1. Project 3 Build an Astable Multivibrator
11/22/2018
Project 3 Build an Astable Multivibrator

2. Electronic Instrumentation
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 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.
Your circuit won’t include the Reset feature
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Components in each Block A G E C D F
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6. 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: SR Flip Flop
G: 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.
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How does the Astable Multivibrator work?
2 NOR gates can be used to create a SR Flip-Flop
Convince yourself that this works
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Build this circuit
This has all the 555 Timer features except for the reset pin.
You will build it on the protoboard
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But model this circuit
The demo version of Capture won’t model the circuit you will build. It can model this one, which uses 2 transistors to model the SR Flip-flop.
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11. 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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12. 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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13. 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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14. 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.
No reset circuit
Use the cursors to record 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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16. Initial Design Protoboard
Build the circuit on your protoboard
don’t forget to put power on the digital chip
add a bypass capacitor
Record data using Mobile Studio and the IOBoard
Use voltage and time features of scope
Use the cursors on the scope
Make sure you have labeled the plots with the numerical values recorded
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Final Design
Modify the inside of the timer to make it switch at different voltages.
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, plots, and discussion
comparison of voltages and times
PSpice
Protoboard
Theory
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Project Report
Final Design
Determine new threshold and trigger voltages
Come up with the new timing 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: Plots of data from Mobile Studio for initial design
Appendix E: Final design (circuit diagram, calculations, PSpice and Mobile Studio plots)
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