Project 3 Build an Astable Multivibrator

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Project 3Build an Astable Multivibrator
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Purpose

• The purpose of this project is to build an
  Astable multivibrator without the 555-timer chip.
• This means you will have to assemble your own
  components to mimic the behavior of the inside of
  the chip.
• You will create a PSpice simulation and a working
  circuit.
• You will then determine how to modify the 555
  timer chip model so that it cycles over a
  different part of the capacitor charge curve.
• You will modify your PSpice simulation and
  circuit to demonstrate that your new model works
  as predicted.

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The Animation
Animation applet
Your initial design will be a PSpice simulation
and working circuit based on this animation.
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Block Diagram

• Circuits are often represented by block diagrams
  that show the flow of the signal between
  different functional blocks.
• Above is a block diagram of the astable
  multivibrator.

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Components in each Block
C
A
H
E
G
B
D
F
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Components in each Block

• A R-R-C Combination
• B Voltage Divider
• C Threshold Comparator
• D Trigger Comparator
• E Reset Logic Chip (NAND gate)
• F J-K Flip Flop
• G LED Circuit
• H Transistor Circuit

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How does the Astable Multivibrator work?
What makes this circuit generate a string of
pulses? This is discussed in detail in the
experiment 7 notes.
Animation applet
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How does the Astable Multivibrator work?
These equations determine the characteristics of
your output pulses based on the values you choose
for R1, R2 and C1.
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How does the Astable Multivibrator work?

• The frequency of the pulses and their duty cycle
  are dependent upon the RC network values.
• The capacitor C charges through the series
  resistors R1 and R2 with a time constant of
• tON (R1 R2)C1.
• The capacitor discharges
• through R2 with a time
• constant of tOFF R2C1

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Where do the equations come from?
The equations that determine the on and off time
of the output pulses are based on the charge and
discharge time of the capacitor. The capacitor
equations are
charging
discharging
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Relating charge equations to time
How much time should it take to charge between
1/3 and 2/3 of V0?
Time to charge up to 2/3V0 is
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Initial Design PSpice

• Build the PSpice circuit and look at the signals
  at the input and output of each block in the
  diagram.
• ignore timing errors from the simulation
• Use the cursors to mark important voltage levels
  and times
• high and low on digital signals
• important points on analog signals (like 1/3 and
  2/3 of Vcc)
• on and off time of the pulses

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Initial Design Protoboard

• Build the circuit on your protoboard
• tie pin 13 of flip flop to 5V
• dont forget to put power on the digital chips
• add a bypass capacitor
• use a 1k pot as a variable pull-down resistor
• set clock to 100k hertz
• Take pictures with Agilent
• Use voltage and time features of scope
• Use the cursors on the scope
• Make sure you have actual numerical values on the
  pictures that you take

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Useful Scope Features

• VOLTAGE
• Vave (DC)
• Vp-p (AC)
• TIME
• Freq
• Period
• Duty Cy
• CURSOR
• T1, T2, DT
• V1, V2, DV
• moves cursors

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Final Design

• How would you modify the inside of the timer to
  make it charge between ¼VCC and ¾ VCC?
• What are the new equations for TON and TOFF?
• What are the new on and off times for the pulses
  in your circuit?
• Modify the PSpice and the circuit on your
  protoboard and show that your results are
  consistent with those predicted by the equations.

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Project Report

• Introduction
• What is the objective of the project?
• At least two relevant topics
• Theory
• Describe the function of the components in the
  circuit
• How does the multivibrator work? Give details.
• Where do the equations for TON and TOFF come
  from?
• What should TON and TOFF be for the circuit you
  are building?

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Project Report

• Initial Design
• PSpice simulation, plots, and discussion
• Protoboard implementation, pictures, and
  discussion
• comparison of voltages and times
• PSpice
• Protoboard
• Theory

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Project Report

• Final Design
• Determine how to change circuit.
• Come up with new equations
• Modify PSpice
• Modify Circuit
• Comparison of voltages and times
• voltage levels affected by redesign
• new on and off times

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Project Report

• Conclusion
• Is it an astable multivibrator?
• Conclusions that can be drawn from your voltage
  comparisons
• Discuss the on and off times of the initial and
  final design. Are they as expected?
• Sources of error
• General Conclusions

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Appendices

• Appendix A Make you own task list.
• Appendix B References and initial design
  equations.
• Appendix C PSpice plots of initial design
• Appendix D Agilent plots of initial design
• Appendix E Final design (circuit diagram,
  calculations, PSpice and Agilent plots)