost hardware “in the open, so that everyone can take inspiration and suggest changes,” he says. Eventually Wimmer wants to put all the functions of the reflectometer into a chip that’s small and inexpensive so that TDR can become an interface designer’s ultimate tool for touch.


Related Links

Technology Review article about the TDR touch project:


“Modular and Deformable Touch-sensitive Surfaces Based on Time Domain Reflectometry,” in Proceedings of UIST, 2011, pp. 517-526.:


UIST 2011:


TDR touch open-source code:


TDR touch public wiki:


Raphael Wimmer:


Patrick Baudisch:


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