Visible Light Transceivers

Wireless optical communication using visible light is one of the emerging green technologies that has not been fully utilized. Many versions of white-LED transceivers have been built, but mainly due to the high cost of good photodiodes and the existing popularity of fluorescent lamps, visible-light communication (VLC) only receives luke-warm attention to date. In this 'instructable', I shall introduce two low-cost and efficient transceiver circuit designs that can be constructed using on-the-market components. They can be applied in moderate-speed data communications such as smart phones and tablets for ad-hoc transmission of photos and files. The potentials for these designs are wide if we are able to scale down the circuits to tiny PCBs or manufacture them at IC level.

Introduction
LED is a very green technology. Since very little heat is produced, it can reduce interior temperatures by 1 to 2 degrees, thus lowering air-conditioning costs and carbon dioxide emissions. LED lighting is also much safer for the living and working environment because it is mercury free and does not produce IR or UV rays which can be harmful to human eyes and skin [1].
    White LED communication uses light in the visible spectrum as the carrier medium. The functional duality of LEDs - both as a light source and a communication medium - creates many new and interesting applications [2][3][4][5][6][7][8][9] based on the fast-switching characteristic of LEDs and the ability to modulate lightwave for free-space communications. In this kind of technology, it is possible to achieve high-speed data transmission for high data loads with low implementation complexity. Also, lightwave cannot penetrate walls, thus making it easy to secure transmissions against casual eavesdropping. Furthermore, unlike radio frequencies, the visible-light spectrum does not need licensing.
    Given the strengths of LEDs [10] - long lifetime, high tolerance to humidity, small size, and low power consumption - a white-LED communication system is therefore potentially feasible for indoor wireless networks. VLC is somewhere between Bluetooth and WLAN, but in the near future, it will replace these two technologies if the transceiver circuit can be fabricated with the LED and photodiode together on a single chip. White-LED communication is likely to be implemented in the next generation of personal computers using IBM and Intel's optical core processors. We will probably have optical I/O ports alongside our RJ45 (LAN) and USB interfaces.
    In the following sections, two types of transceiver circuit prototypes are introduced. The design principles in this work are based on cost cutting, simplicity, and the most common electronic components on the market. The intended application is serial peer-to-peer, ad-hoc communication.

Hardware Description: Prototype 1
The components of Prototype 1 (Fig. 1) consist mainly of a microcontroller, a repeater, and a USB-RS232 converter. The transmitter consists of a microcontroller PIC12F508 which is used for the modulation of the TX signal from the μUSB-MB5 (USB-RS232 converter). When the TX pin transits from logic low to logic high, the 12F508 generates a 40 kHz carrier. During low periods, the carrier is suppressed. After the modulation of the signal, the output of the microcontroller is passed on to an NPN darlington transistor to drive the white LEDs.
    The repeater receives white-light signals using the BS520 eye-response photodiode and retransmits them at a higher power. Signals received by the BS520 is usually weak and easily affected by ambient light. The repeater circuit is used to shape and boost the received signal from the photodiode before passing it to the IR transmitter. The other use of the repeater is to fine-tune the signal to the correct frequency, so that it can be readily accepted by the IR receiver. The path between the IR transmitter and the IR receiver must be enclosed to ensure that the receiver does not receive reflected signals from the transmitter. Alternatively, an optoisolator IC (e.g. 4N25) can also be used in place of the IR transmitter and receiver.
    After the IR receiver receives signals from the repeater and demodulates them, the demodulated signals are sent to the RX pin of the μUSB-MB5, which then passes the converted signals back to the computer. The μUSB-MB5 RS232-to-USB converter is one of the more expensive items in the transceiver (USD $58), but there should be less expensive serial converters out there in the market.
    The design of Prototype 1 is based on the established IR technology by creating a hybrid combination (Fig. 2) of IR and visible-light devices. The components are cheap and widely available.

References
[1] EU Commission Directorate-General for Health & Consumers, Scientific
Committee on Emerging and Newly Identified Health Risks - Light
Sensitivity, 2008.
[2] T. D. C. Little, P. Dib, K. Shah, N. Barraford, and B. Gallagher.
“Using LED Lighting for Ubiquitous Indoor Wireless Networking”. IEEE
International Conference on Wireless & Mobile Computing, Networking
& Communication, pp. 373-378, 12-14 Oct 2008.
[3] G. Pang, T. Kwan, H. Liu, and C. H. Chan. “LED Wireless - A Novel
Use of LEDs to Transmit Audio and Digital Signals”. IEEE Industry
Applications Magazine, pp. 21-28, Jan/Feb 2002.
[4] A. B. Sebitosi and P. Pillay. “White LEDs for Rural Lighting”. IEEE
Power Engineering Society General Meeting, pp. 2619-2623, 2003.
[5] Y. Tanaka, S. Haruyama, and M. Nakagawa. “Wireless Optical
Transmission with White Colored LED for Wireless Home Links”. The
11th IEEE International Symposium on Personal, Indoor and Mobile
Radio Communications (PIMRC), vol. 2, pp. 1325-1329, 2000.
[6] G. Pang and G. Poon. “A Portable Multimedia Information Device in a
Wireless Optical Data Link”. IEEE Trans. on Consumer Electronics, vol.
47, no. 1, pp. 87-95, 2001.
[7] G. Pang. “Information Technology based on Visible LEDs for Optical
Wireless Communications”. 2004 TENCON IEEE Region 10 Conference,
vol. B, pp. 395-398, 2004.
[8] S. Iwasaki, M. Wada, T. Endo, T. Fuji, and M. Tanimoto. “Basic
Experiments on Parallel Wireless Optical Communication for ITS”.
Proceedings of the 2007 IEEE Intelligent Vehicle Symposium, pp. 321-
326, 13-15 June 2007.
[9] G. Pang; T. Kwan, C. H. Chan, and H. Liu. “LED Traffic Light
as a Communication Device”. Proceedings of 1999 IEEE/IEEJ/JSAI
International Conference on Intelligent Transportation Systems, pp. 788-
793, 1999.
[10] T. Komine and M. Nakagawa. “Fundamental Analysis for Visible-Light
Communication System using LED Lights”. IEEE Trans. on Consumer
Electronics, vol. 50, no. 1, pp. 100-107, 2004

Step 1: VLC Transceiver: Prototype 1

The repeater can be built pretty easily or you may purchase it here. The metal enclosure on the repeater houses the IR photodiode. Cut away the metal and remove the IR photodiode. Replace it with the BS520 photodiode.

The codes for the microcontroller can be found here.

Comments

[osudan]

Why do you have the separate 6v Vcc for part of the circuit when the components can tolerate 5v? If the power source needs to be separate, couldn't you just add capacitors and a transistor?

[ethanpng]

Thanks for your pointers, osudan. Yup, there is always more than one way to skin a cat, but do take note that the one-shot IC consumes a lot of power. =)