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Electronic InstrumentationProject 3

Build 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

D

C

F

G

H

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Components in each Block

A: R-R-C CombinationB: Voltage DividerC: Threshold ComparatorD: Trigger ComparatorE: Reset Logic Chip (NAND gate)F: J-K Flip FlopG: LED CircuitH: Transistor Circuit

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How does the Astable Multivibrator work?

Animation applet

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?

These equations determine the characteristics of your output pulses based on the values you choose for R1, R2 and C1.

1)21(693.0 CRRTon 1)2(693.0 CRToff

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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

ON = (R1 + R2)C1.

The capacitor discharges

through R2 with a time

constant of OFF = 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:

t

C eVV 10

t

C eVV 0

charging discharging

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Relating charge equations to time

tt

C eeVVV

3

21132

00

How much time should it take to charge between 1/3 and 2/3 of V0?

sec0986.1321ln tTime 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• 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 FeaturesVOLTAGE

* Vave (DC)

* Vp-p (AC)

TIME

* Freq

* Period

* Duty Cy

CURSOR

* T1, T2, T

* V1, V2, V

* 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)