sensors
Communication

A Touch Sensing Technique Using the Effects ofExtremely Low Frequency Fields on the Human Body
Hatem Elfekey 1, *, Hany Ayad Bastawrous 2 and Shogo Okamoto 1
1
2*

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan;
okamoto-shogo@mech.nagoya-u.ac.jp
Electrical Engineering Department, Faculty of Engineering, The British University in Egypt,
Cairo 11837, Egypt; hany.bastawrous@bue.edu.eg
Correspondence: elfekey.hatem.mahmoud@h.mbox.nagoya-u.ac.jp; Tel.: +20-10-6759-8408

Academic Editor: Vittorio M. N. Passaro
Received: 17 August 2016; Accepted: 21 November 2016; Published: 2 December 2016

Abstract: Touch sensing is a fundamental approach in human-to-machine interfaces, and is currently
under widespread use. Many current applications use active touch sensing technologies. Passive
touch sensing technologies are, however, more adequate to implement low power or energy
harvesting touch sensing interfaces. This paper presents a passive touch sensing techniquebased on the fact that the human body is affected by the surrounding extremely low frequency(ELF) electromagnetic fields, such as those of AC power lines. These external ELF fields induce
electric potentials on the human body—because human tissues exhibit some conductivity at thesefrequencies—resulting in what is called AC hum. We therefore propose a passive touch sensing
system that detects this hum noise when a human touch occurs, thus distinguishing between touchand non-touch events. The effectiveness of the proposed technique is validated by designing andimplementing a flexible touch sensing keyboard.
Keywords: human touch sensing; ELF electromagnetic fields; flexible touch sensing surfaces

1. Introduction
Touch sensing technologies are used in many applications, comprising smartphones and tablets,
laptops, information kiosks, etc. Touch screens are very intuitive and easy to use; they also save
space, because of their screen and interface spatial integration. Many touch sensing technologies havetherefore been developed for commercial purposes. Examples include technologies based on infra-red
sensing elements [1–4], resistive [5,6] and capacitive touch panels [7–9], cameras [10], the acoustic
and deflection characteristics of touch panels [11–13], and others [14–16]. One of the widely usedtechniques is the mutual capacitive method, which is used in almost all smartphones and tablets [17].
In this method, the touch screen interface is built or constructed by rows and columns of transparent
wires made from indium tin oxide. The row and column wires are separated by a thin glass layer.
Each row/column is electronically charged by an individual driver circuit. When the user touches thescreen at a specific position, the capacitance at the intersection between the row and the column at thisposition changes; the point of pressure on the panel can thus by localized by scanning all the othernon-energized rows and columns and computing the capacitance at all intersections [18].
Most of the aforementioned commercially available touch sensing techniques can be classified as
active sensing techniques, because touch detection depends on transmitting and receiving a signal thatis perturbed by a touch. For example, in the surface acoustic wave (SAW) touch sensing, transmitters
mounted on the edges of the touch screen emit sound waves that propagate through the touch screen
to be received by ultrasound receivers. At the onset of touch, the human finger absorbs some ofthe sound waves. As a result, the signal characteristics at the receiver are altered, as the signal’s
Sensors 2016, 16, 2049; doi:10.3390/s16122049

www.mdpi.com/journal/sensors

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2 of 10power is reduced [19]. This change in signal power is used to detect a touch. Therefore, active touchsensing techniques such as SAW use a transmitter and a receiver. On the other hand, passive sensing
involves the measurement of an external signal—which is not predefined—resulting from a touch.
An example of a passive touch technique is acoustic pulse recognition where sound wave receivers areused to detect a touch and no transmitters are required. Passive sensing approaches are usually low
power techniques, because processing and sensing occur only at the onset of touch; in contrast, active
sensing involves the continuous transmission, reception, and measurement of the detected signal,and further requires higher power levels. Some low power passive touch sensing techniques havebeen proposed, for which driver circuits are not required. However, some of these techniques may
not be suitable to detect the touch of human fingers [20,21]. Other passive touch sensing techniques
are based on piezoelectric and pyroelectric effects, as well as bending waves that propagate throughthe touch panel [22,23]. Nonetheless, they lack the “touch and hold” capability, which is an essential
manipulation feature in touch sensing interfaces [17].
In this study, we focus on another sensing technique, called humantenna. This touch techniquedetects the voltage induced on the human body by nearby AC power lines. The humantenna touchsensing technique has been used for gesture recognition and as an ultralow power human body motion
detector [24–26]. However, with the exception of our previous proposals [27,28], few passive touchsensing interfaces based on this technique have been proposed as viable alternatives to build practicaldevices such as keyboards or touch screens. In this paper, we propose and explore the humantenna
touch technique as a touch sensing interface. The device based on this technique is passive, andrequires only a signal conditioning circuit. In particular, we present a flexible touch keyboard design
as an example of a low power passive touch interface based on the humantenna technique.
The rest of this paper is organized as follows. In Section 2, we introduce the operating principleof the proposed sensing technique and the associated detection scheme. In Section 3, we present thedesign and implementation of a keyboard based on the proposed touch sensing technique. Section 4
discusses the results of the tests made to the developed keyboard, and Section 5 concludes the paperand discusses future work.
2. Operation Principle
2.1. Effects of External Electric Fields on the Human Body
Various sources of electric and magnetic fields are present in our daily environment, suchas geomagnetic sources, electrical power (AC) generation and transmission, domestic appliances,industrial equipment, telecommunications, and media broadcasting. All these sources contribute to adaily exposure to a very diverse mix of electromagnetic waves, including extremely low frequency
electric and magnetic fields. Moreover, the human body’s nervous system induces the so-calledbiopotentials on the outer skin layer, which are in the low-frequency range [29]. The combined effectof both phenomena (external electromagnetic fields and biopotentials) creates a resultant potential on
the human body that can be detected and used as a touch sensing trigger.
The voltage induced from biopotentials and the other mentioned sources of electromagnetic fields
on the human body is negligible when compared with the voltage induced by the electromagnetic
fields of nearby AC power lines [30]. Therefore, when developing touch sensors, the focus is placedon this latter external source of electromagnetic fields. This touch sensing technique is known as
humantenna [24].
The frequency of AC power lines is very low (50/60 Hz). Therefore, a quasi-static approximation
can be used, and the effects of the external electric and magnetic fields on the human body can becomputed independently [31]. The magnetic permeability of the human body is similar to that of freespace [32]. As a consequence, the magnetic field’s effect on the human body can be neglected, when
considering indoor applications. Therefore, the electric fields produced by AC power lines are theSensors 2016, 16, 2049

3 of 10main contributors to the voltage induced on the human body. The electric field induced by external
electric fields can be obtained with the following equation:
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3 of 10ωεoEnormal,bio = ELF Enormal,air= σbio
,

,

(1)
(1)

where Enormal,bio and Enormal,air are the normal components of the electric field in the human bodywhere Enormal,bio and Enormal,air are the normal components of the electric field in the human body and air,and air, respectively, and ω ELF , σbio , and εo are the AC line frequency, whole body conductivity, andrespectively, and ωELF, σbio, and εo are the AC line frequency, whole body conductivity, and electricalelectricalpermittivity
space, respectively[33]. Thiscan
equationcanbe usedthe
tovaluecalculatevalue ofpermittivityof space,ofrespectively[33]. This equation
be used to
calculateof thetheinducedthe potential
induced on
potential
on
the
human
body
when
exposed
to50/60
Hz
fieldsfrom
typical220/110the human body when exposed to 50/60 Hz fields from typical 220/110 V AC appliances, V
AC theappliances,the major
sourcesof inelectric
fieldsin life.
common daily life.
major sourcesof electricfieldscommondaily
2.2. 2.2.
AC AC
Hum
Touch
SignalAcquisition
Hum
Touch
SignalAcquisitionand
andProcessing
Processing
A diagramof ofthethehumantenna
herecancanbebeseen
seen
Figurea humanA diagram
humantennaapproach
approach asas implemented
implemented here
inin
Figure1: a1:humanbodytouchesthethesurfaceof aofpanel,andthetheresultingAC
bodytouchessurfacea panel,andresultingACsignal
signalisisthen
thenamplified
amplifiedand
andconditioned
conditioned forfor processing
by a controller.
processing
by a controller.

Figure
1. Operation
principleofofaatouch
touch sensing
sensing device
approach.
Figure
1. Operation
principle
deviceusing
usingthethehumantenna
humantenna
approach.

The detection circuit must be capable of processing low power sinusoidal signals. Therefore, a
The detection circuit must be capable of processing low power sinusoidal signals. Therefore, aDarlington amplifier was used, because of its good sensitivity, linearity, and large amplification
Darlington
amplifierofwas
used,hundreds
because oforitsthousands
good sensitivity,
linearity,and large
amplification
factor (typicallyseveral
A/A) [34].
The wholeamplifying
circuit factoris(typicallyofseveralhundredsor
thousands
A/A)
[34].
The
whole
amplifying
circuitiscomposedcomposed of one Darlington pair, two resistors, and an LED, as shown in Figure 2. The LED is used of
oneforDarlington
two resistors,
andLED,
as shown
Figure 2. The
Thebranch
LED iscontaining
used for indication
indicationpair,
purposes,with R2 (50
Ω)an
being
used
for LEDin
protection.
R2 andpurposes,
with
R
(50
Ω)
being
used
for
LED
protection.
The
branchcontaining
the LED is
the LED is placedin parallel with the controller input, whereas R1 (1 MΩ) is used Ras2 anda pull-down
2
placedin parallelwiththe controller
whereas R1 (1 MΩ) is used as a pull-down resistor toresistor
to nullify
the parasitic
effects input,
in the circuit.
Most
controllers
havea smallnumber of analogue inputs. Therefore, for convenience, digitalnullify theparasitic
effectsin thecircuit.
controller
inputs wereused.
In addition,
analogueprocessing
would
imply analogue-to-digital(A/D)
Most controllers
havea smallnumberof analogue
inputs.
Therefore,
for convenience,
digitalconversions
andfiltering(implyingeitherextra
modules,oramore
advanced
controller),
whereascontroller inputs were used. In addition, analogue processing would imply analogue-to-digital (A/D)
the use ofand
digitalinputs(implying
avoids theeither
aforementioned
needs,orallowingand simpler
processing
conversions
filteringextra modules,
a morefasteradvanced
controller),
whereasalgorithms.
Therefore,
leveraging
the
fact
that
the
analogamplitude
informationisnotnecessaryforthe use of digital inputs avoids the aforementioned needs, allowing faster and simpler processing
the intended detection, both the electrical signal conditioning and the processing algorithm werealgorithms. Therefore, leveraging the fact that the analog amplitude information is not necessary
designed such that the obtained signals could be connected directly to the controller’s digital input
for the intended detection, both the electrical signal conditioning and the processing algorithm were
pins. Consequently, the signal delivered to the controller at each input pin is a square wave oscillating
designed such that the obtained signals could be connected directly to the controller’s digital input
between logic zero and logic one, instead of an analogue sinusoidal wave.

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4 of 10pins.
Consequently, the signal delivered to the controller at each input pin is a square wave oscillating
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betweenzero
Sensorslogic
2016, 16,2049and logic one, instead of an analogue sinusoidal wave.
4 of 10Figure2.2.Amplifier
Amplifier cell schematic.
schematic.
FigureFigure2. Amplifier cell
cell schematic.

Figures
3a–cshow
thesignal
signalwaveforms
waveformsat
atthe
the output
output of theandFigures
3a–c
show
the
the amplifier
amplifierininthe
thecases
casesofoftouch
touch
and
Figure3a–c logic
showsquare
the signal
waveforms
at the output
the amplifierin the casesof touch andtouch,
wavereadreadbybythe
thecontroller
controller
in the
theof
case
no no
touch,
thethelogic squarewave
incase ofof aatouch
touchevent,
event,and
andthethefrequency
frequency
no touch,
the oflogic squarewave respectively.
read by the controller
inthe caseof a touch
event, toand thefrequency
spectrumtouch
signal,Asshownshown inin Figure
Figure3a,spectrumof thethetouchsignal,respectively. As
3a, thethe input
inputsignal
signal tothe
thecontroller
controller
spectrumoftheatouchsignal,respectively.
As voltage
shown
3a,
the
input
signaltothe controllerinin thecasea toucheventhashasa amaximum
maximum
voltageinin Figureexcess ofin the
case
of of
touch
event
in
excessof 44 V,V, whereas
whereasthe
themaximum
maximumsignal
signal
the
case
of
a
touch
event
has
a
maximum
voltageinexcessof
4
V,
whereasthemaximumsignalvoltagevoltageintheno-touch
case
is
below
0.5
V.
For
a
typicalcontroller
withasupplyvoltageof5
V,thevoltage in the no-touch case is below 0.5 V. For a typical controller with a supply voltage of 5 V, themaximumallowable
voltage
for
logic
V,
whereas withthe threshold
is is
3.53.5VV
inmaximum
theno-touchcase isvoltage
below
0.5
V.
Forzeroazero
typicalcontrollera supplyvoltage
voltagefor
oflogic
5logicV, onethemaximumallowablefor
logic
isis11V,
whereasthethresholdvoltage
for
one
[35].
allowable
voltage for logic zero is 1 V, whereas the threshold voltage for logic one is 3.5 V [35].
[35].
A timeris used in
the controllertotodifferentiate
differentiate between
between touch
situations.
As As
seenseen
AAtimertouchandandno-touch
no-touchsituations.
timerisisusedusedininthe
thecontroller
controller to differentiate between
touch
and
no-touchsituations.
As seenin Figure 3b,
a touch-induced signalisisreceived
received byby thecontroller
of of
logic
oneoneimpulses,
ininFigure
the controller
controllerasasaaatrain
train
logic
impulses,
Figure3b,3b,aatouch-induced
touch-induced signal
signal is received by
the
as
train
of logic
one impulses,
whereastheno-touchsituationcorresponds
toastream
of
logic
zero.
A
touch
is
preliminarily
whereasno-touchsituationcorresponds
to atostreamof logictouch
preliminarily
detected
whereasthe
theno-touchsituationcorresponds
a streamof zero.
logic A
zero.
A is
touch
is preliminarily
detected whenever a logic one is detected. When a logic zero is detected after a logic one, the timer iswhenever
a
logic
one
is
detected.
When
a
logic
zero
is
detected
after
a
logic
one,
the
timer
is
activated.
detected
whenever a logic one is detected. When a logic zero is detected after a logic one, the timer isactivated. If only logic zero is read within the specific timer duration, a final no-touch decision isIfactivated.
only logicIfzero
is readwithinspecifictimer
a finalno-toucha decision
is made;otherwise,
only
logic
zero logic
isthe
read
the duration,
specific
timer
duration,
final
no-touchmade; otherwise,
if further
onewithinis detected
within thetimer
duration, theexistenceof adecision
touch is is
ifmade;further
logic
one
is
detected
withinthetimer
duration,theexistence
of
a
touch
is
confirmed,
andotherwise,iffurther
logic
one
is
detected
withinthetimer
duration,theexistence
of
a
touch
is a
confirmed, and a final decision (touch detection) is accordingly made. If a logic one is detected within
final
decision(touchdetection)isaccordingly
made.
If
a
logic
one
is
detected
withinthesubsequentconfirmed,
and a final
accordingly
made. The
If a logic
is detected
withinthe subsequent
timerdecisionperiod, a(touch“touchdetection)and hold” isoperation
is detected.
timer oneduration
was chosen
timer
period,a “touchhold”
operation
is detected.
timer
durationwaschosento be
30 ms,
thetosubsequent
timer period,
a “touchandhold”
is detected.
The
timer
durationwaschosenbe 30 ms,
which
isand
a one-and-a-halfperiodofoperation
a 50 Hz The
signal.
Figure3b shows
thatthe maximum
which
is
a
one-and-a-halfperiodofa50Hzsignal.
Figure
3b
shows
that
the
maximumtime
between
to be
30
ms,
which
is
a
one-and-a-halfperiodofa50Hzsignal.
Figure
3b
shows
that
the
maximumtime between two consecutive ones is 26.1 ms.
time
between two
consecutive
twoconsecutive
ones
is 26.1 ms.ones is 26.1 ms.
5

Signal to the controller at touch event

5

Signal to the controller
touch
event
Signalcontrolleratatno
touch
event

4

Signal to the controller at no touch event

4
3

3
2

2
1

1
0

0

0

0

0.1

0.1

0.2

0.2

0.3

0.4
0.5
Time [s]

0.6

0.7

0.8

0.3

0.4 (a) 0.5
Time [s]

0.6

0.7

0.8

(a)

Figure 3. Cont.

0.9

0.9

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5 of 10

1

0

0

0.05

0.1

0.15

0.2

0.25
0.3
Time [s]

0.35

0.4

0.45

0.5

(b)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0

0

10

20

30
40
Frequency [Hz]

50

60

(c)
Figure 3. Touch signal characteristics. (a) Input signal to the controller before and after the touch
Figure 3. Touch signal characteristics. (a) Input signal to the controller before and after the touch event;
event; (b) Digital signal received by the controller in the case of a touch event; (c) Frequency spectrum(b) Digital signal received by the controller in the case of a touch event; (c) Frequency spectrum of theof the signal resulting from the touch event.
signal resulting from the touch event.

3. Design and Implementation of a Keyboard Based on the Proposed Touch Sensing Technique
3. Design and Implementation of a Keyboard Based on the Proposed Touch Sensing TechniqueUsing the proposed touch sensing technique, a touch keyboard was designed and implemented.
the proposed
touch
technique,a touchkeyboard
was designed
TheUsing
keyboard
is comprisedof asensing
touch keypadinterface,a signal
conditioning
circuitand
andimplemented.
a controller.
The
keyboard iscomprisedof a touch keypadinterface,a signal
circuitand a controller.
A controller
capableof supporting
universal
serial bus(USB)conditioning
protocols was
selected,
both forAconvenience
controller capableofsupporting
universal
serialbus(USB)protocolswasselected,
bothforconvenienceand to have enough digital input channels (a few tens of channels are needed in thisandto have enough digital input channels (a few tens of channels are needed in this application).
application).
To
without thethe needneedforforan
anintermediate
intermediate
Toallow
allowusers
userstotodirectly
directlytouch
touchthethekeyboard’s
keyboard’s interface
interface without
medium,the
proposed
keyboard
interfaceusesamatrixofdryelectrodes
[36].
The
interfaceconsists
medium, the proposed keyboard interface uses a matrix of dry electrodes [36]. The interface consists
mainlyofbarecopperwires
andinsulators,
which
are
arrangedinfourlayers,as
shown
inFigure4a.
mainly of bare copper wires and insulators,
arranged in four layers, as shown in Figure 4a.
Bothinsulatorlayer
layerareare1-mm1-mmthick.
thick.The
The
bottomtransparent
layer,
which
Boththethecopper
copperwires
wires andand the insulator
bottomtransparent
layer,
whichisisreferredreferredtotoas
asthe
thesubstrate
substratelayer,
layer,isisaastandard
standardplastic
plasticsheet.
sheet.The
Thenext
nextlayer,
layer,which
whichisisreferredreferredtotoasas
theThe third
third layer
layercontains
contains15
15verticalverticaltherow
rowlayer,layer,isiscomposedcomposedofofsix
sixhorizontal
horizontal barebare copper
copper wires. The
insulators,
thethelastlast
layer—referredto aslayer—iscomposedof verticalbare copper
insulators,whereas
whereaslayer—referredtothe
as column
the column
layer—iscomposedof verticalbarecopperby thelines
dashedlines 4a.
in Figure4a. The construction
is comprisedofwires,
as wires,shownasbyshown
the dashedin Figure
The construction
process is process
comprisedof three steps.
threethesteps.
row layer(withis mounted(withrows)
equidistantthe plastic
substrate,
fixedFirst,
rowFirst,
layertheis mounted
equidistanton therows)
plasticonsubstrate,andfixed inand
place
by a
in place by a cyanoacrylateadhesive. layer
An insulator
layer
made of cyanoacrylate
is thencyanoacrylateadhesive. An insulator
also madeof alsocyanoacrylate
is then mounted
onmounted
top of the
on topof Finally,
the row thelayer.
Finally,
the iscolumnlayeronis themountedon theinsulator
layer.
Withthethiscolumns
setup,
row
layer.
columnlayer
mountedinsulator
layer.
With thissetup,
thecolumns
and
rows
form
a
matrixofcoordinates.
Each
touch
buttonistheintersectionofa row
and rows form a matrix of coordinates. Each touch button is the intersection of a row and a column;
column;a totalof 90aredistinct
inputsare therefore
Whentoucheduser,willaa andtotalaof
90 distinct
inputstherefore
provided.
Whenprovided.
touched bya humanuser,bya a50human
Hz signal
50 Hz signal will be induced in the corresponding row and column, thus enabling touch detection
and the identification of the touched intersection.

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beSensors
inducedin the corresponding row and column, thus enabling touch detection and the identification
2016, 16, 2049
6 of 10ofSensors
the touchedintersection.
2016, 16, 2049
6 of 10The
proposed
identification ofofthe
theactivatedactivatedtouch
touchbutton,
button,becauseThe
proposedstructure
structureguarantees
guaranteesaa uniqueunique identification
becauseTheis
proposedstructure
guarantees
a uniqueidentificationof Moreover,theactivatedtouchbutton,becausenono
buttonbythanrow
column.
Moreover,thisinterfacedesignhas
buttonisrepresented
represented
bymoremorethan oneone
row andone column.
thisinterfacedesignhas
nobuttonisrepresented
by
more
than
one
row
and
one
column.
Moreover,thisinterfacedesignhas
somedesirablefeatures:itisflexible
and
easy
to
store,
asshown
inFigure
4b;
its
size
can
easilysome desirable features: it is flexible andstore, as shown in Figure 4b; its size can easily bebe
somedesirablefeatures:itisflexible
and
easy
to
store,as shown
Figure4b;asitsshown
size caneasilybecustomized
mountedany
surface,asshown
Figurecustomizedtotothe
theapplication;
application;finally,
finally,ititcan
canbebeeasilyeasilymountedon inany
surface,
ininFigure
5.5.
customized to the application; finally, it can be easily mounted on any surface, as shown in Figure 5.

Figure 4. Flexible layered keyboard. (a) Layer structure; (b) Flexibility demonstration.
Figurestructure;(b)(b)Flexibility
Flexibilitydemonstration.
demonstration.
Figure4.4.Flexible
Flexiblelayered
layeredkeyboard.
keyboard. (a)(a) Layer
Layer structure;

Figure 5. Complete keyboard structure being tested in the laboratory.
Figure 5. Complete keyboard structure being tested in the laboratory.
Figure 5. Complete keyboard structure being tested in the laboratory.

The whole detection and signal conditioning circuit consisted of the 21 Darlington cells for the
The sensors
whole detection
andandsignalconditioning
circuitcan
consistedof theDarlington
cells
for the21 input(15 columns
6 rows).
The controller
determine
the21touch
positionby simply
21 input sensors (15 columns and 6 rows). The controller can determine the touch position by simply

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The whole detection and signal conditioning circuit consisted of the 21 Darlington cells for theSensors 2016, 16, 2049
7 of 1021 input sensors (15 columns and 6 rows). The controller can determine the touch position by simply
determining on which inputs the touch signal is present, and comparing the input pattern withdetermining on which inputs the touch signal is present, and comparing the input pattern with aapredefinedpredefinedlook-up
look-uptable.
table.The
Theoperational
operationalalgorithm
algorithmisistherefore
thereforevery
verysimple,
simple,and
andthetheinputs
inputsarearereadread
asasstandard
digitalsignals,
which
is
a
major
advantageof
thiskeyboard
implementation.
standard digital signals, which is a major advantage of this keyboard implementation.
4. Testing the Keyboard
4. Testing the Keyboard
To show the response of the proposed technique to noisy signals (which are not touch events),
To show the response of the proposed technique to noisy signals (which are not touch events),
Figure 6a shows a comparison of the sensor voltage during a touch event with the one caused by a
Figure 6a shows a comparison of the sensor voltage during a touch event with the one caused by afinger proximity at various distances from the keyboard (non-touch events). The minimum detectablefinger proximity at various distances from the keyboard (non-touch events). The minimum detectable
voltageappeared at a distance below 0.5 mm from the keyboard interface, which indicates that thevoltage appeared at a distance below 0.5 mm from the keyboard interface, which indicates that theimplemented
implementedkeyboard
keyboardisissensitive
sensitive only
only toto touch
touch events.
events.
To
characterizethehysteresis
phenomenon,
Figure6a shows
the voltage
readings
obtainedwhen
To characterize the hysteresis phenomenon,
Figure6a shows
the voltage
readings
obtained
theuser’sfingerisbothapproachingandretreatingfromthekeyboard
interface.
Fortheimplemented
when the user’s finger is both approaching and retreating from the keyboard interface. For the
keyboard,
the hysteresis
maximum
differencein therecorded
voltagevaluesas thevalues
fingerimplemented
keyboard, error—thethe hysteresis
error—themaximum
differencein therecorded
voltageapproachedandbacked
away
from
the
keyboard—was
found
to
be
less
than
1%.
In
addition,as the finger approached and backed away from the keyboard—was found to be less than 1%. theIn
keyboard
tested inwas
the tested
variousenvironments
where its whereuse is its
mostaddition, was
the keyboard
in conditions
the various andconditions
and environments
useexpected
is most(e.g.,
libraries,
rooms,
units andlaboratories).
The keyboard’s
sensitivity
and
expected(e.g., study
libraries,
studyresidential
rooms, residential
units
and laboratories).
The keyboard’s
sensitivity
performance
showednomajorvariations
under
the
different
environmental
conditions,asshownand performance showed no major variations under the different environmental conditions, asin
Figureshown6b.
in Figure 6b.

Figure6.6.Voltage
Voltagemagnitude
magnitude vs.
vs. distancedistance between
between finger
Figurefinger andand keyboard.
keyboard. (a)(a)Voltage
Voltagedetected
detectedwhile
while
approachingandbacking
away
from
the
keyboard;(b)
Voltagedetectedinvariousenvironments.
approaching and backing away from the keyboard; (b) Voltage detected in various environments.

The repeatability error for the implemented keyboard—defined as the maximum difference
between sensor outputs recorded for the same input values, under the same conditions—was lessthan 6.5% for all the conducted tests, with a relative standard deviation value below 5.7%.

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The repeatability error for the implemented keyboard—defined as the maximum difference
between sensor outputs recorded for the same input values, under the same conditions—was less than
6.5% for all the conducted tests, with a relative standard deviation value below 5.7%.
All the previous measurements and statistical data show the potential of the proposed touchsensing technique as a promising technology, both in terms of reliability and of cost-effectiveness,when compared to the other popular touch sensing techniques mentioned in the Introduction.
5. Conclusions and Future WorkIn this paper, a novel touch sensing technique was proposed, based on the detection of the voltage
induced on the human body by the electric fields of AC power lines.
To demonstrate the effectiveness of the proposed technique, a touch sensing keyboard was
designed and implemented. The keyboard was constructed from simple components (a plastic
interface, Darlington pair amplifiers, and a typical controller), and therefore did not require complex
or expensive hardware. Moreover, the implemented keyboard has many desirable features: it is highly
flexible, it is shock-, dust-, and water-resistant, and last but foremost it has very low implementationand operation costs. Taking into consideration that the proposed touch technique is passive and does
not therefore require external generators, other possible important applications may include touch
floors and safety sensors [37–39].
To extend our method to other contexts beyond the detection of binary events (e.g., to the
localization of touch on an object surface [39]), the repeatability error mentioned above should bespecifically considered. Possible causes for this error can be found in the existence of other ELF waves
such as Schumann resonance [24], switched mode power supply interference noise [40], other external
noises imposed by the main supply line [25], and biopotential signals [33]. In future work, the authors
will investigate possible solutions to this problem, such as using a power supply with a higher-quality
factor and lower total harmonic distortion (THD) to remove the power source-induced noises, applying
a high quality factor band pass filter for the 48 to 62 Hz frequency band, and/or using other amplifier
with lower THD and thermal noise. Furthermore, the precise energy consumption of our technique isalso a topic for further investigation.
Acknowledgments: This study was partially funded by MEXT Kakenhi (15H05923).
Author Contributions: Hatem Elfekey and Hany Ayad Bastawrous contributed equally to the work.
Hatem Elfekey, Hany Ayad Bastawrous, and Shogo Okamoto were involved in the technical discussions andmanuscript refinement.
Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript:
ELF
LED
USB

Extremely low frequency
Light emitting diode
Universal serial bus

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