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E-field sensor yields nonmechanical touch (or not) switch

Targeted at the home appliance, boating, and industrial control market as an alternative to traditional mechanical pushbutton switches, the MC34940 “Electric Field Imaging Device” from Freescale Semiconductor gives you the core of a non-contact switching. The IC supports up to 28 touch-pad sensors and 7 electrodes, and detects a wide range of objects and materials including people, metallic, and even non-metallic materials which disturb the electric field surrounding the capacitive plates of the user-defined “switch”. The plates, or electrodes, can formed by PCB material or other metal, and are driven by an internal, low-distortion 5V, 120 kHz sine wave which reduces undesired harmonics (especially in the AM radio band); the frequency is resistor adjustable. Typical capacitance of the electrodes is on the order of 50-100 pF, and the sensing operates both through direct contact as well as physical proximity such as through glass, depending on the electrode shape and size.


In addition, the IC provides a shield driver for applications where the connection between the IC and the electrodes is a coaxial cable. The electrodes can be arranged as a single electrode with a virtual ground, or electrode pairs with a conductive path to ground. Output of the imaging device goes to a low-end microcontroller. Applications are not limited to obvious “switches”, since the capacitance-based e-field can also sense liquid level, ice buildup in a freezer, ambient moisture, or other physical changes.

The MC34940 is $1.85 (10,000 pieces, with more details are available at from Freescale Semiconductor, Inc. www. freescale.com/files/pr/efield.html

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