Printing Multi-Key Touch InterfacesCagdas Karatas

Rutgers UniversityNorth Brunswick, NJ 08902,

USAcagdas@winlab.rutgers.edu

Marco GruteserRutgers University

North Brunswick, NJ 08902,USA

gruteser@winlab.rutgers.edu

ABSTRACTWe present a technique for creating multi-key conductiveink touch user interfaces that can be printed on paper in asingle pass. While 3D printing and open-source electron-ics platforms have led to enormous creativity in creatingsmart objects, the means for user interaction with such ob-jects are often limited and require remote interaction througha smartphone app. Paper-based touch circuits are a conve-nient medium for exploring custom touch sensors that can beattached to numerous objects in our environment. The chal-lenge lies in creating a reliable and customizable touch cir-cuit that is easy to produce. Specifically, it should not requireassembly of multiple layers and it should support multipletouch points without needing separate connections to a mi-crocontroller for each touch point.

We address this through a resistive touch sensor that exploitsthe inherently high resistance of printed traces to create multi-ple detectable touch points. The finger closes the circuit whenin contact with the touch point and the sensor uses a polarity-switching technique to cancel out the effect of the unknownskin resistance. We evaluated the touch sensor using keypadswith 10, 15 and 20 touch points and achieved 99.6%, 93.5%,and 91% touch detection accuracy, respectively. We also ob-served touch detection rates of up to 154 touches per minute.

Author KeywordsPaper Keyboard; Conductive Ink Circuits; Flexible InputInterfaces;

ACM Classification KeywordsH.5.m. Information Interfaces and Presentation : Miscel-laneous; H.4 Information Systems Applications: Miscella-neous

INTRODUCTIONEmbedding an ever increasing number of smart devices intoour surroundings will create a need for more ubiquitous userinterfaces that let us interact with these devices. As we havetransitioned from PCs and laptops to small mobile devices,touch sensing has proven itself to be an especially compelling

Permission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full cita-tion on the first page. Copyrights for components of this work owned by others thanACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re-publish, to post on servers or to redistribute to lists, requires prior specific permissionand/or a fee. Request permissions from Permissions@acm.org.UbiComp ’15, September 7-11, 2015, Osaka, Japan.Copyright 2015 c©2015 ACM 978-1-4503-3574-4/15/09$15.00.http://dx.doi.org/10.1145/2750858.2804285

means of interaction with smaller devices. What if we couldtouch-enable a much richer set of objects and surfaces in ourenvironment? One could imagine custom buttons on car dash-boards that control devices designed long after the car wasmanufactured, remote control functions in clothing or coucharmrests, lighting control and telepresence functions throughbuttons on the conference table, or new generations of inter-active children’s books that do not require bulky reading de-vices.

Challenges. The main challenges lie in creating touch sen-sors that can be easily produced and customized for differ-ent applications. The touch sensor design should enable ex-ploration in this area, similar to how 3D printing and openelectronics platforms have led to enormous creativity in thedesign of smart objects. In particular, touch sensors shouldbe customizable in size and shape, so that they can easily beaffixed to objects of different proportions. Moreover, a touchsensor should accommodate various number of distinguish-able touch points to allow creation of interfaces with multi-ple buttons or keys. Design, production, and particularly re-production of these keyboards should not require specializedhardware knowledge. They should be straightforward to in-tegrate with custom electronics or smartphone apps such asIFTTT [1].

Existing solutions. Earlier research has studied a variety oftechnologies for touch-enabling surfaces in our environment.First, projection and camera systems have been used to createvirtual touch points on surfaces [12, 19]. This usually re-quires careful setup and alignment of the camera system andis susceptible to occlusion. Second, touch surfaces have beenembedded into objects with custom circuitry. Examples arewearable keyboards [4] or touch points on tables. This againrequires careful engineering and instrumentation for a spe-cific object. Third, audio sensing of touches has been used tocreate a virtual keyboard on a table[18] but it requires carefulfingerprinting of the audio environment during initializationtime and the process will need to be repeated if the nearbyobjects are moved. Only recently, researchers have exploredprinting of touch sensors through conductive ink. The exist-ing work is limited to a single button interface [16], or re-quires assembly of multiple printed layers and connection ofa large number of ADC lines to an electronic circuit [5]. Ei-ther the level of customization or the ease of production andreproduction is therefore limited in these approaches.

Approach. To address these challenges, we propose a resis-tive polarity switched touch sensor design that can be printedin one pass on paper using conductive ink. It separates the

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(a) (b) (c)

Figure 1: Example prototypes of printed touch interfaces.

necessary readout electronics components into a small attach-able clip device in order to avoid mounting electronic com-ponents on paper. Single layer printing is possible through aresistive touch sensor design wherein the finger acts to closethe circuit. A key advantage of the resistive polarity switcheddesign is that the layout and number of touch points can bemodified, without requiring any redesign of the clip deviceor a change in the number of interface lines between the de-vice and the paper. Specifically, the current prototype al-lows the creation of up to 10-15 reliable touch points withthe three GPIO pins on the clip device. This is in contrast toother more common touch sensor designs, where adding moretouch points requires more GPIO pins on the readout inter-face. The approach supports multiple touch points by exploit-ing the inherently high resistivity of printed traces (0.19Ω/2in our configuration)1 to create a voltage divider circuit layoutthat allows distinguishing touch points based on different readout voltages. This requires a sensing technique that is inde-pendent of the unknown skin resistance at the finger, however.Polarity switched sensing achieves this by implicitly cancel-ing out the effect of finger resistance.

The approach builds on advances in conductive ink printingwhich now allow for printing of conductive traces on plainphotographic paper with standard home-use inkjet printers atrelatively low cost (ink prices of a few hundred US dollarsper 100ml). This makes it possible to create touch circuits ona paper substrate that is flexible and thus able to conform toobjects that are not flat. We have also developed layout designsoftware that can create the necessary printed traces basedon a simple specification of the location of the touch points.The keystroke recognition method is also resource-efficient interms of computing power and hardware, allowing the designof long-lasting, small form factor clip devices that wirelesslyconnect to the devices they control (e.g., a smartphone or lightswitch).

Example applications. To illustrate some of the possible ap-plications for prototype printed touch sensors, we have pre-sented three sample prototypes in Figure 1. Many printed cir-

1The standard unit for sheet resistance is Ω/2 which is dimension-ally equal to Ω. However, 2 is used to discriminate it from regularresistance.

cuits replace or extend the capabilities of existing input solu-tions, such as car dashboards, bulky tablet add-on keyboards,or DJ controllers. Users may want to create their own physi-cal shortcut keys to be placed in a convenient location. Thisis illustrated through the customized interface for a driver’sphone on the steering wheel 1(a). While some cars alreadyinclude certain functions to control the phone over Bluetooth,printed interfaces allow for a more personalized experienceand allowing older cars to keep up with newer phone func-tions. A touch sensor in an interactive children’s book isdemonstrated in Fig 1(b). Other examples in this class mayinclude touch-sensing surveys and ballots. Fig 1(c) stretchesthe support for multiple touch points to create an entire roll-able and foldable keyboard. These examples also illustratehow it can be convenient to use the conductive ink to printtouch points in the form of recognizable icons. While notrecommended by the manufacturer of the conductive ink weused, other inks and printing processes may allow for over-laying the touch circuits with printed imagery using regularink.

In summary, the salient contributions of our work are item-ized below:

• simplifying the use of conductive ink printing for multi-button custom touch input interfaces that augment objectsin our environment.

• introducing a touch circuit design that can be printed inone pass and supports multiple touch points (i.e., buttons)without changes to the external device’s hardware or its 3pin interface to the paper circuit.

• designing a resistive polarity switching touch detectiontechnique to improve touch detection accuracy and allowscaling to 20 touch points with the three pin connectionfrom the paper to the external hardware module.

• developing a prototype readout circuit and layout guidanceto create custom paper user interfaces.

• evaluating the accuracy of touch detection and its depen-dence on several factors such as skin resistance and size ofthe touch point.

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Voltage Divider

VCC

R1

R2 ... Rn

Rfinger

...VADCRb

ADC

(a) Basic voltage divider touch circuit.

(b) Printed voltage divider with padstacks for buttons.

(c) Connector clips

Figure 2: Implemented circuit.

BACKGROUND AND RELATED WORKThe need for reliable and accurate input interfaces has led toa tremendous effort in the research community. The researchin this field has resulted in great improvements in touch-screen technology including analog resistive, surface capaci-tive, projected capacitive, surface acoustic wave, infrared(IR)and optical technology, to mention a few. However, powerand portability requirements limit the screen size of mobiledevices and the interaction surfaces on them.

Therefore, there has been a great amount of work put intomoving touch interfaces out of the smartphone footprint [12,4, 3]. In Canesta keyboard[12], users input text by pressingkeys on a projected image of a keyboard, and a sensor mod-ule captures the intersection of fingers with an IR light planeemitted from IR light source. Several works proposed wear-able keyboards (e.g., [4]) that detect key presses from kine-matic sensors placed on fingers. Moreover, Elfekey and thecolleagues [3] proposed a 4-layer thin keyboard that exploitsthe AC hum phenomenon that leads to a small AC currenton the human body due to AC noise from the environment.However, sufficient AC noise may not exist in all locations.

Analog resistive keypads and touchscreens have been widelyused by the industry [15] mainly because their recognitionapproach enables detecting multiple keys with a single sens-ing line. In a typical circuit, a reference voltage is appliedacross a voltage divider resistor with multiple stages in whichpressing a key closes the circuit and brings a stage in contactwith the sensing line. Since, the voltage is different at everystage (Vi at ith stage), the key that is pressed can be found bymeasuring the voltage on the ADC line. A similar principle isapplied to touch screens to find the 2D position of the fingerby using two resistive sheets separated by spacers. Unfortu-nately, mechanical switches and two layer approaches requireextra assembly and production effort.

To provide ubiquitous keyboards and touch surfaces, re-searchers have also explored vision and audio sensing ap-proaches. Visual panel [19] uses computer vision techniquesto detect a paper keyboard and the relative position of a fin-

ger tapping a key on the paper. This method, however, iscomputationally intensive and requires a carefully positionedcamera. Wang and colleagues proposed using [18] usingtwo microphones employed in a smartphone and detecting akeystroke’s location from multipath fading features of the au-dio signals. While this technique could enable a new methodof human-computer interaction, it requires keyboard softwaretraining with every movement of the keyboard or nearby ob-jects. Overall, none of these approaches lends themselveswell to instrumenting our environment with touch surfaces.

More recent work has also explored conductive ink to printspecial conductive patterns onto a paper to detect finger touchfor creating touch user interfaces. Touch events on the pat-tern can be detected with capacitive [13, 16, 10, 11] sensingtechniques as well as through resistive graphs [7]. Resistivegraphs use a similar approach as our method and they al-low touch sensing in 2D. Although these techniques comeinto prominence with different features such as integrateddisplays [11], they all require special multi-layer substratesto find the exact touch position and, therefore, the ease ofproduction and the level of customization are limited. Ourpolarity-switching technique, in comparison, enables fingertouch detection in single layer, such as on a sheet of paperprinted on one side. An alternative capacitive sensing tech-nique [5, 8] uses similar conductive ink and a single-layerpaper substrate as in our proposed interface. Gong and col-leagues [5] are also able to sense pressure by using skin resis-tivity. However, it needs a separate set of input pins from themicrocontroller for every touch point to detect the position ofthe finger. This requirement means connecting a large num-ber of tracks from the paper to the off-paper microcontroller,which has implications on the ease of connection and the sizeof the off-paper device. Hodges and colleagues [6] achievedlimiting the number of the tracks by connecting touch pointswith one-wire and I2C bus-based protocols. Still, this re-quires additional electrical components for extra touch points,which limits scalability. We describe how we address the lim-itations of these earlier works next.

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VCC

(n− i)Rs

iRs

Rfinger

VADC1

Rb

ADC

iRs

(n− i)Rs

VCC

Rfinger

ADC

VADC2Rb

Figure 3: Key recognition and Finger resistance calculationby switching voltage divider reference voltage

SYSTEM OVERVIEWIn this section, we detail the challenges unique to the printingmedium and introduce the polarity-switched resistive touchsensor system as a way to overcome these challenges.

ChallengesDesigning customizable and easily printable touch sensors in-volves several challenges due to restrictions in the printingmedium.

Single Layer. The touch sensing circuit should be in onelayer so that it can be printed from an inkjet printer in onepass. Several layers would introduce additional complex-ity in combining these layers. In particular, connectionsacross layers are difficult since vias and drilling are noteasily achieved on a paper substrate.

No mounting of circuit components. The sensor should notrequire mounting of external circuit components, whichconsumes excess time, restricts flexibility (the paper be-comes less bendable), and introduces protrusions that maybe easily ripped off during use.

Multiple touch points or keys. The touch sensor designshould support printing multiple distinguishable touchpoints or button on the same circuit. This will allow, forexample, creating the five icons with different functions asdepicted in Fig. 1(a).

Easy connectivity. The touch sensor design should allow forstraightforward connectivity with mobile devices or de-vices in our environment. Existing printable touch sensorsoften connect printed touch points to an off-paper readoutand communication circuit to accomplish this task. Thisoff-paper device can include radios to connect to other de-vices, but the connection between the paper sensor and theoff-paper device introduces a new scalability challenge. Asthe number of touch points increases, the number of re-quired sensing lines from the paper to the off-paper devicegoes up. The touch sensor design should minimize thiscomplexity.

ApproachTo eliminate mounting of circuit components, we have movedall other necessary items, such as the microcontroller, theamplifying transistors, and the high-value resistors, into aseparate off-paper module. This module could be designed

12345678910 20 30 40 50 60

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uenc

y

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8

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Figure 4: Histogram of finger resistance values taken from 15people at different times, 27MΩ and 65MΩ are taken fromthe same person.

in the form of a binder clip that can be attached directly tothe printed tracks. Unfortunately, a typical resistive touchsensor design requires two layers of electrodes, which arebrought into contact through the pressure from the finger. Toallow for single layer printing, we designed a resistive ap-proach where the sensing circuit is open at the touch pointand closed through the finger when touched. While straight-forward in principle, the challenge lies in allowing for multi-ple distinguishable touch points without increasing the num-ber of printed tracks that need to be connected back to theoff-paper module. We address this through two key ideas.

Touch Detection with Polarity SwitchingFirst, we exploit the inherently relatively high resistivity ofprinted tracks to create a voltage divider circuit. The sheet re-sistance (0.19Ω/2 ) of the conductive inks is sufficiently lowenough to use as a conductive trace while still being higherthan regular copper tracks. While often considered a disad-vantage, this allows us to print the basic voltage divider touchcircuit shown in Fig. 2a without requiring the mounting ofexternal resistor components. We only need to ensure thatthe printed tracks are long enough to create the desired resis-tances R1 to Rn. To elongate the tracks, we printed a snakingtrack, as shown in Fig. 2b (note the square-wave-like patternat the top). Given constant skin resistance, this basic voltagedivider circuit will allow for measurement of distinguishablevoltages at the ADC line, depending on which touch pointwas pressed. This means that multiple touch points can becreated with only three tracks to the off-paper module (Vcc,GND, and ADC). In practice, however, skin resistance variessignificantly over time, from person to person, and for differ-ent tapping gestures.2

Second, we therefore designed a polarity-switching techniquethat can cancel out the effect of skin resistance. Its key ideais that the ratio of two voltage measurements, the regular oneand the one where Vcc and GND are swapped, is independentof the skin resistance. We will detail this technique furtherbelow.

Together, these techniques allow us to create multiple touchpoints by printing only a single layer and clipping a deviceonto the paper so that it connects to the three interface tracks.It thus addresses the challenges above. The number of tracks2We have observed skin resistances between 0.5 - 65 MΩ.

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Figure 5: Voltage measurements

on the connection interface remains the same, regardless ofthe layout and number of touch points on the printed cir-cuit, which allows for the designing of different touch sen-sors without changing the clip hardware. We have also ex-perimented with capacitive sensing but ultimately chose theresistive approach because we found it to be more robust withmultiple touch points and only require a single sensing line tothe ADC.

In our solution, we use the finger as a circuit element in placeof mechanical switches. The finger acts as an inconsistentand high-value resistor Rfinger that connects the stage of thevoltage divider to the ADC line. Both the inconsistency andhigh-value characteristics of Rfinger create challenges. First,even a small current drawn from the analog to digital con-verter (ADC) pin of the microcontroller will create a signifi-cant voltage drop across the finger due to the high resistanceof the finger. Therefore, the voltage drop is proportional withfinger resistance. The measured voltage is also affected by thepull-down resistor Rb, which is used to ensure zero voltage atthe ADC when there is no finger connection. The measuredvoltage by the ADC is thus equal to VADC = ViRb

Rfinger+Rb

where Vi is the voltage at the ith stage of the voltage divider.To illustrate this, Figure 5 shows the voltage measurementswhen two different touch points are each pressed three times.We can observe that the voltage for touch point 2 varies be-tween 1.05 and 1.35 volts. Unfortunately, the voltages fortouch point 3 are within the same range. This means thechanging finger resistance makes it difficult to distinguish thetouch points based on these voltage measurements alone.

Estimating finger resistance is difficult since there are manypsychological [9], physiological [17], and environmental fac-tors that affect skin resistance. During our experiments weencountered a wide range of finger resistance values. In Fig-ure 4, we show the histogram of finger resistances encoun-tered when about 15 students have touched our keypad atdifferent times. As we can see, more than 93% of the fin-ger resistance values were less than 10MΩ but two outliermeasurements of 27MΩ and 65MΩ were observed (from thesame person incidentally). Therefore, we extend the methodto better accommodate different skin resistances.

Our method eliminates the dependence on skin resistance byswitching the polarity of the reference voltage lines that feedthe voltage divider and taking a second measurement. Themeasurements are time multiplexed during a single touch.Using these two measurements it is possible to determine thetouch point independent of finger resistance. We use this ap-proach since simply calibration Rfinger with a separate in-dependent circuit suffers from two problems. First, the resis-tance measuring circuit should not affect the measurementsof the primary touch point detection circuit. Second, thereis no room to route a second independent circuit on a singlelayer paper.

Figure 3 illustrates the polarity switching approach. It showstwo copies of the circuit from Figure 2a with opposite polar-ities. Note that the voltage divider is abstracted here for thecase where the ith touch point is pressed on a n touch pointinput interface. If we neglect the current drawn by the ADCand assume that the finger is not grounded, we can representthe measured voltages as follows.

VADC1 =ViRb

Rfinger + Rb(1)

Similarly, we obtain the following from the second measure-ment.

VADC2 =Vn−iRb

Rfinger + Rb(2)

Combining and simplifying the equations (1) and (2), yields

VADC1

VADC2=

i

n− i(3)

This ratio of the two ADC voltage readings only depends onthe point that the finger touches and eliminates the depen-dence on skin resistance and pull up resistor. To illustrate

this Figure 6 shows theVADC1

VADC2ratio for the voltage mea-

surements from Figure 5. Note that the ratio values are con-siderable more consistent across touches and remain between

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Figure 6: Corresponding ratio values of consecutive voltage measurements.

0.4 and 0.5 for touch point 2. Note also that these values cannow clearly be distinguished from those at touch point 3.

AlgorithmsWe first use a threshold detection algorithm to detect when afinger touches any touch point. Specifically, we detect a touchif VADC1 + VADC2 > t(Vcc − 2Vbe), where t is a thresholdparameter that we chose as 0.8 based on empirical data andVbe is the voltage drop across the transistor. In theory, thefollowing should hold.

VADC1 + VADC2 = VccRb

Rfinger− 2Vbe

The conservative threshold therefore accounts for noise andthe simplified model due to unknown finger resistance. Whena touch is detected we consider VADC1 and VADC2 a validsample pair and process it further.

The second stage in our pipeline is a touch point recogni-tion algorithm. Touch point recognition algorithm first esti-mates the touch point for each valid sample pair by finding thethe key with the closest expected ratio value to the measuredvalue. The expected ratio values are determined in calibrationstage after the keyboard is printed. It further applies filteringover multiple samples to improve robustness. Estimates forinvalid sample pairs are assigned to zero. The last n samplesboth estimates for both invalid sample pairs and estimates forvalid sample pairs are held in a buffer. The touch recognitionalgorithm recognizes a key only if at least half of the mem-bers in the sample buffer are valid and agree on a key otherthan the previously recognized key. Therefore, the key recog-nition algorithm will not produce duplicate results when theuser holds his or her finger on a key constantly.

The third stage applies temporal filtering. The temporal filterholds the output of the key recognition algorithm temporar-ily and releasing it only if no other key is recognized withina certain time period, the temporal window. If another keypress is detected within this period, the earlier key detectionis discarded and the new one is placed on hold. Therefore, thetemporal filter ensures that only the last key recognized is re-leased. This temporal filtering approach reduces spurious keydetections at the beginning of a key press event. While the fin-ger is still moving and the measured resistance is changing,false detections are especially likely. We have observed that

Time [ms]

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ts]

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samples

Figure 7: Signal at ADC and AC Hum noise

the last recognized key, when the finger has settled, is usuallythe correct one and hence the algorithm gives prioritizes thelast detection.

The response time of the current system is affected by themeasurement period, buffer size and temporal window. In ourcurrent settings, we have observed best results when the mea-surement period is 16.67ms, the prediction buffer is 6 validsamples requiring at least 4 predictions to agree on a key, andthe temporal window is 20ms. Therefore the time it takesto recognize a touch point is 153ms. In order to recognizeanother key, the prediction buffer needs to be flushed by therelease of the finger and needs to be refilled for the values ofthe following touch event. In theory, the current system canrecognize consecutive touch events every 286ms. In practice,our experiments show a minimum of 389ms.

CalibrationUnfortunately, the printed voltage divider pattern is quite non-linear in terms of resistance. As a consequence, it is not pos-sible to calculate precise expected voltages and the ratio val-ues for each touch point at design time. These voltage ratiovalues need to be measured after the circuit is printed. Fortu-nately, the system needs to be calibrated only once. For thispurpose, we have created a small calibration procedure forthe microcontroller. During the calibration process, the useris instructed to short the sensing line with each touch pointone after another. After ratio values for each touch point arecalculated, consecutive ratios are averaged to create safe mar-gins between predefined ratio values.

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Output1 Roffset R1 R2 ... Rn

RoffsetOutput2

Rfinger

...Rb

MCU

Vcc

Re

External Board On Paper

Stand

ard3-p

inconn

ection

Figure 8: Final Circuit

Layout ConsiderationsLet us first consider placement of the voltage divider. Thevoltage divider is in reality a very long trace. To create sucha long trace within a small area we have used a square wavepattern. We have chosen the top or bottom of the page for thevoltage divider, since we found it less distracting to the restof the design. It is also important to know the number of thekeys that will be placed on the paper. The number of keys isused to find the most efficient strategy for dividing the paperinto stages.

Due to the aforementioned voltage drop across the amplifiercircuit, the first and last stage should output 0.5V in the firstand second measurement periods, respectively. On the otherhand, the nonlinear resistivity of a printed voltage divider pat-tern forces us to use a safer voltage of 0.7V. Since our refer-ence voltage is 5V, most of the first and last 0.7/5 portions ofthe voltage divider are left as offset margins. The rest of thevoltage divider is split equally based on the number of keys.

The last step of the design is placing touch points on the paperand connecting them to the voltage divider pattern. Key pat-terns must be composed of at least two terminals that will beconnected with only a finger touch. We have observed that theminimum space between two terminals should be 0.88mm,otherwise a leakage current can occur between terminals af-ter a touch due to moisture residue from the finger.

IMPLEMENTATIONImplementing this method requires addressing several practi-cal issues that we ignored so far.

First, we assumed that Rfinger does not change between con-secutive voltage readings. This assumption holds when thefinger remains steady on a touch point, but Rfinger actuallychanges during finger movement due to the change in the con-tact area. For example, the contact area is very small at thebeginning of a touch event, largest at full press and then de-creases when finger is leaving the touch point, which con-tinuously changes the resistance. To reduce the change, thetime between the consecutive measurements should be mini-mized. The change of finger resistance between consecutivemeasurements can also be neglected if the change is relativelysmall with respect to Rfinger + Rb. For that reason, we useda high-value resistor Rb of 100MΩ in our circuit.

Second, our analysis neglected the current that the ADCdraws. In order to minimize this current, we used a Sziklaipair transistor configuration with a current gain of approxi-mately 10000. Therefore, the current that passes through thefinger due to ADC is at the 20 − 30nA level. However, thevoltage drop across the finger due to the ADC current can stillbe significant for very large finger resistances. The currentdrawn by the base of the Sziklai pair can be also taken into ac-count by merging equivalent resistance seen from the base ofthe pair with Rb. To compensate the voltage drop across theSziklai pair, a 0.5V bias voltage needs to be introduced to keyvoltages which can be done by connecting two serial resistorsto both ends of the voltage divider. In our design, we pre-ferred integrating these resistors into the voltage divider onthe paper circuit. Therefore, the layout should start placingkeys after leaving a certain margin at both ends of the volt-age divider. Third, we assume the paper is non-conductive;however, the paper between terminals is highly resistive witha resistance around 500MΩ when they are open. This resis-tance of the paper becomes smaller and eventually compara-ble to the finger resistance as the gap between touch pointsdecreases and the number of the keys increase. Therefore,the paper keypad design should maintain a minimum gap sizeand increase the gap between terminals as the number of keysare increased.

Finally, we assumed so far that the finger acts purely as a re-sistor rather than a voltage source. Unfortunately, the humanbody can act as an antenna and can introduce a voltage (noise)into the circuit. The primary source of noise is from the elec-tricity mains which is also called AC hum noise. Figure 7shows the voltage at the sensing line when a key is pressed.Note the clearly visible sine wave due to AC hum noise. Inour design, we have solved this problem with a simple trick.By setting the polarity switching frequency to 60Hz, we canobserve exactly one full cycle of AC noise within our mea-surement period. Since the average of noise within a full cy-cle is zero, we can eliminate the AC hum noise by takingmultiple samples within the measurement period and calcu-lating the average of those samples. Our sampling frequencyis 8160Hz therefore 136 samples are taken in each measure-ment period. Since AC hum was the primary noise sourcein our experiments, we did not use any filter. However, thedesign could be further improved with band-pass filtering toreduce noise at other frequencies.

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Figure 9: The change in accuracy for different users and num-ber of keys

Hardware PlatformWe have used an Arduino Duemilanove development board torun the touch point recognition algorithms. The developmentboard has ATmega328 microcontroller and provides 6 ADCinput pins and 14 Digital I/O pins. We use 2 of these digi-tal I/O pins for polarity switching and 1 of the ADC pins assensing line. The polarity switching is implemented by sim-ply setting one of the digital I/O pins high while setting theother one low and then alternating them during the next mea-surement periods. The amplifier stage is implemented with aSziklai pair. The Sziklai pair is a compound unit of two tran-sistors of opposite polarities. The current gain of the Sziklaipair equals to the product of the gains of the two transistors.The main advantage of the Sziklai pair over the well-known,equivalent Darlington pair is that the base turn-on voltage isonly half of the Darlington’s nominal turn-on voltage. Thisadvantage is very important for our application since the turn-on voltage is left as a margin in the 5V Vcc voltage.

In order to connect paper circuits with the Arduino, we imple-mented a clip-like connector as in Figure 2c. The connector iscomposed of a bulldog clip and a copper board and was firstintroduced in [14]. The copper board is scored with threeseparations for each connection and serves as a pad.

PrintingTo print circuits we have used an Epson WF30 printer,NBSIJ-MU01 ink and resin coated paper from MitsubishiImaging. The printed voltage divider resistor (snaking line)is 0.5cm high and 19.5cm long and has approximately 3kΩresistance. Based on equation (3), however, the actual resis-tance of the printed voltage divider is not very important aslong as it is lower than the resistance of the finger, which istypically on the order of MΩs. We print touch circuits withdifferent numbers of keys by spacing keys equally along theentire voltage divider.

EVALUATIONThe performance of the system is evaluated based on key fac-tors, namely finger resistance, key patterns and their size,number of keys, as well as key press frequency. First, westudy the limits of our touch detection circuitry in terms of thenumber of touch points it can support and the detection delayafter touch. Next, we evaluate the detection accuracy withdifferent touch point designs to provide guidance to printable

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touch user interface designers. Finally, we validate the pro-posed method by testing it with different finger resistancesand showing the range of finger resistances it can accommo-date.

The experiments were carried out in a standard office environ-ment and the paper interface was placed on a wooden desk.The experiments were performed by three male and two fe-male users in their 20s and early 30s. During the experiments,users did not receive visual or audio feedback from the elec-tronic device. They pressed the keys five times and are askedto touch the key fully.

Metrics. Generally, the experiments consider three types oferrors. Missed detection errors happen when no key is de-tected after a key press. False detection errors happen whenthe wrong key is detected after a key press. In theory, falsedetections can also occur when a key press is reported eventhough no key is pressed. In practice, however, we have neverobserved this type of error in our experiments after propercalibration. All reported false detections are therefore errorswhere the wrong key was reported. Finally, multiple detec-tion errors happen when the algorithm produces multiple keysafter a single key press. We define the overall accuracy asthe percentage of key touches correctly recognized, that istouches where none of these errors occur.

Number of keysIn order to understand the limits of the paper keypads interms of number of keys, we have conducted experimentswith printed circuits with 1, 5, 10, 15, 20, and 25 keys. Thetouch points use a rectangle pattern and the 10 key setup isshown in Figure 2b. Each user presses every key five times.

Due to performance variations between users, Figure 9 illus-trates the effect of different number of keys on accuracy fordifferent subjects. We observe that across all five users theaccuracy remains above 96% for up to 10 keys. For largernumbers of keys, accuracy decreases to as low as 72% forsome users but remains above 94% for two of the users.Tounderstand the root cause of this accuracy decline, rates fordifferent error types are given in Figure 10. As the numberof keys is increased, false detection and multiple detection er-rors increase significantly. This is likely due to smaller volt-

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age margins between the keys (or voltage divider stages) asthe number of keys increases. As the finger’s touch surfacechanges during a key press, the measured finger resistancevaries and the prediction algorithm’s output can swing be-tween keys. As a consequence, the key recognition algorithmeither recognizes the key incorrectly or outputs multiple keysfor a single key press. We believe that the main reason for in-stantaneous changes in the measured finger resistance is thefingers motion during a key press.

When considering the average accuracy across all users, weobtain 100% for 1 key, 99.2% (standard deviation [SD] 1.6)for 5 keys, 99.6% (SD 0.8) for 10 keys, 93.5% (SD 4.64) for15 keys, 91% (SD 6) for 20 keys and 85.1% (SD 10) for 25keys.

PatternsNext, let us examine how the shape of the keys affects theperformance. We chose four shapes for comparison. Therectangle key pattern is chosen over a square pattern becauseit enables placing enough space between keys and still pro-vides a decent contact area. The second pattern, which wecall circle-in-box was chosen since its uniform shape ensuresa connection between terminals. The third pattern, interdig-itated pattern, is commonly used on printed circuit boards(PCB) for mechanical buttons. Finally, the last pattern, linepattern, can be used in designs where minimalist key pat-terns are desired. On the other hand, the gap between ter-minals had to be increased slightly for this pattern in order tomake it distinguishable from regular traces. The patterns areillustrated in Figure 13. We also studied how performancechanges with size. The aforementioned patterns are printedin small (4.4x8.8mm) and large (6.6x13.2mm) versions. Thefive users pressed every key 5 times on 10-key keypads.

The best performance attained with the rectangle key patternfor both sizes was 99.6% of the keys are correctly recognizedand only two missed detection errors occurred over 500 keypresses. The circle-in-a-box pattern and Interdigitated pat-terns show similar results for both small and large footprints.The circle-in-a-box yielded 95.2% (SD 6.65) in small sizeand 99.6% (SD 0.8) in large size for different people. Theinterdigitated pattern resulted in 95.6% (SD 4.08) and 99%(SD 1) accuracy for small and large sizes, respectively. Weobserved wrong key error in 0.4% of key presses and misseddetection errors in 2.65% of the key presses. Although, the

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Figure 13: Different button patterns a) Rectangle pattern b)Circle-in-a-box pattern c) Interdigitated pattern d) Line pat-tern

line pattern had a similar shape to the rectangle pattern, itperformed worst with 94.8% (SD 4.66) and 95.2 (SD 2.99)%accuracy for small and large key sizes, respectively. False de-tection errors occurred only three times and the rest of theerrors were missed detection errors. The accuracy resultsare illustrated as a bar graph in Figure 12. We also experi-mented with the gap height factor on the rectangular patternand its dependence on terminal size. For this purpose, we pre-pared 10-key touchpads with the following width, height, andgap height dimensions: 1.1mm to 5.5mm with 0.55mm incre-ments for touchpad width, 1.1mm and 1.65mm for touchpadheight, and 0.44, 0.88, 1.32, 1.76,2.2,3.3, 4.4,6.6 and 8.8mmfor gap heights .The accuracy was not very sensitive to ter-minal width and height. For gap height, the average accuracyresults are given in Figure 11 for the rectangular pattern.Notethat the accuracy remains high for gaps between 0.88-2mm.However, when gaps become larger than about 2.5mm, theaccuracy starts to decline and the system misses some touchevents. This is expected since the chance increases that thefinger no longer covers both terminals sufficiently as the gapgets larger. A second interesting observation is that for gapheights lower than 0.88mm the system no longer reliably de-tects the release of the finger. We believe that this occursbecause of finger moisture left on the paper which compro-mises the isolating properties of the gap. Although the effectlasts only a few seconds, missed detection or multiple key er-rors occur during this time. In our experiments, this 0.88mmlower bound on the gap height was independent of the termi-nals height and width.

In summary, we observed the following. First, large patternsshowed better performance than small patterns. Second, thefirst three patterns showed quite similar performance withover 99% accuracy in large patterns. Third, higher misseddetection errors can occur as a consequence of the smallercontact area and larger gap between terminals, such as in theline pattern.

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Typing SpeedIn our tests, our 10-key keypad was able to detect up to 154key presses per minute. We believe this rate is enough formost touch user interface applications and would even sup-port keyboard typing at moderate speeds. The typing speedof an average smartphone user is 9.94 words per minute(WPM ) and 75.85 WPM for a desktop QWERTY key-board user [2].

Skin resistanceWe also examined how effectively our method can cancel outchanges in finger resistance. The experiments are performedby imitating the finger with resistors of a known value. In or-der to achieve this the resistor is connected to the sensing linein series, and the finger touch is simulated by a finger cov-ered with aluminum foil and an isolation layer between thefoil and the finger. The aluminum foil connects the two ter-minals of the key when pressed and the isolation layer elimi-nates any electrical noise that might leak from the body. Wehave used a keypad with 10 rectangular keys as in Figure 2a& b). This particular keypad was chosen because of its per-formance on our key pattern, size, and number of keys exper-iments. In this setup, every key is pressed 5 times, yieldinga total of 50 touches, for each of the 10, 20, 30, 40, 50, 60,70, and 80MΩ resistors. The results up to 50MΩ are shownin Table 1. Although we observed missed detection errorsa few times for each resistance value, these errors occurredwhen the finger did not stay on the contact area long enough.The other error types are not as frequently observed duringthese experiments. Overall, the performance is not very sensi-tive to different finger resistance values up to 50MΩ. Abovethis threshold, however, the voltage drop across the finger be-comes too significant and we were no longer able to detecttouches for 60, 70, and 80MΩ.

LIMITATIONS AND FUTURE WORKWhile we believe that this technique simplifies the construc-tion of prototype touch interfaces, it also imposes several lim-itations and presents opportunities for further research.

Multi-touch. The current system is limited to a single touchat a time. If n touch points are pressed simultaneously, nfinger resistance values and n stage values would need to becalculated. Such a system could potentially be solved with2n linearly independent equations but obtaining indepen-dent measurements would require more sophisticated sensingtechniques.

Special electrical conditions. As previously highlighted, thesystem cannot detect finger touch events when the finger actslike a voltage source, is grounded, or shows extremely highskin resistance. When the finger is grounded, the voltagereadings on any key will be zero and therefore it will be im-possible to identify the touch point. In addition, electrostaticdischarge related problems can occur when the finger acts likea voltage source. We have also been unable to detect toucheswhen the finger resistance exceeds 50MΩ, however this lat-ter limit could be extended by using an ADC with lower sinkcurrent.

Finger Resistance [MΩ] Accuracy [%]10 9620 9630 9840 9650 98

Table 1: Finger resistance and accuracy.

Robustness and ink issues. There are also substrate and inkrelated problems that may arise. The printed interfaces are notvery durable to due to the nature of paper and ink. The paperinterface can be easily broken by folding or tearing the paperand the ink can be scratched from the surface. Air moistureor residue from the finger may connect exposed terminals oftouch points. However, the problem might be easily solvedby increasing the gap height between terminals. Last but notleast, potential health implications of long-term exposure tonanoparticle-based inks may need to be further studied.

These issues do not impede prototyping use, however.

CONCLUSIONSWe have demonstrated a method for quick prototyping ofubiquitous touch sensors by exploiting conductive ink print-ing. In particular, the method allows for multiple distincttouch points to be printed using a single layer circuit that canbe printed in one pass. It eliminates the need for assembly orwire connections to a large number of conductive tracks onthe paper. Instead, the printed touch sensor can be completedby simply attaching a device in the form of a binder clip tothe paper.

We introduced a polarity-switched resistive touch identifica-tion technique that supports multiple touch points with onlythree connections to the binder clip device. Since it is able tocancel out the effect of changing finger resistance, it allowsthe fingers to be used to close the circuit at the touch point.This in turn enables printing the entire circuit in a single layer.

We have created a prototype of the readout circuit and printedcustom touch sensor designs. Our experiments indicate thatthe sensor achieves touch detection accuracy above 99% withup to ten different touch points and above 90% with 15 dif-ferent touch points. We found that the effect of the shape ofthe touch point is relatively small, as long as it is reasonablylarge enough to be touched with a finger and includes a gap ofabout 1-2mm. This enables a wide range of arbitrary shapesand touch sensor designs.

We hope that this technology inspires creativity in interactiondesign and touch-enables our environment, similar to how 3Dprinting and open electronics platforms have led to an abun-dance of smart object designs.

ACKNOWLEDGEMENTThis material is based in part upon work that is supportedby the National Science Foundation under Grant No. CNS-1409811 and a Google Faculty Research award. The authorswould also like to thank Emiliano Miluzzo for discussionsthat started this work, Rich Howard for his insights on circuitdesign, and Philip Chan for help with the implementation andexperiments.

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