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IDescriptionThisblogintroducesandanalyzes4simpleandeasy74LS00NandGatecircuitdiagrams.Itsincluding:SquareWaveGeneratorCircuit,PulseGeneratorCircuit,LEDLightCircuit.Andintheend,wewillanalyzethecircuitthatturnsthetimerintoacountdowntimerindetail.ThisVideoisAnIntroductionof7400LogicDevicesCatalogIDescriptionIISquareWaveGeneratorCircuitIIIPulseGeneratorCircuitIVLEDLightCircuitVTurnTimerintoCountdownTimer5.1SchemeDesign5.2ImplementationofSchemeDesignOrdering&QuantityIISquareWaveGeneratorCircuitLetstakealookatthefigurebelow.Itsasquarewavegeneratorcircuit.Thiscircuitcontainsa74LS00NandGateintegratedcircuit.Figure1.SquareWaveGeneratorCircuitDiagramAmongthiscircuitdiagram:NANDgates1,2andexternalRCtimeconstantcomponentsformanoscillatorcircuitNANDgate3isabufferoutputstage.AslongasthecapacityofCischanged,squarewaveoutputsofdifferentfrequenciescanbeobtained.IIIPulseGeneratorCircuitFigure2.PulseSignalGeneratorCircuitDiagramThecircuitdiagramisshowninFigure2anditsasimplepulsesignalgeneratorcircuit.ThesignalgeneratormainlyusestwoTTLintegratedcircuits(74LS00and74LS221).Sowhychoosethesetwocircuits?Thatisbecause,thesetwocircuitscanbeusedtogenerateapulsesignalof=4s.Besides,itusesfewercomponentsandisconvenientfordebuggingandmaintenance.IVLEDLightCircuitThiscircuitismadewithNE555,74LS00,74LS154,74LS193andLEDlights,andtheproductionprocessisverysimple.Whenweturnonthepower,hereishowitworksis:WhentheoutputQ0ofthe74LS154decoderislow,the74LS193isapositivecounter.Atthistime,theLEDsareindividuallylitfromD1...D16;WhentheoutputQ15ofthe74LS154decoderislow,the74LS193isacountdowncounter.Atthistime,theLEDsareindividuallylitfromD16...D1.Figure3.LEDLightCircuitDiagramFromtheabovewecanseethat:theLEDlightsturnonfromD1toD16,andthenbacktoD1fromD16,andsoon.VTurnTimerintoCountdownTimerGenerally,therearetwodesignideasforturningatimerintoacountdowntimer:First,changethecountingchipinthetimer;Second,resetthefunctionofthechip.Besides,thereisactuallyanotherwaytoachievethisgoal:Byappliyingthe74LS00and74LS20chipstoreversetheresultsonthedisplay,soastoachievethepurposeofcountingdown.5.1SchemeDesignTheresultdisplayedbyeachdigitofthetimerisanincrementalvalue,suchas0.1.2.3.4.5.6.7.8.9.Yet,thecountdowntimerdisplaysadecreasingvalue,suchas9.8.7.6.5.4.3.2.1.0.Aslongasthedisplayresultconversioniscompletedwithasuitablelogiccircuit,thetimercanbeturnedintoacountdowntimer.Atfirst,weneedtofindthelogicalrelationshipbetweenthetimerdisplayresultandthecountdowntimerdisplayresult.Table1belowliststheBCDcodescorrespondingtoeachdisplayresultofthetimerandcountdowntimer.Fromthistable,youcaneasilyfindtheBCDcodesofthetimerandcountdowntimer:ThelowestbitQ1andY1areopposite;WhileQ2andY2arethesame;RelationshipbetweenQ3andY3:Y3ofthecountdowntimeristheexclusiveORoftimerQ3andQ2;RelationshipbetweenQ4andY4:TheY4bitofthecountdowntimeristheoppositevalueoftheORofQ4,Q3,andQ2ofthetimer,whichisalsoequaltothenon-re-ANDofQ4,Q3,andQ2.Table1.CorrespondingBCDCodeDisplayedby(Down)TimerTheabovelogicalexpressionis:Therefore,aslongasyouchooseacircuitthatcancompletetheabovelogicconversionrelationship,youcanrealizethedesignfromatimertoacountdowntimer.Thefigure?showsatwo-digittimercircuit.Afteraddingtheaboveconversioncircuit,itbecomesthefigure3showsthecountdowncircuit.Figure4.TimerCircuitDisplaying2DigitsFigure5.CountdownCircuitDiagram5.2ImplementationofSchemeDesignTwokindsofchips74LS00and74LS20areuesdhere.Theformerarefourtwo-inputNANDgates,whichareusedtocompletetheconversionofY1andgeneratethenegationofQ4,Q3,andQ2.Thelatteraretwofour-inputNANDgates,whichareusedtoobtainY4fromthenon-reANDofQ4,Q3,andQ2.Insummary,wecanfollowthelogicalrelationshipasfollows:ThelogicdiagramisshowninFigure6.Figure6.LogicDiagramKnowingthattheXORgateoperationcanbecompleted,theY3conversioncanbecompleted.Theconnectioncircuitdiagramoftheabove-mentionedY4,Y3,Y1conversionspecificphysicalobjectsisshowninFigure7.Figure7.Y4,Y3,Y1ConversionSpecificPhysicalConnectionDiagramFigure8showstheactualpictureofthecountdowntimer.Figure8.CountdownTimerSofar,thetimerhasbecomeacountdowntimer.Throughthisdesignmethod,thereisnoneedtochangetheoriginalcountercircuit,isitparticularlytrouble-free?

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