Touch screens are great, but big touchscreens are expensive and irregular touchscreens are not easy to make at all. Electrik is a method developed by several researchers at Carnegie Mellon University that makes almost any solid object into a touch surface using tomography. The catch is that a conductive coating — in the form of conductive sheets, 3D plastic, or paint — is necessary. You can see a demonstration and many unique applications in the video below. They’ve even made a touch-sensitive brain out of Jell-O and a touchable snowman out of Play-Doh.
The concept is simple. Multiple electrodes surround the surface. The system injects a current using a pair of electrodes and then senses the output at the other terminals. A finger touch will change the output of several of the electrodes. Upon detection, the system will change the injection electrodes and repeat the sensing. By using multiple electrode pairs and tomography techniques, the system can determine the location of touch and even do rough motion tracking like a low-resolution touch pad mouse.
In addition to the video, there is a lecture video and a paper available. There are enough details that you should be able to experiment with this technique yourself. The prototype used a Teensy board with Bluetooth to allow objects to run wirelessly. The excitation signal was a 200 kHz sine wave driving a voltage-controlled current source, providing a constant AC current. Multiplexers allow the use of many electrodes, which is key to being able to get a reasonable resolution. At least, that’s true to a point. The researchers found that after a certain number of electrode pairs, the accuracy actually got worse for a particular size object.
Some of the more interesting use cases included a touch sensitive wall, a computer desk with hot spots, and a touch-enabled steering wheel. The system will operate through ordinary paint or even stickers.