Download Experiment 2

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Electronic Instrumentation
Project 3
Build an Astable Multivibrator
Purpose
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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
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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
A
C
H
E
G
B
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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?
Ton  0.693( R1  R2)C1
Toff  0.693( R 2)C1
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.
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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
t 

VC  V0 1  e t 


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t 

VC  V0  e t 
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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?
t 

VC  2 V0  V0 1  e t 
3


1 2  e
3

Time to charge up to 2/3V0 is: t   ln 1  2
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t
t
t  1.0986t sec
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Initial Design PSpice
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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
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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
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Build the circuit on your protoboard
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tie pin 13 of flip flop to 5V
don’t 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
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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
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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
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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
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Final Design
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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
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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
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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)
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