Sensing Proximity & Touch

Nearly all interactive devices use buttons as a means of user input, even if it is not the main mode of interaction. Simple buttons work by using a physical action to create an electrical connection between two conductors. In the case of user interface elements, many different variants of this principle exist.

  • Tactile switches — Small momentary switches that are attached directly to a printed circuit board. A visually attractive button cover is usually incorporated into the design of the product’s housing.
  • Rubber dome switches — These switches use a deformable, rubber dome with a conductive pad on the inside. They can be moulded in various shapes, offering more design freedom.
  • Membrane switches — Membrane switches are constructed from a sandwich of flexible conductive/nonconductive layers. These switches are typically used in appliances such as microwave ovens. Multiple membrane switches are often integrated into a single interface panel complete with graphics. Displays and button backlights are sometimes also integrated into the panel.

Capacitive touch

Technology advances in recent years have led to the development of more advanced ways to sense touch input. One of the most prominent examples is capacitive touch sensing. This technology relies on the fact that human skin is faintly electrically conductive. As a finger approaches a capacitive touch pad, the skin and the touch pad form a small capacitor, and the change in capacitance can be measured. Direct contact between the touch pad and the skin is not required, and this type of sensor can even be used to sense proximity. The figure below shows a schematic overview of the working principle behind capacitive touch sensing. The capacitance CF increases as the finger approaches the touch pad, and this change can be measured.

Capacitive touch sensors can be made waterproof, they afford richer interaction, and allow a great degree of design freedom. The downside is the increased cost and complexity compared to traditional buttons. However, the technology is quite mature, and this is reflected in the availability of low-cost touch-sensing chips. Capacitive touch sensor technology can be used for much more than just buttons. Multiple touch pads can be combined in various configurations to create more advanced input elements, such as sliders, rotary knobs, or X-Y grids. The image below shows examples of different touch pad designs.

GestIC

GestIC is a proprietary technology by Microchip that extends the principle of capacitive sensing in order to do 3D gesture tracking. Similar to regular capacitive touch sensing, the technology uses electrodes on a printed circuit board. In contrast, the GestIC system can not only detect touch, but also proximity, gestures (e.g. swiping), and 3D position. Development boards can be bought directly from microchip. Alternatively, the Pimoroni Skywriter HAT offers the same technology as an add-on board for Raspberry Pi.

Further reading

AN3863, Designing Touch Sensing Electrodes – Application Notes [PDF]
Capacitive Touch Sensing Layout Guidelines [PDF]

LeapMotion

The LeapMotion is a type of depth-camera that is optimized for hand and finger position tracking. The controller is approximately 80mm x 30mm x 11mm large, and can be connected to a computer using a USB cable. The device is primarily intended for use with desktop and laptop computers, though an API for Android phones is also under development. The LeapMotion can either function as a general purpose input device (placed on a desktop facing upward), or it can be mounted directly on a virtual reality headset.