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Laser Audio Transmitter

Jehhal Liu (jl589), Daniel Zuo

(dyz2)Shared by:

http://hello-engineers.blogspot.com/

Introduction

Project DetailsDesign Details

Results

Conclusions

Appendices

Introduction

Thisprojectisaproof-of-conceptdevicethattransmitsanaudiosignalusinga

laserbeam,whileremovingtheneedfortheusertoalignthebeamthemselves.

Forourproject,wecreatedamono-axialtransmitter/receiversetupthatconvert ananalogaudiosignal,viaastandard3.5mmjack,andtransmitsitviaalasertoareceiver,whichconvertsthesignalbackintoaudio.Thereceivercanberotatedwithinitshorizontalplane,andusesaservomotorcontrolledbyamicrocontrollertoautomaticallyalignitwiththereceiver.Theendresultisawirelessaudiosignalthatcannotbeoverheardbyotherdevices.

ProjectDetails

Rationale

Lightisalreadybecomingapopularmeansofcommunication,thankstofiberoptics,whichcanguideopticaldatamuchlikeawiretransmitscurrent.Itmightseemimpractical,then,touselaserswithoutaguidingmediumtotransmitinformation.However,incontextswhereaphysicalconnectionisimpossibleor

unfeasible,andtheneedforafocusedbeamarises,itwouldseemlogicaltouselaserlight.Inparticular,freespacelasercommunicationhasusefulapplicationst

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militarylogistics,whereinformationonthefrontmustbekeptlimitedtofriendlyears,andground-to-airlinksareimportant.Wedecidedtocreateasimpleandinexpensiveproof-of-concepttodemonstratetheadvantagesofthisseeminglyimpracticalscheme.

InspirationforthisprojectcamefromadesigninthebookGonzoGizmos:Projects&DevicestoChannelYourInnerGeek,abookwrittenbySimonQuellenField,inwhichalaser/solarcellcombinationallowedforthetransmissionofanaudiosignalusinglight.AtopicofresearchatMITLincolnLaboratory(http://www.ll.mit.edu/news/freespace-lasercom.html )inspiredustoincorporatetrackingcapabilitywhilekeepingtheprojectbudgetaffordableatahobbyistlevel.

LogicalStructure

Ourprojectisdividedintotwodistinctsections:audiotransmissionviahardware andalignmentcontrolviasoftware.Theformercanbeaccomplishedoncethelatterhassucceeded,makingthetwotasksmutuallydependentforoverallfunctionality.Thealignmentisaccomplishedusingaservomotortorotatethetransmitteruntilitalignswiththereceiver.Thisisdonebydetectingthelightemittedfromthereceiverslaserusingaphototransistor.Thetransmitterscans

overitsfreerangeuntilitfindsthereceiver,atwhichpointthetransmittinglaseristurnedonandtheaudiosignalispickedupbyanarrayofphotodiodes.Thedetailsofthisprocessareexploredbelow.

SafetyandStandards

Sinceweareusinglasers,theeffectsofexposureneedtobeconsidered.Thelasersweusedwerepurchasedcommercially,andcomplywithclassIIIApowerandsafetyspecifications,asdesignatedbytheANSIZ136.1consensus.IntheUS,ANSIandOHSAstandardsspecifytheparametersunderwhichlaserscanbesafelyoperated.ForaclassIIIAlaser,thebeamsaregenerallynothazardouswithouttheuseoffocusingequipment,thoughdirectexposuretotheeyeshouldbeavoided.Withthisinmind,weoperatedthedeviceinaplanethatlackedreflectiveobjectsandwaswellbeloweyelevel.Additionally,thetransmittinglaseristurnedoffwhentheexactdirectionofthelaserisnotknown(i.e.whenth transmitterisscanningthearea).Thereceiverslaserislimitedtobejustbrightenoughtobedetectedbythetransmitter,butnotatthepeakamplitude.

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Hardware/SoftwareTradeoffs

Theaudiosignalsentbythetransmitterisprovidedexternallyratherthanviathemicrocontroller,sinceexternaldevicesrepresentmoreflexibilityandmobilityintheinformationthatcanbesent.HavingtheaudiosynthesizedbytheMega644wouldrequiresomeformofexternalmemoryandamethodofflashingfromanexternalsourceanyways,sowedecidedtocutoutthemiddleman.

Theaudiosignalissentviaananalogtransmissioninsteadofdecodingitintoadigitalsignal.Opticaltransmissionisusuallydoneusingwithdigitalsignals,requiringhardwarefordecodingatthereceiverend.Bytransmittingtheanalogaudiosignalthroughlaseramplitudemodulation,wecutdownonthetransmitterandreceiverhardwarebyeliminatingtheneedforencodinganddecoding

processors.

DesignDetails

Hardware

Transmitter

Figure1:Transmittercircuit,readyforoperation.

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Thetransmitterisbrokendownintothreecircuits.Thefirstcircuit,whichissimilartothereceiverend,isthelaseraudiotransmissioncircuit.Thistransmitterworksbymodulatingtheamplitudeofthelaserbasedontheamplitudeoftheaudiosignalthatwearetryingtotransmit.

Theaudio,whichisfedtothecircuitthroughastandard3.5mmaudiojack,issenttoanaddercircuit.Theadder,shownbelow,consistsoftwoinputsandanegativ feedbackresistorthatwillbesummedtogether.Thetwoconnectionstothenon-invertinginputoftheop-amparetheaudiosignalandaDCbiassignalthatisgenerallykeptataround3V.Thisvoltagecanbemodifiedbyadjustingapotentiometer.

Thesignaltotheinvertinginputoftheop-ampishalfthatoftheoutput,whichisobtainedbyusingavoltagedivideroftwoequalresistors.Thisfeedbackallowstheop-amptofollowtheamplitudeofthenon-invertinginputasitattemptstoequatethevoltagesatthetwoinputs.

Theoutputoftheop-ampissentdirectlytothelaser.Theamplitudeofthelaseri proportionaltothevoltagethatisappliedtoit.Theamplitudelinearlyfollowsthevoltageuptoroughly4V,atwhichpointtheresponseismoreexponential.Wear takingadvantageofthelinearregionoftheresponsetosendamostlyunmodified

audiosignaltothereceiverforplayback.

Thelaserneedstobeturnedoffwhilethetransmitterisscanningforthereceiver mainlyforsafetyandpowerreasons.Toachievethis,the9Vsupplytotheop-am iscontrolledbyanNPNtransistor,whichcanactasaswitchcontrolledbythemicrocontroller.

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Figure2:Maintransmittercircuit.Modulateslaseramplitudebasedonaudioinput.

Thesecondcircuitinthetransmitterisbasedonaphototransistorthatresponds

toredlight.WhenVCEofthetransistorissetto5volts,thecurrentflowingthroughthetransistorisproportionaltotheamountofredlightthatisincidentuponthedetector.Thecurrentflowsthroughthe10Kresistor,resultinginameasurablevoltagethatisproportionaltotheincidentlight.Thisvoltageisfedtotheanalog-to-digitalconverterofthemicrocontroller,anddetermineswhetherthereceiverandtransmitterareproperlyalignedandreadyfortransmission.

Figure3:Transmitterphotodetectorusedforalignmentpurposes.

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Thelastpieceofthetransmitteristhemotorcontrollingcircuit.Weareusingaservomotortocontrolthedirectionofthetransmission.Themotorisopto-isolatedfromtherestofcircuit,andparticularly,themicrocontroller.Thismeansthatthemotorispoweredbyadifferentpowersupply,andhasnodirectconnectionswithanyterminalofthemicrocontroller.

Theservomotoriscontrolledbyapulse,whoselengthcorrespondswiththedesiredpositionofthemotor.Servoscanachieveabsolutepositioning,whileothertypesofmotorscanonlybepositionedrelativetothecurrentstate.The

preciselytimedpulseissentthrougha330resistortotheinputoftheisolator.Theoutputoftheisolatorissimplythesamepulsewitha5voltamplitude,whichisthensenttothemotorforpositioning.

Figure4:Motoropto-isolatorcircuit.Pulseoutputgoestomotorforanglecontrol

Receiver

Thereceivercircuitsomewhatresemblesthetransmittercircuit.Ratherthanasinglephototransistor,however,itinsteadusesthreephotodiodes,whichhavemuchlargersensitiveareascomparedtothetransistor.Sincetheresponseofthediodesdirectlyaffectstheaudioquality,amorecomplexcircuitiscalledfor.Thediodesthemselvesareplacedbetweenthetwoterminalsofanop-amp,whoseoutputvoltageisdeterminedbythecurrentthatflowsthroughthediodes.Usinganop-ampinsteadofbiasingthediodesallowsustoutilizeanear-idealshort-circuitcurrent.Withthreediodesinparallel,weeffectivelytripletheareaupon

whichwecanreceiveasignal.Afteramplifyingthesignalwithasecondop-amp,

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theresultisthenfeddirectlytoanaudiojack,wherethesignalcanbeheardusin anycompatibledevice.

Figure5:Receivermaincircuit.Readsdetectorvoltageandamplifiesbeforesendingtoaudiojack.

Figure6(left):Receiverlasercircuit.Usedforalignmentpurposes.Figure7(right):Receivercircuit,readytooperate.

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Software

Thesoftwareisdesignedtobecompletelyinterrupt-driven.TimingisbasedaroundtheabilitytocontrolanoutputpinwithaprecisePWMoutput,allowingustoaccuratelycontrolthemotorforscanningpurposes.Theinterruptserviceroutineiscalledonceeveryfivemicroseconds.Uponeachinterrupt,theprogramupdatesitstimingcounters,readstheoutputoftheanalog-to-digitalconverterwhennecessary,andperformsstateupdatesforreceivertrackingpurposes.

TheStateMachine

Figure8:Softwarestatemachine,usedfortransmitterscanning.

WeuseasimpleMooremachinetocontroltheactionsofthetransmitter.Thediagramaboveoutlinesitsbehavior.

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HardwareControl

Throughourtimingcontrolourhardwareisrelativelysimpletomanipulate.Theservomotorweusedusesthedurationofapulsetodetermineitsangularposition.Tocontrolthisposition,wesetapinhigh,andturnitoffwhenavariableamountoftimehaselapsed.Sincetheexecutionofotherinstructionscanhaveanoticeableeffectonthetruepulsewidth,thistogglingishandledpurelybythetimercompareinterrupt.

Enablingordisablingthelaserisassimpleastogglingaseparatepin.

AttemptedDesigns

Weattemptedtoimplementahandfulofdesignsthatdidn tworkoutintheend.

Hardwareinparticularwentthroughalot.Initiallywethought(naively)thataservomotordidnotneedtobeopto-isolated.Ourmotordisagreed,andvibratederratically.

Thereceiverwentthroughmultiplephotoreceptivedevices.Weweregoingtous asolarcell,butsawthattheresponseofaphotodiodewasgoodenough,soweusedoneofthoseinstead.Whenwesawthatthesensitiveareaofthephototransistormadealignmentusingtwoofthemnexttoimpossible,we

switchedtoanarrayoflargerphotodiodes.

Thepowersupplyforthemotorwasoriginallyanother9Vbattery,butwequickl foundthatthecurrentitsuppliedwasinsufficient.

Lastly,topoursaltonourwounds,ourcustomMega644proto-boardmysteriouslydiedonceeverythingelsehadbeencompleted,takingourmicrocontrollerwithit(orviceversa?).WedecidedtoswitchtoanSTK500asourcontrolsource,lackingthetimeorresourcestomakeanotherproto-board.

Thetransmitterslasercontrolalsowentthroughseveraliterationsbeforewelandedonthefinaldesign.Specifically,thecircuitdesignedtothelaseronoroffduringthescanstagewasmodified.Originally,theDCbiasinputtotheadderwouldbecontrolledbyapotentiometer,whichcouldbeadjustedtozerowhenduringscanning.Howeverthismethodwasnotautomatic.Wethentriedtocontroltheop-ampsupplyvoltagewithanoutputpinofthemicrocontroller,butfoundthatsupplying5voltscausedcircuittobeunabletooutputgreaterthan3

voltsattheoutput,whereasweneededanaudiosignalcenteredatroughly3.5volts.WealsooriginallyhadaBJTthatcontrolledabattery s9voltconnectionto

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theop-ampsupply,butfoundthatifthetransistor sbasevoltagewaskeptat5volts,thenthesupplycouldnotexceed5volts.Wefinallyimplementedthedesig describedaboveandkeptitasourfinaldesign.

Results

HardwarePerformance

Audio

Whendealingwithaudiotransmission,animportantconsiderationishowwellarangeoffrequenciesispreservedintheoutput.Generally,theaudiblefrequencie rangefrom20to20,000Hz.Tothatend,werantestscomparinginputwaveform

withthesignalsgeneratedbythereceiverforvariousfrequencieswithinthisrange.Anexampleoscilloscopetraceisshown(barely)below.

Figure9:Oscilloscopetraceofinputvs.output(smallersignal).Ataudiblefrequencythecorrelationishigh.

Throughourtestswefoundthattheoutputfrequencyremainsthesameastheinput,andthattheshapeofthesignal(asinusoidinourtests)waspreservedwithintheaudiblerange.Ataround50kHztheresponsetimeofthephotodiodeisnolongernegligibleandsomedistortioncanbeseen,asshownbelow.

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Figure10:Asthefrequencyclimbsbeyondtheaudiblerange,thecircuithastroubleresponding.

Therelativeattenuationofdifferentfrequencieswasalsomeasured.Thegraphbelowshowshowtherelativeamplitudedependsonfrequency.

Figure11:Receiversfrequencyresponse.

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Whilehighfrequencysignalsaretransmitteduniformly,theresponsedropsoffatthelowend.Thesystemactsonawholeasahigh-passfilterwithacriticalfrequencynear100Hz.Therefore,audiowithheavybasscomponentswillsufferthemostwhileothercomponentswillbelargelyunaffected.

MotorandAlignment

Thesmallservomotorperformsadmirablyundertheloadofthetransmittercircuit,operatingsmoothlyandconsistently.However,thephysicalconnectionstothepowersupplyforthemotorandthestk500causesthebasetoslideonallbuttheroughestofsurfaceswithoutadhesives.Onceanchored,thisisanon-issue

Thetransmitterandreceiveralignmoderatelywell.Caremustbetakeninplacingthereceiver,asitslasermustpointtowardsthecenterofrotationofthetransmitter.Evenwiththissatisfied,thereremainsadegree(nopunintended)offlexibilityintheangleofthetransmitterwhileintheLOCKstate,whichmeansthatthetransmitterbeamcanmissthephotodiodesofthereceiverasthedistancebetweenthetwoincreases.Whenweswitchedtothearrayofthreephotodiodes,thefrequencyofthismisalignmentoccurringdecreasednoticeably.Partialmissescanalsooccurwhenthebeamisbetweentwodiodesorslightlyoff

oneofthem.Theresultisthatdifferentvolumesarerequiredinthesourcetopreventdistortion.Theimpactonsafetyisnegligible,however,giventhatthenormalprecautionsassociatedwithlasersarekeptinmind.

Interference

Wefoundthattheonlyinterferencecamefromaudiocontaminationduetonearbyelectronics.Thelevelsofnoisedidnotseemgreaterthanwhatyoumighthearincheapaudioproducts,however.

SoftwarePerformance

Onitsown,thesoftwarecontrollingthetrackinglogicworkswithoutanyflaws.Thetimingofthemotorsignalexecutesconsistently,asevidencedbytheoperationofthemotoritself.Thedeviceiscapableoftrackingamovingbeamfairlywell,thoughitcaneasilyloseabeamthatdisplacesconsistentlyfasterthanapproximately5degreespersecond.Ifitguessesincorrectlywhichdirectionthebeamwent,thebeamcanbelostmoreeasily.Acquiringalockworksconsistentlyhowever,andwedidnotwitnessanyoccurrencesoffalsepositivesinourtesting.

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Usability

Ingeneral,thedevicecanbesuccessfullyoperatedbyanyonewithtwohands,functioningeyesight,andasurplusofpatience.Theamountofconstantvoltagesuppliedtobothlaserscanbemanuallyadjustedviatrimpotstocompensateforvariousvisibilityconditions,andtheaudiosignalscanbesuppliedandreceivedwithanythingusingastandard3.5mmstereojack(thoughtheoutputismono).

Conclusions

Expectations,Retrospect,andPossibleImprovements

Ourfinalproductnearlymetourinitialexpectations.Wedidn texpecttheaudioqualitytobeasgoodasitwasgiventherelativeeaseofdesigningthecircuits,buttheunreliablenatureofthealignmentwassomethingwehadn tanticipated,anddidnthaveasolutionreadilyavailablefor.Thesoftwaretooklessgriefandmiserythanwethought,andgenerallyworkedwithoutmuchcajoling.

Withthesepointsinmind,thereareafewthingswewouldhavedonedifferently.ThesoftwarecouldbeexpandedtoallowforthetransmissionofadigitalI/Ostream,usingtheRS232capabilitiesoftheMega644andmoresymmetryinthetransmitter/receivermodules.Thiswouldallowustotapmoreofthepotentialof

themicrocontrollersandbroadentheusefulnessofthedevice.

Asforhardware,wefoundthatconsideringeverydeviceasiftheoperatingconditionswereidealcostustimeandmoneythatcouldhaveotherwisebeenmoreproductive.Notconsideringtheareaofthephototransistors,forexample,wasanavoidableoversight.Additionally,withmoretimeandanexpandedbudge wecoulddealwiththealignmentissueandbuildonit:withmoreinvestmentinopticalhardwarethelaserscouldbesecuredwithmoreprecision,andtheir

beamscouldbebroadenedandre-collimated.Additionally,withamorecostlygimbalsetupanotherdimensioncouldbeaddedtotherangeofthedevice,andwithinfraredimagingwecouldmakethealignmentprocessmoreaccurate.

Thelast-minutefailureoftheproto-boardcameasaheavyblowtouslateinthegame.BeingmorepreparedfortheworstmighthavesavedussomegriefifwedsampledmorethanoneMega644andmademorethanoneproto-board.

Standards

Ourlasersarealwaysoperatedatpowerlevelsequaltoorlessthanthatoftheiroriginaldesign.Toreducethechancesofstraybeams,thetransmitterisdisabled

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untilitacquiresalock.Additionally,thelasersthemselvesscatterafairamounto redlightfromtheircavities,makingtheiroperationobvious.Giventhesefactors,andthelowpowerofthelasersthemselves,webelievewehavefollowedthelase safetystandards.

IntellectualPropertyConsiderations

Allofourcodeisoriginal,andthedesignsforourcircuitswerenottakenfromanoutsidesource.Sincetheideaforlasersignaltransmissionisalreadyalargepartofmanyapplications,weareconfidentthatourdevicedoesnotinfringeupontheintellectualpropertyrightsofotherparties.Bearingthatinmind,wedonotfeelourdevicemeritsapatentclaim.Theresultsoftheproject,whilesomewhatnovel,arenotexactlyunheardofanddonotmeritpublication,saveasaDIY.

Ethics

ReferringtotheIEEECodeofEthics,webelievewehaveactedresponsibleaselectricalengineerstotheextentthatourprojectallows.Byfollowingstrictprecautionsinusingourlasers,wehaveavoidedsafetyriskstoourselvesandothers.Sinceourworkwasindependentofoutsidepartiesandpurelyintheinterestoffunandlearning,weavoidedconflictsofinterestandbribery,aswellasslanderorintellectualdishonesty.Lastly,wehopethatourdeviceisableto

demonstratetoothersthecapabilitiesandlimitationsofopticalcommunication.LegalConsiderations

Sinceourdeviceisalowpowersetupthatproducesminimalinterferingnoise,therearenolegalissuesathand.

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Appendices

SourceCode

SourceCode

SchematicsTransmitter:MainCircuit

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TransmitterAlignmentUnit

MotorOpto-isolator

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Receiver:MainCircuit

ReceiverAlignmentUnit

ProjectBudgeting

Source PartNo. Description UnitPrice

Quantity

ECE4760Lab -- STK500 $15.00 1

ECE4760Lab -- 9VBattery $2.00 2

ECE4760Lab -- SolderBoard $1.00 2

ECE4760Lab -- DIPSocket $0.50 3

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ECE4760Lab -- 1-pinJumperCable

$0.50 5

ECE4760Lab -- 3.5mmStereoJack -- 2

AdamShapiro -- Adjustableclamp -- 1dealextreme.com 22334 RedLaserPointer $1.91 2

jameco.com hs-244 ServoMotor $19.95 1

digikey.com BPW46 Photodiode $1.01 3

digikey.com LM358ANFS-ND

Op-amp $0.39 2

digikey.com 160-1304-5-ND

Opto-isolator $0.32 1

digikey.com 2N3904FS-ND

NPNTransistor $0.11 1

digikey.com 3386F-103-ND

10kPotentiometer $1.14 2

digikey.com AE9919-ND 3.5mmAudioCable

$5.10 1

allelectronics.com LT9593-91-0125

IRPhototransistor $0.65 1

Total $61.04

DivisionofLabor

Task TeamMember

ProtoboardAssembly Dan

PhotodiodeTesting Dan&Jehhal

SolarCellTesting Jehhal

TransmitterCircuit JehhalReceiverCircuit Dan

TransmitterBase Dan

HardwareDebugging Dan&Jehhal

SoftwareDesign Dan&Jehhal

SoftwareDebugging Dan&Jehhal

WeeklyUpdates Dan&Jehhal

FinalReport Dan&Jehhal

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References

Mega644DataSheet(http://instruct1.cit.cornell.edu/courses/ee476/AtmelStuff/mega644full.pdf)

LM358Op-ampDataSheet(http://instruct1.cit.cornell.edu/courses/ee476/labs/s2009/LM358.pdf)

LT9593-91-0125Phototransistor(www.ledtechusa.com/data/datasheets/LT7-9X/LT959X-91-0125.pdf)

BPW46Photodiode(http://www.vishay.com/docs/81524/bpw46.pdf)

HS-422ServoMotor(http://www.imagesco.com/servo/motors/HS-422.html )

Inspirationforthisdevicecameinpartfromthebook GonzoGizmos:Projects&DevicestoChannelYourInnerGeek,bySimonQuellenField.Itsanexcellentintroductionintovarioususefulscientificphenomena.

(http://www.amazon.com/Gonzo-Gizmos-Projects-Devices-Channel/dp/1556525206)

MITLincolnLaboratoryisworkingonasimilar,butmuchmorecomplex,conceptforlong-rangelasercommunication.(http://www.ll.mit.edu/news/freespace-lasercom.html )

ThanksgotoAdamShapiro,whoprovidedsomehonestfeedbacktoourinitialideaandprovidedausefulstandforourreceiver.

Additionalthankstothefollowingpartsvendors(youcansamplemostofthecomponentswepurchasedforfreeifyouarepatient):

(http://www.digikey.com/)

(http://www.jameco.com/)

(http://www.dealextreme.com/)

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