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1.INTRODUCTIONToday’s high school students may lack interest in math and science due to insufficient connections between why they learn in classes and real-life applications. However, the same students are attracted by new electronic devices, such as portable game players, smartphones and computer games, which rely on engineering principles that integrates math, physics, sciences and technology. One popular application for such electronic devices relies on social networking, such as Facebook, with a high prevalence of young users. The challenge is to attract students’ interests to engineering applications and make them aware of the engineering potential in their future life. In order to provide a smooth transition from high school to engineering for those who do choose engineering, outreach programs engaging high school and graduated engineering students are one of the proposed methods1 to overcome those difficulties. Bearing these in mind, the project “On-Light - Optical Social Network”, was born of a partnership between three secondary schools: Amato Lusitano High School in Castelo Branco, Quinta das Palmeiras High School in Covilha, and the High School cluster in Fundao, and the Polytechnic Institute of Castelo Branco, that coordinates the project (all the schools are from Portugal). The concept of social networking serves as a motto for this project, which aims to create a simple communication network, using light as the transmission medium. Mixed team, composed by students from secondary and higher education, are responsible for the development, production and testing of an optical transceiver. When several transceiver modules are aligned with each other in a ring topology, as depicted in Fig. 1a, this configuration will create a communication network, enabling the exchange of messages between users. This way, some concepts addressed in physics classes from secondary schools (photoelectric phenomena and properties of light) are experimentally verified, and combined with other subjects from undergraduate courses related with Telecommunications, such as communication protocols, digital modulation techniques, and electronics. The project is funded under a national program, financed by “Ciencia Viva”,2 a state agency focused on the dissemination of science activities among young people. This program aims to promote closer ties between high school and graduate education, with a view to share resources and encourage young students to pursue studies in science and technology areas. 1.1Related WorkThe dissemination of Visible Light Communication (VLC) technology and its principles among young students is scarce. A recent work uses lasers as optical transmitter3 for undergraduate courses, which may raise concern on eye safety when manipulating such devices. Even if Physics curriculum from secondary cchool courses include the photoelectric effect and the properties of light, little time is available for teachers and educators to bring these subjects into practical experiments. However, using visible light has the transmission support for wireless communication is not a recent subject,4 and has drawn great interest as a new communication medium due to the following recent developments. Firstly, solid-state light sources, like Light Emitting Diodes (LED) are rapidly replacing conventional ones in signaling, illumination and display infrastructures. It thus becomes possible to carry communication data on such light sources.5 Secondly, the visible band is free from frequency regulation and RF interference so that it is well suited to RF crowded or RF restricted environments. Thirdly, the unique feature of visibility can enhance the physical-layer security and offer intuitive usage. Given the growing expectation of ubiquitous connectivity in all settings and environments, the need for unlicensed, high bandwidth, easy to use wireless communications technology has never been greater. Since the appearance of the LED, several researcher have successfully create free-space optical transmission links with high bit rates and long reach.6 Recently, IEEE has created the working group 802.15.7 to produce a standard for WPANs (Wireless Personal Area Networks) using VLC.7 The rest of this paper is structured as follows. Section 2 defines the methodology of the project. Hardware and software development are described in details in Section 3 and 4, respectively. Outreach activities are presented in Section 5, and preliminary results and assessment in Section 6. Finally, Section 7 concludes the paper. 2.METHODOLOGYEach task in this project is described in the following sub sections. Fig. 2 highlights the task interdependencies. 2.1Task 1 - Team creation and assignmentThe project coordinator was responsible for the process of selecting students to participate in this project. Students from three educational level are involved: students from secondary schools, post secondary and undergraduate courses (the last two groups are from the Polytechnic Institute of Castelo Branco). The selection process was different between each educational level. In secondary schools, the selection usually follows a request for volunteers during sciences classes, where the project was previously presented and described by an educator. One exception occurs in Quinta das Palmeiras secondary school, where the selection targeted a group of students, leaded by a science teacher, that had already formed a club for extra curricular activities on electronics and software programming. After the student selection, the project coordinator creates student teams from the three educational levels previously described. Each team is responsible for the development, production and testing of one optical transceiver. 2.2Task 2 - Development and production of the transceiverTask 2 implements the required hardware and software, to obtain an optical transceiver with communication capabilities. More details are given in Sections 3 and 4. Each team is responsible for the selection of the necessary electronic component, the bill of material (BOM) and the programming of the Arduino micro-controller.8 The first prototypes are tested individually, and in a latter stage incorporated into the ring network for performance evaluation. In each team, the tasks are distributed in the following temporal order:
2.3Task 3 - Evaluation and field trialsTask 3 is where the trial of the optical network occurs, to assess the limitations and future improvements of the optical transceivers. Interaction between Task 2 and Task 3 will allow tuning of the transceivers architecture and software. This is where secondary school student will have their most significative participation, collaborating in the field trials and giving their feedback on the results. 2.4Task 4 - Promotion and managementTask 4 will disseminate the results of this project hence, will flow from all previous tasks. These activities include:
3.HARDWAREThis section presents a overall description of the electronic circuits of the transceiver presented in Fig. 1b. 3.1TransmitterThe transmitter may be subdivided into 4 subsystems, as depicted in Fig. 3. First, the Arduino micro-controller processes the information received from the PC, and send a corresponding signal to a digital port. This signal is used to control the state of a electronic switch (MC14066). The Arduino also generated an analog voltage to feed a voltage-to-frequency converter (LM311). Depending on the logical state of the control signal, the output of the electronic switch will generate an ASK signal. Finally, the ASK signal is used as at the excitation signal at the base of a power transistor (TIP41C), set in a common emitter configuration. This last subsystem is meant to keep the lamp always on, and imprint an optical signal variation proportional to the ASK signal. 3.2ReceiverThe diagram of the receiver circuit is depicted in Fig. 4. The photodiode is first connected to a transimpedance amplifier, to convert the current produced from the photoelectric effect into a voltage. Then, a bandpass filter removes the spectral component outside the frequency band of interest. The central frequency of the filter is the same frequency used in the voltage-to-frequency subsystem, on the transmitter. This operation introduces losses, so the resulting signal is once again amplified. The next step is to demodulate the ASK signal, remove the carrier and recover the original information. Thus, we implement a simple envelope detection using a diode and a capacitor. A threshold subsystem is needed to recover the mean value of the signal, and used as one of the inputs of a comparator. The threshold subsystem is implemented with a low-pass filter with cutoff frequency equal to 10 Hz. The final stage is a comparator, that delivers a binary signal according to the input signals levels (envelope and mean value signals). The resulting signal is then adjusted to 0-5 V levels and sent to one of the digital input of the Arduino micro-controller. One of the key parameters in optical system is the speed with which the optical sensor responds to light variations. Therefore, we tested the frequency response of a set of optical sensors. We choose three solar panels with surface areas of 2400, 3000 and 30000 mm2, that we call solar panel 1, 2 and 3, respectively. We also tested a photo diode with an integrated transimpedance amplifier (OPT101) and a sensitive area of 9 mm2. For the experiment, we use a LED lamp connected to an electrical signal source with variable frequency as the optical transmitter. The distance between the transmitter and the optical sensor was 30 cm. The results of spectral measurements presented in Fig. 5 show that the photo diode presents an increased bandwidth when compared to other components, mainly because it has much smaller dimensions than a solar panel, and a faster response to variations in amplitude modulated light from the LED lamp. Therefore, this photodiode will be the optical receiver of the transceiver. 4.SOFTWAREThe Arduino platform offers a simple interface and a multitude of functions to interact with the micro-controller, all based on the universal C language. Nowadays, this is a commonly used language, teached in secondary school. To keep the project as simple as possible, we use Hyperterminal from Windows (or a similar tool from other operating systems) as the graphical user interface on the computer. Regarding the communication protocol, the student are working on a proposal based on the Token Ring protocol, often used for ring network topologies. It is a protocol for Local Access Networks (LANs) defined in IEEE 802.5,10 where all stations are connected in ring and each station can only receive direct transmissions from the nearest neighbor. Such networks carries a short bit sequence, designated as a token. Possession of the token grants the right to transmit. If a transceiver receives the token and has data to send, it captures the token, changes a bit, adds the information to convey and sends this information to the next transceiver on the ring. While the token circulates on the network, other stations wishing to transmit must wait. Therefore, collisions doesn’t occur in this type of networks. The token circulates on the ring until it reaches the intended destination station, which copies the information for further processing. The token continues to circulate in the network and is removed as it arrives at the destination station. The sending station can check for the token to see if it was received and consequently copied by the destination target. 5.OUTREACHThis section describes outreach activities with the visible light transceivers and the path toward exploring the associated communication network curriculum more broadly. 5.1Hands-on sessions in secondary schoolsOne of the most rewarding activities is the interaction between students of different levels of education, made possible through the creation of collaborative teams. Fig. 6a shows the project coordinator, together with students form the Polytechnic Institute of Castelo Branco, that are involved with the project and have developed the first prototype of the optical transceivers, using electronic breadboards. Fig. 6b to 6d shows some of the hands-on sessions that took place in secondary schools. Here, the role of the project coordinator is minimized, to give room to a direct interaction between students form secondary schools and higher education. The hands-on sessions were designed to focus on the photoelectric effect and the properties of light subjects. 5.2Young scientists and researchers science fairEach year, the Museum of Electricity in Lisbon, gathers hundred of young researchers to participate in a national science fair, organized by a youth foundation.11 The project “On-Light: Optical Social Network” was represented by a team composed by three students, one from the Polytechnic Institute of Castelo Branco (higher education) and two students from Amato Lusitano secondary school, and accompanied by two educators. The stand was composed by a poster, describing the theoretical aspect related to the photoelectric effect and the properties of light, and a table with two optical transceivers, as depicted in Fig. 7b. The participants could exchange messages bi-directionally, using portable computers connected to each optical transceiver, as depicted in Fig. 7a. 6.RESULTS AND ANALYSISAlthough the participation in science fairs and hands-on sessions is a priori a good indicator of the relative success of the project, the only way to ascertain whether this is true is by means of an opinion pool about the project with the general population. Therefore, a survey has been administered during the participation on the Lisbon science fair, in May 2013 (described in Section 5). After each participant has exchanged messages with the optical network (e.g. Fig. 7a), we followed with a survey to collect their opinion. Fig. 8 shows the questions along with the responses gathered during the event. The total intake was 38 answers. Since the participants on the science fair were mainly students from secondary school, the sample mode is 18 years old (Q6). However, the science fair is opened for the general public, and received attendees from all ages. As a result of the participant responses, the following conclusions were drawn:
7.FUTURE WORK AND CONCLUSIONSThe development of the transceivers is still in its early stages, and an optical ring network is not yet a reality. However, the project has already reached a few milestones, and probably the most significant ones: The continuous cooperation between students from secondary schools and the Polytechnic Institute in Castelo Branco district has become a reality. The hands-on sessions and the joint participation in science fair are the result of that. The question is how to maintain this cooperation in the future, when the project is concluded? The opinion pool about the project, during the science fair in Lisbon, is a good indicator that we should continue with this approach. In October 2013, a international science Fair, called “Ciência en Acción”, will be held in Bilbao, Spain. The project team has already filled the application form, and hope to participate in this event, that will certainly motivate even more our student teams to continue their cooperation, and hopefully find a motivation to enroll on engineering courses. ACKNOWLEDGMENTSThe author gratefully acknowledges the Portuguese program Ciência Viva (Project PEC41/2013) and the Polytechnic Institute of Castelo Branco for funding this research. REFERENCESHabash, R. and Suurtamm, C.,
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