Field Effect Touch Controls

The “go-anywhere” switch

As product design has evolved since the dawn of the Age of Electricity, so has the execution of the user interface. Switches and controls have been everything from toggle switches to rotated knobs of wood, Bakelite and stainless steel; from slide rheostats to touchscreens. In many cases, there is a necessity for a control sealed from the environment, while in others, the demand is for a clean design.

Material Sciences Corporation subsidiary Electronic Materials and Devices Group Inc. (EMD), Elk Grove Village, IL, has introduced Mirus detector cells into a component line of user interface touch controls. The cell generates an electric field that extends through a dielectric substrate above it, allowing a finger or gloved hand to activate the cell by coming within a predefined distance or by touching the substrate. This permits industrial designers to add seamless, intuitive user interface controls behind a wide variety of materials including plastic, glass, wood, isolated metal, cloth, foam and leather, to create flat or elegantly curved “intelligent surfaces.” 

An active circuit (ASIC) provides an oscillating electric field to two nearby electrodes, creating an electric field about the cell. The active device senses when a stimulus interrupts the electric field, and responds by outputting a digital signal in 160 microseconds without the use of software. Stimulus covering both electrodes is rejected and there is no response. 

The stimulus to activate the detector cell can be a metallic object, a liquid, or a dielectric material above a 2.0 dielectric constant for electronic sensor applications. Some applications have demonstrated sensitivity to lower dielectric constant stimulus. Sensitivity is controlled by the design of the electrodes, the gain, and the ASIC. In this way, cells are designed to meet each application’s specific needs.

Because they have no moving, bending or flexing parts to degrade, short or fail, these detector cells provide a crisp, consistently high performance over time with a virtually unlimited cycle life as compared to electromechanical switches. The cells are powered by 5 VDC +/-10% (there’s a 3.3 V option), outputting 4.0 to 4.9V for active high output, ±0.15V for standard active low output. The cell requires only 16 µA of current in their inactive state, making them suited for portable electronics, battery-operated devices and other products designed for low energy consumption. The detector cells have a high signal-to-noise ratio through the use of high-level signals with a dynamic range of over 3V, and meet FCC/CE requirements with their low pulse amplitude (5V) and low frequency (typically 60 kHz) operation. A low impedance-to-ground level increases immunity to induced electrical noise. Field effect technology essentially eliminates false positives induced by low-level circuit noise. 

Detector cell size varies by substrate type, thickness and desired sensitivity. Typical sizes range from 5 x 6 mm to 30 x 30 mm. Shape of cell (round, square, rectangular, etc.) varies by application. Since these detector cells are sealed behind a protective substrate, they are unaffected by environmental factors that can degrade conventional switches. They are available on printed circuit boards or flex circuits for curved substrates. Specifications call for 3 mm minimum distance between touch cells. 

“Mirus detector cells give industrial designers a new level of creative freedom and source of cost-effective brand differentiation and loyalty,” says EMD president Andrew Blake. “Designers can easily integrate colored backlighting, LEDs and sound with the cells to create an enhanced user experience. Product engineers like the ‘one surface design,’ which eliminates the problems of color matching and the precise alignment of mechanical switches in manufacturing. Mirus detector cells also save space, are easy to clean, and provide the solid-state durability and digital accuracy that electromechanical switches cannot.” 

—RM