Pushing the Boundaries of IoT: Building andTesting Self-powered Batteryless Switch

Nikolaos Kouvelas, Ajay Keshava K, Sujay Narayana, R. Venkatesha PrasadEmbedded and Networked Systems Group

Delft University of Technology, The Netherlands

Abstract—Battery operated systems are bulky, expensive, andoften add unnecessary burden because of their maintenance.They are also harmful to the environment. However, the designand development of batteryless systems are highly challengingas the energy needs to be harvested from user’s activities or theenvironment. The harvested energy also varies with the activity,environment, and other aspects. In this paper, we present a systememploying an energy harvesting switch to power a low-powerradio, which transmits data wirelessly in 2.4 GHz ISM band. Weprovide the details of the design of our system and modules. Weevaluate our energy harvesting switch which we built in-house.With evaluations, we show that our system works well, and wedemonstrate the transmission of 27 bytes at 200 kbps data rate.Further, by varying the transmission power between -10 dBmand 5 dBm, we transmit data packets of length between 19 and27 bytes with a single press of the switch.

Index Terms—batteryless, energy harvesting, switch, zero-energy

I. INTRODUCTION

Internet of Things (IoT) is an important enabler for manysmart applications. Many systems are becoming smarter inseveral verticals, such as smart cities and smart agriculture.These systems are seen as the future way of living andimplementation/work [1], [2]. The unbounded growth of IoTis also causing more stress on the use of batteries andmaintenance. One of the major issues is the use of batteriesas it is not easy to power tiny IoT devices using mainspower. The concept of the batteryless system is not new.Techniques for energy harvesting, either by renewable sources(e.g., heat, light, vibration) or by human intervention (e.g.,spinning, shaking) have been investigated since decades [3]–[5]. Further, there is some work on movement-based energyharvesting for wireless switches [6]. Similarly, piezoelectricenergy scavengers, i.e., systems converting piezoelectric vi-brations to electricity, have long been considered as criticalenablers of self-powered wireless systems [3], [7]. However,the interest regarding batteryless communication systems isgrowing unprecedentedly due to these two main reasons:(i) Low-Power Wide-Area Network (LPWAN) protocols whichsupport the wireless communication of energy constraineddevices are gaining increasing popularity [8] and the numberof IoT-devices handled by one base station in LPWANsincreases. (ii) Forecasting of intermittent energy sources –mostly by using means of machine learning– is becoming

mature enough to support such batteryless devices [9]. Thiscan help in the rollout of batteryless systems.

Batteryless systems can be used in several places likeaeroplanes, buses and trains. For example, calling a flightattendant requires pushing a button connected through wiresto actuators [10]. The ‘stop’ indication by the passengers inbuses is also activated in a similar way. Such simple systems ofsensors and actuators can be replaced by batteryless systems,also using wireless communication. This will not only reducethe cost of wires, but also lower the amount of fuel thatis needed for the operation. Regarding the maintenance ofsystems powered by batteries, frequent battery replacement islabour intensive and, in many situations, even impractical dueto physical or deployment conditions (e.g., pillars of bridges,on top of wind turbines, etc.). Furthermore, batteries generatea considerable carbon footprint and greenhouse gases.

One of the main components of IoT devices is a commu-nication module. When we replace the batteries with energyharvesting, we need to take extra care since the amount ofenergy is limited and transient. In this paper, we present abatteryless wireless switch for IoT communication applica-tions that is able to transmit up to 27 bytes of data reliablyover the air. The system employs an electromechanical switchthat harvests energy through the application of force varyingbetween 2.7 N, to 3.9 N on its shaft and can produce up to210µJ of energy. This energy is converted to DC and storedin supercapacitors to power up an RF transmitter operating at2.4 GHz. Our system has a small footprint and is inexpensivecosting approximately $10. It can be used in multiple domains,like Smart Cities, Industry 4.0, etc.

Owing to this, we developed a batteryless energy harvestingwireless switch. The contributions of this paper are:

• We provide the complete design of a batteryless portableand lightweight system able to transmit data frames ofvariable sizes wirelessly.

• We achieved wireless transmission of 27 B frames (data,header, and checksum) with 100% reliability.

• We performed extensive evaluations of our system, con-sidering the different factors that influence its function-ality.

• We demonstrated communication distance of 60 m withour proposed system.978-1-5386-4980-0/19/$31.00 ©2019 IEEE

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The rest of the paper is structured as follows. In Sec. II wediscuss the related works and in Sec. III we describe the designof our system. We present the evaluation results of our systemin Sec. IV and we conclude in Sec. V.

II. RELATED WORK

The current works using piezoelectricity focus on creatingsmall footprint systems that enable batteryless transmission ofdata and reduce wiring [11]–[14]. Takemura et al. installeda piezoelectric sensor/generator inside drum pads [11]. Everyhit to the pad with the drumstick produces vibration (energy),which is harvested by the piezo generator. The signal getsmodulated and transmitted wirelessly to a receiver installed ina synthesizer. In this way, not only the signal is transmittedwith no connecting wires, but also the wireless transmitteroperates without batteries.

In [12], [13], contrary to our work, the piezoelectric har-vesters possess a button with spring(s). Every button pressresults in a short duration of vibration (AC pulse) which isthen converted into DC and stored in a capacitor. The controlcircuit guarantees enough power for the capacitor to supportthe batteryless operation of the wireless transmitter. In [12], thetransmitter produces On-Off Keying (OOK) signals, designedto support simple home automation applications, while in [13],the transmitter can also perform Frequency Shift Keying (FSK)modulation.

Different from [11]–[13] which transmit at 315 MHz and433 MHz, our system uses RF transmitter operating at2.4 GHz, guaranteeing lower energy consumption, higher ro-bustness and higher data rates (further dependent on the sizeof the transmitted frames).

Finally, in [14], which is the most relevant to our work,Yang et al. perform remote, batteryless on-off switching ofdevices. The user presses a piezoelectric button, creatinga single bipolar analogue pulse. The energy of this pulsepasses through a bias flip rectifier to minimize the losses andcharges six in-series capacitors. Then, the capacitors switch toparallel connection to perform down conversion of the chargedvoltage. 4 B length data is transmitted using Pulse FrequencyModulation to operate a device located 10 m away.

In our work, we tested frames of sizes up to 27 B overthe same distance of 10 m with 80% of the transmissionsbeing correctly received; and for reduced packet sizes of19 B, we achieved 100% reliable transmissions. The number oftransmitted bytes depends mainly on the amount of harvestedenergy. To this point, contrary to the 160µJ at 2.5 V that can beachieved by [14] or the 20µJ (roughly) in [12], our harvestingsystem can scavenge up to 210µJ of energy at 2 V when similarforce is applied.

III. SYSTEM DESIGN

In this section, we explain our system design in detail.First, we present the characterization of the amount of energyharvested by the switch, and then we explain the hardware andsoftware design.

A. Energy harvesterIn our system, we employ an energy harvesting switch

ECO 200, manufactured by EnOcean [6]. This switch is atransducer that converts mechanical energy (linear motion)into electric energy, based on the principle of electromagneticinduction. The generated energy is stored in a capacitor actingas a power bank for an RF transmitter. Figure 1 shows ourbatteryless wireless transmitter system, housing the switch andan RF transmitter module. The switch is capable of producing120 – 210µJ at 2 V for actuation forces between 2.7 – 3.9 Nrespectively. As the energy pulse that is produced by the switchlasts only for a few milliseconds, it has to be utilized for theRF communication effectively. The no-load voltage generated

Fig. 1. Batteryless transmitter system

Fig. 2. No-load voltage generated on switch actuation

after an actuation of the switch is shown in Figure 2. Withoutloss of generality, when the shaft is moved in one direction, apositive voltage pulse is generated, and when it is pushed backto its original position, a negative pulse is generated. Figure 3is the magnified image of Figure 2, focusing on the positivepulse. As observed in this figure, the maximum voltage of12.2 V is generated for a single actuation and the pulse lastsonly for around 3.26 ms.

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Fig. 3. Positive voltage generated on switch actuation

Our goal with this system is to reliably transmit a highernumber of bytes per given distance with a single actuation ofthe switch.

B. Hardware Design

The block diagram of the system hardware is shown inFigure 4. The main building components include (a) an ECO200 switch as energy harvester, (b) a bridge wave rectifierto obtain non-negative DC voltage, so that the system isfunctional irrespective of the shaft actuation direction, (c) aCC2652 RF System on Chip (SoC), housing an ARM CortexM4F core and 2.4 GHz RF front-end, and (d) a DC-DCconverter to provide regulated 3.3 V output to the CC2652RF SoC.

Fig. 4. Block diagram of the system hardware

CC2652 SoC is equipped with a 48 MHz Cortex M4FMicrocontroller Unit (MCU) and 2.4 GHz multiprotocol radio(BLE 5.0, IEEE 108.15.4 and proprietary RF). The mainreasons for choosing this chip are its low-power operationand the availability of proprietary RF protocol where datacan be transmitted over 2.4 GHz with a user defined datapacket format. In our system, we employ Gaussian FrequencyShift Keying (GFSK) modulating scheme with the frequencydeviation of 50 kHz for transmission that is fixed by themanufacturer. As we measured, the SoC consumes around18 mW active power at 48 MHz when the radio is OFF. Totransmit data at 200 kbps with 5 dBm transmission power, theSoC consumes 45 mW at the maximum. With our modifiedfirmware for CC2652 proprietary RF transmission, the chiprequires around 10 ms to initialize MCU and radio, and thento transmit 30 bytes at 200 kbps with -10 dBm transmissionpower.

Although the switch can generate higher voltage than re-quired by a low-power RF chip (operating at 3.3 V), theduration of voltage (> 1V) generation that can be utilized forharvesting is less (around 1.5 ms). We also observe in Figure 2that the negative pulse has slightly higher amplitude (around15.4 V) than that of the positive pulse. Empirically, we foundthat this phenomenon is random in all the switches that weevaluated. After analyzing ten different switches, we observedthat the utilizable voltage of 3.3 V is generated for a durationof at least 1.5 ms, when the minimum required force is appliedto the shaft.

The energy generated by the harvesting device has to bestored and utilized efficiently in order to get the maximumthroughput from the system. Considering the energy harvestedby the switch, the time for which the voltage is generated isonly around 1.5 ms, while CC2652 requires at least 10 ms toinitialize the ARM core and radio, and transmit 30 bytes. Toresolve this issue, a capacitor is used to store the short termenergy generated by the switch and power up the SoC for atleast 10 ms. Additionally, a bridge wave rectifier connectedbefore the DC-DC converter causes a voltage drop of 0.14 Vat 10 mA load current. The DC-DC block consists of an inputcapacitor that stores the harvested energy output from therectifier. It is obvious that, as the capacitance increases, thecharging time increases, and as the capacitance decreases,the amount of energy that can be stored decreases. Hence,to completely utilize the power output from the switch, weempirically chose the input capacitor value to be 37.7µF. Weused LTC3245 DC-DC buck regulator to provide continuous3.3 V to CC2652 SoC using an output capacitor with value3.3µF. A chip antenna 2450AT from Johnson Technologyis connected to the RF port of CC2652. Figure 5 showsthe voltage output from DC-DC block when CC2652 wasoperational, transmitting 27 bytes at 200 kbps with -9 dBmtransmission power. We also observe that regulated DC supplyof 3.3V is available for CC2652 for at least 10 ms as designed.

Fig. 5. Regulated voltage output from LTC3245

C. Software Development

We adapt TI’s proprietary RF transmission format in oursoftware architecture. The data packet format is of three mainfields - header, payload, and checksum. The structure of a

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single data packet is shown in Figure 6. The details on thepacket fields are as follows.

Fig. 6. Data packet format

1) Header comprises of Preamble, Sync word and datalength.

• Preamble consists of 4 bytes, used to synchronizetransmission timing with the receiver. In our system,we are fixing 0x55555555 as the preamble.

• Sync word is comprised of 4 bytes, used to indicatethe beginning of data. It was chosen as 0x930b51de.

• Length is a one-byte field which indicates the size ofthe payload.

2) Payload is the data to be transmitted.3) Checksum is a two-byte CRC to validate the data recep-

tion.

IV. EVALUATION

We evaluated the performance of our system for differentfactors such as the number of bytes transmitted, data rate,reliability, and transmission distance under different condi-tions. We also analyzed the critical parameters that affectthe performance of our system, such as the amount of forceexerted on the switch, the duration for which the shaft is beingpressed, and the transmission power. In all the experiments,TI’s CC2652 evaluation board was used as receiver to validatethe transmitted packets.

A. Amount of harvested energy

The number of bytes that can be transmitted dependsmainly on the amount of the harvested energy. Evidently,higher amounts of generated energy guarantee more bytesto be transmitted. Hence, we analyze the factors affectingthe amount of energy generated by the switch. The amountof harvested energy from the switch depends mainly on theapplied force and the number of turns in the switch coil. Sincethe latter is fixed by the manufacturer, we focus only on theimpact of the applied force.

To analyze the effect of the magnitude of the force appliedon the switch shaft, we considered the switch alone, anda 100 kΩ resistor connected across its terminals. Using anenergy meter, we recorded the amount of energy harvested fordifferent magnitudes of force applied. The varying force wasemulated by placing metal blocks of different weights on theswitch shaft. For the extreme acceptable switch actuation forceof 2.7 N and 3 N, we observed the maximum harvested voltagelevels 12.4 V and 16.2 V respectively. Hence, we designedour system to function for the lowest harvested voltage,approximately 12 V.

B. Transmission power vs number of bytes transmitted

By fixing the data rate at 200 kbps and the switch actuatingforce to the maximum (3 N), the transmission power of thetransmitter was varied in each trial. The receiver was placedtwo meters away from the transmitter. At different transmis-sion powers, the maximum number of bytes transmitted (alsoreceived successfully) is shown graphically in Figure 7.

Fig. 7. Number of bytes transmitted at different transmission power

As observed, the transmission power significantly affectsthe number of bytes transmitted. For transmission power of -10 dBm, the maximum number of 16 data bytes (27 includingheader and checksum) were transmitted. As the transmissionpower increases, the energy consumption of the RF front-end also increases. This results in a lower number of bytestransmitted. When the transmission power was at 5 dBm,only 8 data bytes (19 including header and checksum) weretransmitted, which is half the number of bytes at -10 dBm.Hence, there is a trade-off between the transmission power andthe size of the transmitted data packet when targeting longercommunication ranges. However, high gain antennas (around4 dBi) on both transmitter and receiver side can aid to achievelong-range communications at low transmitting power.

C. Data rate vs number of packets

We examined the effect of data rate on the number ofpackets transmitted. To evaluate it, we fixed the transmissionpower to -10 dBm and the data rate was varied from 100 kbpsto 250 kbps in steps of 50 kbps. The actuation force wasmaintained to be at the maximum of 3 N. The receiver wasplaced two meters away from the transmitter. The maximumnumber of bytes transmitted successfully at different data ratesare shown in Figure 8. As observed, higher data rates up to acertain extent (200 kbps) allow more bytes to be transmitted.This is because, as the data rate decreases, the transmitter takesmore time to transmit a fixed number of bytes over the air.As we increase the data rate, higher number of bytes can betransmitted at the same duration for which the harvested poweris available. For instance, at the data rate of 200 kbps, 27 bytescould be transmitted. Further increase in the data rate leads tofewer bytes transmitted as the required operating power ofthe RF chip increases (operating power increases along withthe data rate) while the amount of harvested energy remainsconstant. Hence, there is a prime data rate at which maximum

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Fig. 8. Number of bytes transmitted at different data rates

number of bytes can be transmitted for a fixed design. In ourdesign, it is 200 kbps.

D. Reliability

We evaluated the reliability of our system in terms ofsuccessful reception of the transmitted data. A data packetof 27 bytes (including header and checksum) was transmitted25 times at 200 kbps data rate and -10 dBm transmissionpower. The receiver was placed at various distances for eachtrial considering 3 dB link budget. The number of packetsreceived is shown in Table I. For shorter distances, such as two

TABLE INUMBER OF PACKETS RECEIVED OVER DIFFERENT DISTANCES

Distance between the transmitterand the receiver (metre)

Number of timespackets received

1 252 255 248 2210 20

meters, all the transmitted packets were received successfully,providing 100 % reliability. As the distance increased, therewas a drop in the number of packets received. One of thereasons for packet loss is that the receiver had low gain(around -2 dB) PCB trace antenna compared to that of short2.4 GHz whip antennas (around 4 dB) that are being used indevices such as WiFi routers. However, when the transmissionpower was increased to 5 dBm, we achieved 100 % reliabilityat 10 m communication range. In addition, 19 bytes werereceived over 60 m range at 5 dBm transmission power, whichwas the maximum communication distance we could achieve.However, this has trade-offs with the number of bytes thatcan be transmitted, as explained in Section IV-B. Hence, thecommunication distance depends mainly on the antenna gainof transmitter and receiver, and the transmission power.

E. Transmission power vs communication distance

The communication distance achieved at different levelsof transmission power is shown in Figure 9. It is apparentthat, for low transmission power (for instance -10 dBm), thecommunication distance accomplished is relatively less (26 m).

Fig. 9. Communication distance at different transmission power

As the transmission power increased, communication overlarger distances was attained, for example, 60 m at 5 dBm.However, wider range comes in compromise with reliabilityand data packet size, as described in Section IV-D.

V. CONCLUSION

The deployment and maintenance of wired communicationsystems in IoT applications such as audience polling systemsand smart homes are expensive in terms of cost and carbonburning, especially in the case of battery operated systems.While the usage of wireless communication systems solvesthe problem of wires, the use of batteries becomes apparent,making the system non-ecological. In this paper, we presenteda batteryless communication system that harvests energy froman electromechanical switch. We explained our system designin detail, wherein the harvested energy can be utilized effi-ciently to power up an ultra low-power SoC consisting of anMCU and a 2.4 GHz radio. We also presented the evaluationresults of our system for the maximum number of bytestransmitted and analyzed the different factors that affect theperformance of our system. The outcomes of our evaluationshow that 27 bytes can be transmitted over 10 m with 80 %reliability at 200 kbps data rate and -10 dBm transmissionpower for a single actuation of the switch. Further, we couldachieve 100% reliability by increasing the transmission powerto 5 dBm and decreasing the number of bytes transmitted to19. We also accomplished long range transmissions up to 60 mat 5 dBm transmission power.

ACKNOWLEDGEMENT

This project is supported by SCOTT http://www.scott-project.eu that has received funding from the Electronic Com-ponent Systems for European Leadership Joint Undertakingunder grant agreement No 737422. This joint undertakingreceives support from the European Unions Horizon 2020research and innovation program and Austria, Spain, Finland,Ireland, Sweden, Germany, Poland, Portugal, Netherlands,Belgium, Norway.

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