Multivibrators and the 555 Timer

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Chapter 14

• Multivibrators and the 555 Timer

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Objectives

• You should be able to
• Calculate capacitor charging and discharging
  rates in series RC timing circuits.
• Sketch the waveforms and calculator voltage and
  time values for astable and monostable
  multivibrators.
• Connect IC monostable multivibrators to output a
  waveform with a specific pulse width.

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Objectives

• (Continued)
• Explain the operation of the internal components
  of the 555 IC timer.
• Connect a 555 IC timer as an astable
  multivibrator and as a monostable multivibrator.
• Discuss the operation and application of crystal
  oscillator circuits.

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Multivibrators

• Changes between two digital levels
• Continuous, free-running
• On demand
• Three types
• Bistable (S-R flip-flop)
• Astable
• Monostable (one shot)

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Capacitor Charge and Discharge

• RC circuit charge and discharge curves

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Capacitor Charge and Discharge

• Exponential charge and discharge
• Time constant (?)

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Capacitor Charge and Discharge

• Transposing the curve equation for t

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Astable Multivibrators

• Single Schmitt Inverter and an RC circuit

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Discussion Point

• Describe the operation of an astable
  multivibrator similar to that shown in Figure
  14-5
• How is the frequency of the multivibrator
  controlled?

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Monostable Multivibrators

• Also called a one shot
• Block diagram and waveforms

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Monostable Multivibrators

• Built from NAND gates and RC circuit

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IC Monostable Multivibrators

• 74121
• Nonretriggerable
• Connect RC components for proper pulse width
• Two active-LOW trigger inputs
• One active-HIGH trigger input

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IC Monostable Multivibrators

• 74121 block diagram and function table

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Retriggerable Monostable Multivibrators

• 74123
• new timing cycle each time new trigger applied

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Retriggerable Monostable Multivibrators

• 74123 logic symbol and function table

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Retriggerable Monostable Multivibrators

• 74123
• Dual multivibrator
• Active low reset (RD) which forces Q low
• No internal timing resistor
• Pulse width is determined using
• If Cext is greater than or equal to 1000 pF, the
  graph in Figure 14-18 can be used to select
  components.

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Capacitor Selection Curves (Figure 14-18)
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Astable Operation of the 555 IC Timer

• One shot or astable oscillator
• Voltage divider
• Comparators
• S-R flip-flop
• Discharge transistor

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555 IC Timer Block Diagram
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Discussion Point

• Describe the operation and function of the 555
  pins.
• Describe the operation of the 555 connected in
  astable mode (as in text Figure 14-20)

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Astable Operation of the 555 IC Timer

• 50 Duty Cycle Astable Oscillator
• RA cannot 0 ohms
• RA RB and short RB with a diode

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555 Timer Astable Multivibrator
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555 Timer Monostable Multivibrator
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555 Timer Monostable Multivibrator Waveforms
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Crystal Oscillators

• Quartz crystal
• Size and shape determine specific frequency
• Accurate to more than five significant digits
• Integrated circuit packages or use external
  quartz crystal
• 74S124
• Voltage controlled oscillator

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Crystal-Controlled Oscillators
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Summary

• Multivibrator circuits are used to produce
  free-running clock oscillator waveforms or to
  produce a timed digital level change triggered by
  an external source.
• Capacitor voltage charging and discharging rates
  are the most common way to produce predictable
  time duration for oscillator and timing
  operations.

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Summary

• An astable multivibrator is a free-running
  oscillator whose output oscillates between two
  voltage levels at a rate determined by an
  attached RC circuit.
• A monostable multivibrator is used to produce an
  output pulse that starts when the circuit
  receives an input trigger and lasts for a length
  of time dictated by the attached RC circuit.

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Summary

• The 74121 is an IC monostable multivibrator with
  two active-LOW and one active-HIGH input trigger
  sources and an active-HIGH and an active-LOW
  pulse output terminal.
• Retriggerable monostable multivibrators allow
  multiple input triggers to be acknowledged even
  if the output pulse from the previous trigger had
  not expired.

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Summary

• The 555 IC is a general-purpose timer that can be
  used to make astable and monostable
  multivibrators and perform any number of other
  timing functions.
• Crystal oscillators are much more accurate and
  stable than RC timing circuits. They are used
  most often for microprocessor and digital
  communication timing.

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