Chapter 14Chapter 14

Multivibrators and the 555 Multivibrators and the 555 TimerTimer

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ObjectivesObjectives

You should be able to:You should be able to: Calculate capacitor charging and Calculate capacitor charging and

discharging rates in series RC timing discharging rates in series RC timing circuits.circuits.

Sketch the waveforms and calculator Sketch the waveforms and calculator voltage and time values for astable and voltage and time values for astable and monostable multivibrators.monostable multivibrators.

Connect IC monostable multivibrators Connect IC monostable multivibrators to output a waveform with a specific to output a waveform with a specific pulse width.pulse width.

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ObjectivesObjectives

(Continued)(Continued) Explain the operation of the internal Explain the operation of the internal

components of the 555 IC timer.components of the 555 IC timer. Connect a 555 IC timer as an astable Connect a 555 IC timer as an astable

multivibrator and as a monostable multivibrator and as a monostable multivibrator.multivibrator.

Discuss the operation and application of Discuss the operation and application of crystal oscillator circuits.crystal oscillator circuits.

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MultivibratorsMultivibrators

Changes between two digital levelsChanges between two digital levels Continuous, free-runningContinuous, free-running On demandOn demand

Three typesThree types Bistable (S-R flip-flop)Bistable (S-R flip-flop) AstableAstable Monostable (one shot)Monostable (one shot)

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Capacitor Charge and Capacitor Charge and Discharge Discharge

RC circuit charge and discharge RC circuit charge and discharge curvescurves

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Capacitor Charge and Capacitor Charge and Discharge Discharge

Exponential charge and dischargeExponential charge and discharge Time constant (Time constant ())

RCteEv 1

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Capacitor Charge and Capacitor Charge and DischargeDischarge

Transposing the curve equation for Transposing the curve equation for tt

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Astable MultivibratorsAstable Multivibrators Single Schmitt Inverter and an RC Single Schmitt Inverter and an RC

circuitcircuit

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Discussion PointDiscussion Point

Describe the operation of an astable Describe the operation of an astable multivibrator similar to that shown multivibrator similar to that shown in Figure 14-5in Figure 14-5

How is the frequency of the How is the frequency of the multivibrator controlled?multivibrator controlled?

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Monostable Monostable MultivibratorsMultivibrators

Also called a one shotAlso called a one shot Block diagram and waveformsBlock diagram and waveforms

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Monostable Monostable MultivibratorsMultivibrators

Built from NAND gates and RC Built from NAND gates and RC circuitcircuit

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IC Monostable IC Monostable MultivibratorsMultivibrators

7412174121 NonretriggerableNonretriggerable Connect RC components for proper Connect RC components for proper

pulse widthpulse width Two active-LOW trigger inputsTwo active-LOW trigger inputs One active-HIGH trigger inputOne active-HIGH trigger input

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IC Monostable IC Monostable MultivibratorsMultivibrators

74121 block diagram and function 74121 block diagram and function tabletable

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Retriggerable Retriggerable Monostable Monostable

MultivibratorsMultivibrators 7412374123

new timing cycle each time new new timing cycle each time new trigger appliedtrigger applied

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Retriggerable Monostable Retriggerable Monostable MultivibratorsMultivibrators

74123 logic symbol and function 74123 logic symbol and function tabletable

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Retriggerable Retriggerable Monostable Monostable

MultivibratorsMultivibrators 7412374123

Dual multivibratorDual multivibrator Active low reset (Active low reset (RRDD) which forces Q low) which forces Q low No internal timing resistorNo internal timing resistor Pulse width is determined using:Pulse width is determined using:

If CIf Cextext is greater than or equal to 1000 pF, is greater than or equal to 1000 pF, the graph in Figure 14-18 can be used to the graph in Figure 14-18 can be used to select components.select components.

extextextw RCRt

700128.0

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Capacitor Selection Capacitor Selection Curves (Figure 14-18)Curves (Figure 14-18)

Astable Operation of the Astable Operation of the 555 IC Timer555 IC Timer

One shot or astable oscillatorOne shot or astable oscillator Voltage dividerVoltage divider ComparatorsComparators S-R flip-flopS-R flip-flop Discharge transistorDischarge transistor

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555 IC Timer Block 555 IC Timer Block DiagramDiagram

Discussion PointDiscussion Point

Describe the operation and function Describe the operation and function of the 555 pins.of the 555 pins.

Describe the operation of the 555 Describe the operation of the 555 connected in astable mode (as in connected in astable mode (as in text Figure 14-20)text Figure 14-20)

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Astable Operation of the Astable Operation of the 555 IC Timer555 IC Timer

50% Duty Cycle Astable Oscillator50% Duty Cycle Astable Oscillator RRAA cannot = 0 ohms cannot = 0 ohms

RRAA = R = RBB and short R and short RBB with a diode with a diode

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555 Timer Astable 555 Timer Astable MultivibratorMultivibrator

555 Timer Monostable 555 Timer Monostable MultivibratorMultivibrator

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555 Timer Monostable 555 Timer Monostable Multivibrator WaveformsMultivibrator Waveforms

Crystal OscillatorsCrystal Oscillators Quartz crystalQuartz crystal Size and shape determine specific Size and shape determine specific

frequencyfrequency Accurate to more than five Accurate to more than five

significant digitssignificant digits Integrated circuit packages or use Integrated circuit packages or use

external quartz crystalexternal quartz crystal 74S12474S124

Voltage controlled oscillatorVoltage controlled oscillator25

Crystal-Controlled Crystal-Controlled OscillatorsOscillators

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SummarySummary

Multivibrator circuits are used to Multivibrator circuits are used to produce free-running clock oscillator produce free-running clock oscillator waveforms or to produce a timed digital waveforms or to produce a timed digital level change triggered by an external level change triggered by an external source.source.

Capacitor voltage charging and Capacitor voltage charging and discharging rates are the most common discharging rates are the most common way to produce predictable time way to produce predictable time duration for oscillator and timing duration for oscillator and timing operations.operations.

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SummarySummary

An astable multivibrator is a free-An astable multivibrator is a free-running oscillator whose output running oscillator whose output oscillates between two voltage levels oscillates between two voltage levels at a rate determined by an attached at a rate determined by an attached RC circuit.RC circuit.

A monostable multivibrator is used to A monostable multivibrator is used to produce an output pulse that starts produce an output pulse that starts when the circuit receives an input when the circuit receives an input trigger and lasts for a length of time trigger and lasts for a length of time dictated by the attached RC circuit.dictated by the attached RC circuit.

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SummarySummary

The 74121 is an IC monostable The 74121 is an IC monostable multivibrator with two active-LOW and multivibrator with two active-LOW and one active-HIGH input trigger sources one active-HIGH input trigger sources and an active-HIGH and an active-LOW and an active-HIGH and an active-LOW pulse output terminal.pulse output terminal.

Retriggerable monostable multivibrators Retriggerable monostable multivibrators allow multiple input triggers to be allow multiple input triggers to be acknowledged even if the output pulse acknowledged even if the output pulse from the previous trigger had not expired.from the previous trigger had not expired.

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SummarySummary

The 555 IC is a general-purpose timer The 555 IC is a general-purpose timer that can be used to make astable and that can be used to make astable and monostable multivibrators and perform monostable multivibrators and perform any number of other timing functions.any number of other timing functions.

Crystal oscillators are much more Crystal oscillators are much more accurate and stable than RC timing accurate and stable than RC timing circuits. They are used most often for circuits. They are used most often for microprocessor and digital microprocessor and digital communication timing.communication timing.

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