issue: August 2004 APPLIANCE Magazine
Engineering Control Panels
Designing a Non-Contact Appliance Control Panel
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by Ron DeLong, senior systems and applications engineer, Freescale Semiconductor, and Catherine Booth, digital design engineer, Networking & Communications Systems Group, Freescale Semiconductor, Engineering Rotational Program (ERP)
The design of the human-machine interface (HMI) can strongly impact the perception of an appliance.
The creation of control panels that support
an increasing array of features and layouts has become a key issue for
appliance makers, which requires interfacing new, more complex systems
with the consumer interface. Up until now, choices have typically been
confined to mechanical switches or electronic keyboard-type products connected
to low-end microcontrollers.
However, recently developed electric field (E-field) imaging circuit technology
simplifies the design of low-contact force touchpad sensors and offers
several advantages for appliances. The E-Field sensor can allow immediate
reconfiguration of the user interface in support of new models, implementation
of flat panels for cleanliness, and elimination of all moving and mechanical
products in the user interface. The use of an E-Field sensing IC provides
a switch design that avoids wear-out, contact bounce, corrosion, and arcing
as well as providing a high-tech image to an appliance.
on E-Field Sensing
An E-Field imaging circuit offers designers an alternative to mechanical
pushbuttons for control panel applications. The integrated circuit (IC)
contains the circuitry necessary to generate a low-level E-Field and measure
the field loading caused by objects moving into or out of the field. The
IC can be used to implement non-contact sensing, proximity detection,
and three-dimensional E-Field imaging, and it integrates support for a
microcontroller and up to nine electrodes. The electrodes can be used
independently to determine the size or location of an object in a weak
The E-Field IC also contains circuitry to support two references, a shield
driver to reduce cable capacitance circuit, and a +5.0-V regulator to
power external circuitry . As shown in the application circuit (see
Figure 1), additional capability provided in the design includes the following:
ISO 9141 physical layer interface (ISO TX and RX), lamp driver output,
Watchdog (WD IN), Power-On Reset timer, and high-purity sine wave generator
tunable with external resistor.
With the E-Field IC, membrane switches and resistive touchpads may be
replaced with an array of touchpads consisting of conductive electrodes
embedded beneath an insulating surface. Since the circuitry is able to
detect touch and proximity through the insulating surface without direct
electrical contact to the electrode metal, problems of wear, contamination,
and corrosion are eliminated. This capability is also important for sophisticated
touch-control applications, including a user-interface panel that is sensitive
to different degrees of proximity—enabling the system to go from
standby to active mode as a finger approaches the panel (see Figure 1).
1: Basic E-Field application circuit showing the
IC, MCU interface, and connection to electrodes.
CLICK to see larger graphic.
the Electric Field
An electric field is created by the oscillator circuitry within the E-Field
IC that generates a high-purity, low-frequency, 5-V, peak-to-peak sine
wave . This frequency is tunable by an external resistor and is optimized
for 120 kHz. The a.c. signal is fed through an internal 22-k resistor
to a multiplexer that directs the signal to the selected electrode or
reference pin or to an internal measurement node. Unselected electrodes
are automatically connected to the circuit ground by the IC.
The “unused” electrodes can act as the return path to create
the electric field current, since the current must follow a complete path
out of the electrode pin and back to the common ground of the IC GND pin.
An E-Field will cause a current to flow between the active electrode and
any object with an electrical path to the IC ground, including unselected
The current flowing between the electrode and its surrounding grounds
produces a voltage drop across the internal resistance and results in
a voltage change at the pin. An on-board detector in the IC converts the
a.c. signal to a d.c. level. The d.c. voltage is multiplied, offset, and
sent to the level pin of the IC.
To simplify the evaluation of the capability of the E-Field IC, a reference
design in the form of an evaluation board has been developed. As shown
in Figure 2, the board includes a printed wiring board and an E-Field
IC, a pre-programmed 8-bit microcontroller (MCU), a communication IC,
and other passive components on a printed circuit board. The 8-bit MCU
converts the analog signal from the E-Field IC into an 8-bit byte. The
MCU then transmits this value to a computer via its COM port. A Windows®-type
software program is included in the kit that allows the measured A-to-D
value to be displayed in real-time along with its corresponding analog
bar graph. The program allows the scanning of all electrodes, references,
and internal nodes of the E-Field IC or any combination of them (see Figure
2: The E-Field IC evaluation board requires only an external
power source to connect up to nine electrodes, whose operation
can be observed though the communication (COM) port. CLICK to
see larger graphic.
a Touch-Control Panel
The E-Field IC is able to detect anything that is either conductive
or has different dielectric properties than the electrodes’
surroundings. Human beings are well-suited for E-Field imaging because
the human body is composed mainly of water that has a high dielectric
constant and contains ionic matter, which gives it good conductivity.
The body also provides good electrical coupling to earth ground
that can be connected to the ground return of the IC. When a finger
is brought close to a metal electrode, an electrical path is formed,
producing a change in electric field current that is detected by
the E-Field IC and translated to a different output voltage.
The size of the electrically conductive electrodes must be taken
into account for any design. The larger the electrode, the more
range and sensitivity will be obtained. As the electrode size is
increased, its susceptibility to interference, electrical noise,
and “stray” electric-field paths in its surroundings
also increases. However, the area of the touchpad only needs to
accommodate the contact area of the finger. This limits its useable
size. Therefore, the distance or spacing factor will play a significant
role in how the electrode should be laid out.
The E-Field IC works best when the total capacitance between an
electrode and ground or another electrode is approximately 50 picofarad
(pF) when the finger is in the “activate” range. The
total system capacitance should be below 100 pF and preferably below
75 pF for best sensitivity. This includes the IC pin, PWB trace,
wire, and any other stray capacitance. Large electrodes should be
used when distances are great, and small electrodes when distances
The placement of ground is important. As mentioned earlier, electric
field currents can exist between the active electrode and any grounded
object. By intertwining the electrode with ground, the essential
ground source needed to create an E-Field is directly accessible
to the electrode. This path is less variable than the path through
a body and earth and provides a more predictable and less noisy
3: Intertwined electrode and ground designs and dimensions.
see larger graphic.
To investigate how variables in ground can impact the E-Field measurement,
the ground effect was tested with a two-electrode design, a spiral,
and an interdigitated layout (See Figure 3). In addition, the width
of the ground electrode intertwined with a signal electrode was
varied to determine how much the ground area affects the readings.
One electrode ground test configuration was designed with the ground
having the same width as the electrode, and another with the ground
electrode thinner than the signal electrode. A touchpad with an
area large enough to accommodate a typical finger was designed in
a square shape with a length and width of 0.6 in. The dimensions
and layout of the electrodes are shown in Figure 3. A 4.5-mm (0.0045-in)
thick vinyl film was used as an insulator over the patterns. Subsequent
testing determined that the layout with the narrower ground electrode
provided a slightly greater amount of difference in comparison to
the design with ground having the same width (see Figure 3).
Uses for E-Field Sensing in Appliances
By simplifying the design of E-Field sensing, a number of other applications
may be considered. In addition to touch controls, the E-Field IC is able
to provide additional functionality for appliances, especially those appliances
in which the person contacting the appliance can provide additional data,
such as force, or where protection is required in order to prevent a safety
hazard if the person drops or lets go of the appliance.
When using the E-Field IC for other functions, adding touch control to
the same object may maximize its use—if all of the electrodes are
not used to obtain the other functions.
 Motorola Data Sheet MC33794, “Electric Field Imaging Device.”
 Motorola Application Note AN1985/D, “Touch Panel Applications
Using the MC33794 E-Field IC.”
This is an edited version of the paper that was originally delivered
at the 55th Annual International Appliance Technical Conference (IATC),
held March 29-31, 2004, in Lexington, KY, U.S.
DeLong is a senior systems and applications engineer
for Freescale Semiconductor’s Analog Products
Division of the Transportation & Standard Products
Group, Semiconductor Products Sector (SPS). Mr. DeLong
was involved in reducing to practice an integrated
(IC) to image an occupant in a vehicle and determine
their size and proximity to an airbag. The IC is
now in volume production and has proven to be an
method of preventing airbag injuries to children
and small adults. He is actively involved in expanding
technology into more types of applications.
Booth is digital design engineer, Networking & Communications
Systems Group for Freescale Semiconductor’s
Engineering Rotational Program (ERP), a program that
engineers to explore different disciplines in the
engineering field. Through the ERP program, she was
able to experience
applications and designs in the fields of sensors,
analog, and digital. She received her bachelor of
in Electrical Engineering from Arizona State University.