LED wireless

REPORT ON

LED WIRELESS

A use of LEDs to transmit audio and digital signals

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1. INTRODUCTION & BACKGROUND

Billions of visible LEDs are produced each year, and the emergence ofhigh brightness AlGaAs and AlInGaP devices has given rise to many new markets. Thesurprising growth of activity in, relatively old, LED technology has been spurred by theintroduction of AlInGaP devices. Recently developed AlGaInN materials have led to theimprovements in the performance of bluish-green LEDs, which have luminous efficacypeaks much higher than those for incandescent lamps. This advancement has led to theproduction of large-area full-color outdoors LED displays with diverse industrialapplications.The novel idea of this article is to modulate light waves from visible LEDsfor communication purposes. This concurrent use of visible LEDs for simultaneoussignaling and communication, called iLight, leads to many new and interestingapplications and is based on the idea of fast switching of LEDs and the modulationvisible-light waves for free-space communications. The feasibility of such approach hasbeen examined and hardware has been implemented with experimental results. Theimplementation of an optical link has been carried out using an LED traffic-signal headas a transmitter. The LED traffic light (fig 1 below) can be used for either audio or datatransmission

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Fig 1.

Audio messages can be sent using the LED transmitter, and the receiverlocated at a distance around 20 m away can play back the messages with the speaker.Another prototype that resembles a circular speed-limit sign with a 2-ft diameter wasbuilt. The audio signal can be received in open air over a distance of 59.3 m or 194.5 ft.For data transmission, digital data can be sent using the same LED transmitter, and theexperiments were setup to send a speed limit or location ID information. The work reported in this article differs from the use of infrared (IR)radiation as a medium for short-range wireless communications. Currently, IR links andlocal-area networks available. IR transceivers for use as IR data links are widely availablein the markets. Some systems are comprised of IR transmitters that convey speechmessages to small receivers carried by persons with severe visual impairments. TheTalking Signs system is one such IR remote signage system developed at the Smith-Kettlewell Rehabilitation Engineering Research center. It can provide a repeating,directionally selective voice message that originates at a sign. However, there has beenvery little work on the use of visible light as a communication medium.The availability of high brightness LEDs make the visible-light mediumeven more feasible for communications. All products with visible-LED components (likean LED traffic signal head) can be turned into an information beacon. This iLight

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technology has many characteristics that are different from IR. The iLight transceiversmake use of the direct line-of-sight (LOS) property of visible light, which is ideal inapplications for providing directional guidance to persons with visual impairments. Onthe other hand, IR has the property of bouncing back and forth in a confined environment.Another advantage of iLight is that the transmitter provides easy targets for LOSreception by the receiver. This is because the LEDs, being on at all times, are alsoindicators of the location of the transmitter. A user searching for information has only tolook for lights from an iLight transmitter. Very often, the device is concurrently used forillumination, display, or visual signage. Hence, there is no need to implement an additional transmitter for information broadcasting. Compared with an IR transmitter, aniLight transmitter has to be concerned with even brightness. There should be no apparentdifference to a user on the visible light that emits from an iLight device.

It has long been realized that visible light has the potential to be modulatedand used as a communication channel with entropy. The application has to make use ofthe directional nature of the communication medium because the receiver requires a LOSto the audio system or transmitter. The locations of the audio signal broadcasting systemand the receiver are relatively stationary. Since the relative speed between the receiverand the source are much less than the speed of light, the Doppler frequency shift observedby the receiver can be safely neglected. The transmitter can broadcast with viewing angleclose to 180. The frequency of an ON period followed by an OFF period to transmitinformation is short enough to be humanly unperceivable; so that it does not affect trafficcontrol. This article aims to present an application of high-brightness visible LEDs forestablishing optical free-space links.

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2. COMPARISON TABLE

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The City of Vancouver is currently in the process of converting its 670traffic signals. The upgrade will save the City an estimated $247,500 in annual energysavings and $110,000 in annual traffic light maintenance, a budget reduction of 65%. Atotal of 6.9 GWh of electricity per year will be saved, equal to the amount of energy thatit would take to power almost 700 homes. This shows the reliability and efficiency ofLED traffic light system.

3. TRAFFIC INFORMATION SYSTEM(USING LED TRAFFIC LIGHTS)

Vehicle Information & Communication System (VICS) is starting to

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become practicable. The Infrared System of VICS detects vehicles on the road by usingoptical beacons to control traffic and to supply real time traffic information. But it is anenormous budget because the optical beacon must be located on every lane of the roadthroughout the country.Under this background, the utilization of LED traffic light to transmitinformation has been patented. This is because LED traffic light use low power, havebetter efficiency and have much longer lamp life. The proposed setup of this system isshown in fig2 in which traffic light is the transmitter and receiver is fixed in front of thevehicle.

Fig 2

4. SYSTEM DESCRIPTION

The system description of the Traffic Light Information system consists oftransmitter section and receiver section. Since the proposed system is under research, ablock diagrammatic description of both sections only, has been revealed out byresearchers.

4.1) TRANSMITTER SECTION:A block diagram representation of the schematic diagram of thetransmitter design is shown in fig 3. The audio signal from the cassette tape or CDplayer has small amplitude; hence, amplification of this audio signal is necessary. The

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audio amplifier is used to amplify the weak audio signal and shift the average voltagelevel of the audio signal to an appropriate level so that the signal is within the capturerange of a voltage-controlled oscillator (VCO).A VCO chip is used to modulate the incoming audio signal variationsfrom the audio amplifier and generate the FM signal. The VCO has 2 output pins ( asquare wave and a sine wave output). A square wave VCO is used instead of sine wavebecause there are only two states (on & off) for the LEDs. The carrier frequency is set at100 kHz with a maximum frequency deviation of or 50 kHz. The switching of LEDstransmits the modulated signal. The frequency of switching is high enough that theperceivable light appears to be constantly illuminated to the human eye.

BLOCK DIAGRAM OF TRANSMITTER

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Fig 3

4.1) RECIEVER SECTION: A block diagram schematic of the audio receiver is shown in fig 4. Thephoto-detector is used to detect a light signal from the transmitter and convert it into anelectrical signal. The limiting pre-amplifier is used to amplify the electrical signal fromthe photo-detector for the next stage. The data-reproducing circuit is used to reconstruct

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the square wave. The differentiator circuit is used to produce pulses according to thesquare wave. The pulse generator is used to convert the pulses from the differentiator intosharp pulses for use by the integrator and envelope detector in the next stage fordemodulation of the signal. The band-pass filter is used to smooth out the distortionsfrom the integrator and envelope detector to produce an appropriate waveform. Finally,the power amplifier is used to amplify the weak signal from the band-pass filter so thatthe audio signal would be comfortable for hearing.

BLOCK DIAGRAM OF AUDIO RECEIVER

Fig 4

Below is a more detailed description of the each stage

Photo-Detector Circuit: The photo detector circuit consists of a photo diode and a resistor. One endof the photo diode is coupled to the current limiting resistor with the other end coupled tothe ground. Since the signal from the photo-detector circuit is very small, amplification isneeded for the next stage. The limiting pre-amplifier circuit consists of two op-amplifiersas well as some resistors and diodes. The diodes are used to limit the input voltage levelto a desired level (such as between 0.7 and 0.7 V). This circuit aims to amplify the inputvoltage to a certain level, and a comparator is used to produce rectangular signal pulses.Two pre-amplifiers are used in this circuit because using one pre-amplifier will require a

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very high gain amplifier. Hence, two pre-amplifiers, each with lower gain, are used toachieve high gain with reduced noise.

Data -Reproducing Circuit: Next, a data reproducing circuit, which consists of an operationalamplifier, a resistor, and two NAND Schmitt triggers, is used. Its function is to producerectangular pulses from the amplified signal in the previous stage. An operationalamplifier is used as a comparator, which uses a virtual ground as a reference. The NANDSchmitt trigger gates are used to enhance the noise immunity and to correct the edgesfrom low to high voltage levels due to slew rate of the amplifier. Two NAND Schmitttrigger gates are used instead of one so that the signal will not be inverted. Then, adifferentiator circuit consisting of a capacitor and a resistor is used to detect the leadingedges of the pulse with the trailing edge blocked by the diode. Next, there is a circuit of apulse generator. A Schmitt trigger gate is used as a pulse generator, and the output givesthe inverted version of pulses from the differentiator.

Integrator & Envelope Detector: An integrator and envelope detector can be found next. The integrator isan envelope detector, and double integration is carried out. If the inverted pulses from thepulse generator contain high frequency, the frequency of integration is higher and thevoltage level of output would be higher. However, if the inverted pulses contain lowfrequency, the frequency of integration is lower and the voltage level of output would belower. In this way, the modulated signal would be reconstructed.

Band-Pass Filter: Next, a band-pass filter is used. The output signal from the previous stage,integrator and envelope detector has many distortions. A band-pass filter is used to filterout all the high-frequency distortions. The higher cut-off frequency depends on acapacitor and a resistor. A lower cut-off is also used to filter out the low-frequency noise,such as the 50-Hz power line frequency. The output signal from the band-pass filter is an

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audio signal.

Power Amplifier: The final stage of the receiver circuit is a power amplifier, the output of which isconnected to the speaker. The objective is for the delivery of the audible messagesthrough a speaker or headphone/ear jack.

5. IMPLEMENTATION

An LED traffic-signal head made up of 441 high-brightness LEDs hasbeen implemented in the Industrial Automation Research Laboratory at The University ofHong Kong. Each LED is a Hewlett Packard high intensity AlInGaP type with aluminous intensity of 2000 mcd at 20-mA rated driving current, and the viewing angle is30. The specifications of the LED traffic signal head are given in table below.

LED TRAFFIC SIGNAL HEAD SPECIFICATIONS

The radiation pattern of the LED traffic light is given in fig 5. An HPAudio Analyzer, which has low-distortion signal source with a signal analyzer, is thenused for audio measurement of the visible-light LED audio broadcasting panel. The

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frequency response of the communication channel occupied by the audio signal wasdetermined. Here, the frequency of the audio signal transmitted via the LEDs was varied,and the response was observed using the HP audio analyzer. The frequency response isnot as flat as may be expected from the enormous bandwidth of visible light. This is duethe limitations inherently governed by the VCO in the transmitter and the discriminatorused at the receiver.

Fig 5

6. ADVANTAGES & DISADVANTAGES

As a medium for wireless short-range communication, visible light hasboth advantages and disadvantages when compared with IR, microwave, and radiomedia.

6.1) ADVANTAGES:

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. This system has many advantages1) On one hand, LEDs and photo detectors capable of high-speedoperation are available at low cost.2) Like the IR, the visible spectral region is unregulated worldwide andFCC licenses are not necessary, as the commission does not regulate the visible lightfrequencies.3) Both IR and visible light penetrate through glasses, but not throughwalls. For transmission to be possible there must be no obstructions standing in the wayof the visible-LED light beam as it requires a clear LOS between the sending side (LED)and the receiving side, whereas IR also allows a nondirected and non-LOS link design.4) Like microwaves, visible-LED light beams follow a straight-line pathand are well suited for the wireless delivery of large quantities of voice and datainformation. In practical use, one should take the advantage of this highly directionalfeature of LEDs.5) High directional features of LEDs.

6.1) DISADVANTAGES:

On the other hand, LEDs also have many drawbacks.1) They are suitable for short range only, as the photo-detector current isproportional to the received power.2) Intensity modulation with direct detection seems the only practicaltransmission method.3) It should also be mentioned that the relationship between the radiantintensity and the distance from the receiver follows the inverse square law. Hence, as acommunication medium, it has limited range.4) It is subjected to noise arising from sunlight, incandescent lighting,and fluorescent lighting. It is not suitable for broadcasting signals over a wide coveragearea or over long ranges.

7. APPLICATIONS

1. IN TRAFFIC LIGHT:

High-brightness LEDs are increasingly being used in traffic lights due totheir low power consumption and minimal required maintenance, which can be translated

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into considerable cost savings each year. For example, Philadelphia, Pennsylvania, USA,is replacing all of its 28,000 red signals with LEDs, with an estimated annual cost savingsof $1.2 million. The next stage of development will involve the three-color LED signals.In Singapore, there has been a complete change of traffic signals from the incandescent toLED. The $12.7 million project has replaced all 60,000 incandescent lamps in 1,600intersections of the city. Again, power and maintenance savings, as well as safety, arecited as the reasons for the replacement. An LED traffic signal can use only 18 ultrabrightLEDs and is warranted for five years. LED power consumption is only 8-12 W,compared with around 150 W used by its incandescent counterpart.

2. IN TRAFFIC INFORMATION SYSTEM:

With ideas and developments described in this article, an LED traffic lightcan be used as an audio broadcasting device, in addition to their normal function of beingan indication and signaling device. A receiver some distance away pointing at the trafficlight can receive voice messages. For drivers, the message can announce the time untilthe next signal change. For pedestrians or people with visual impairments, the voicemessage can tell the location or directional information.

3. IN MUSEUM:

Other applications can be found in a museum or exhibit-hall environment.The information on an individual exhibit can be broadcast via a plurality of LEDs, whichis also used for the purpose of illumination. With the guest pointing the receiver to therelevant LEDs on a transmitter, with the head phone or ear jack attached to a portablereceiver, he can listen to the audio message about the specific exhibit item he isinterested. Thus, the indoor environment can remain quiet while the guests stroll in themuseum. This is the major advantage over conventional broadcasting systems in thatindividuals with receivers have the freedom of choice to receive specific messages

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without hearing any unwanted announcement, music, or commercials.

8. CONCLUSION

High brightness LEDs are getting more popular and are opening up anumber of new applications, especially with the improved efficiency and new colors. Inthis article, the novel idea is based on the fast switching of LEDs and the modulation ofvisible light is developed into a new kind of information system. A visible-LED audiosystem that makes use of visual-light rays to transmit audio messages to remotely locatedreceiver is described. Such a system made up of high-brightness visible LEDs canprovide the function of open space, wireless broadcasting of audio signals. It can be usedas an information beacon for short-distance radio communication.Any illumination device making using ofhigh brightness visible LEDs can be used as a kind of short-range information beacon..One example is an LED traffic light for the support of roadside-to-vehiclecommunications. There are many potential novel uses of visible light from LEDs as acommunication medium. This concurrent use of LEDs for simultaneous signaling andcommunications will open up many new applications.

9. REFRENCES:

1. Wireless Networking Survival Guide  - by TechRepublic 2. Marketing Led Sales Driven: How Successful ...  - by Ajay K Sirsi3. Handbook of Algorithms for Wireless Networking ...  - by Azzedine Boukerche4. http://books.google.co.in 5. http://www.sciencedaily.com

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