experiment eb2: ic multivibrator circuitsfoe.mmu.edu.my/lab/lab sheet/labsheet trim2 13-14... ·...

EEN1026 Electronics II: Experiment

Experiment EB2: IC Multivibrator Circuits

Learning Outcomes

LO1: · Explain the principles and operation of amplifiers and switching circuits

LO2: Analyze high and low frequency response of amplifiers

LO3: · Analyze the operation of power amplifiers and switching circuits.

1.0 Objectives

To measure the frequency and duty cycle of an astable 555 timer

To measure the pulse width and duty cycle of a monostable 555 timer

To measure the frequency and duty cycle of a voltage-controlled oscillator

2.0 Apparatus

Equipment required Components required Power Supply – 1 Timer IC 555 – 2

Oscilloscope – 1 Resistor 10k (1/4W) – 2

Multimeter – 1 Resistor 100k (1/4W) – 1

Breadboard – 1 Resistor 33k (1/4W) – 1

Function Generator – 1 Resistor 68k (1/4W) – 1

Resistor 47k (1/4W) – 1

Resistor 1k (1/4W) – 2

Mylar Capacitor 0.01F – 4

Potentiometer (1k) – 1

3.0 Introduction

Multivibrators are circuits that are designed to have zero, one, or two stable output states. The

555 timer is one of the most popular general purpose IC multivibrators. It can be used in a

variety of applications requiring accurate time delays, oscillation, and pulse conditioning.

Signetics Corporation first introduced it as SE555 timer, which is an 8-pin IC that can be

connected with external components for either astable or monostable operation. Figure 1

shows the simplified block diagram of a 555 timer. The circuit’s name is derived from the use

of an internal voltage divider between VCC and ground using three 5k resistors. This divider

chain is used to set a pair of reference voltages for two comparators that drive the set and

reset inputs of an R-S flip-flop.


Figure 1: Block diagram of 555 Timer

Refer to Figure 1, a logic high voltage (+V0) applied to the set S input and a logic low (0V) to

the reset R input forces the output Q to high (VCC) and Q low (0V). This is referred to as the

set condition of the flip-flop. A high reset R and low set S causes the output to switch to a low

Q and a high Q . This is referred to as the reset condition of the flip-flop. The circuit latches

in either of the two states. In other words, a high S input sets Q to high; a high R input resets

Q to low. Output Q remains in a given state until triggered into the opposite state.

The comparators are simply Op-amps. Note that the upper comparator has a threshold input

(pin 6) and a control input (pin 5). In most applications, the control input is not used, so that

the control voltage equals +2VCC/3. However, applying an external voltage to this pin

provides some control over the reference voltages for both comparators. When the voltage of

pin 6 exceeds the control voltage, the high output from the Op-amp will set the flip-flop. The

high Q output from the flip-flop will turn on transistor Q1 and discharge the external timing

capacitor connected to pin 7. The complementary signal (logic low) of the flip-flop goes to

pin 3, the output.

When the external reset (pin 4) is grounded, it inhibits the device. This ON-OFF feature is

useful sometimes. In most applications, however, the external reset is not used and pin 4 is

tied directly to the supply voltage. The inverting input of the lower comparator is called the

trigger (pin 2) and its noninverting input has a fixed voltage of +VCC/3 developed by the three

5k voltage divider. When the trigger input voltage is slightly less than +VCC/3, the Op-amp

output goes high and resets the flip-flop. Lastly, pin 1 is the chip ground, while pin 8 is the

power supply pin. The 555 timer will work with any supply voltage between 4.5 and 16V.

Monostable Operation

Figure 2a shows the 555 timer connected for monostable (one-shot) operation. It produces a

single, fixed voltage, output pulse each time a trigger pulse is applied to pin 2 (Figure 2b).

+

_

+

_ S

R Q _

Q

5k

5k

5k

1

4

6

5

2

7

3

OUTPUT

DISCHARGE

TRIGGER

GROUND

CONTROL

THRESHOLD

+VCC

RESET

Q1

8


(a)

(b)

Figure 2: (a) Monostable operation; (b) ideal waveforms

The trigger input is a narrow pulse with a quiescent value of +VCC. When the trigger input is

slightly less than +VCC/3, the lower Op-amp has a high output and resets the flip-flop. This

cuts off the transistor, allowing the capacitor to start charging up. As the capacitor charges,

the voltage at pin 6 increases. Eventually, the voltage becomes slightly greater than the

control voltage (+2VCC/3). The output of the upper Op-amp then goes high, forcing the RS

flip-flop output to be set. As soon as Q goes high, it turns on the transistor and this quickly

discharges the capacitor. As a result, we get a triangular pulse at pin 6 & 7.

The capacitor C is charged through resistance R. For a larger RC time constant, the capacitor

will take longer time to charge to +2VCC/3. In other words, the RC time constant controls the

width of the output pulse. Solving the exponential equation for capacitor voltage gives the

formula for its pulse width as

RCW 1.1 (1)

Astable Operation

Figure 3a shows the 555 timer connected for astable or free-running operation. The output is

a square-wave signal. When Q is low, the transistor is cut off and the capacitor is charged

through ( BA RR ). Because of this, the charging time constant is CRR BA )( . When the

voltage at pin 6 is slightly greater than +2VCC/3, the upper Op-amp has a high output and this

sets the flip-flop. With Q high, it turns on the transistor and grounds pin 7. Now the capacitor

discharges through BR . The discharging time constant is CRB . When the capacitor voltage

drops slightly below +VCC/3, the lower Op-amp has a high output and this resets the flip-flop.

+

_

+

_ S

R Q _

Q

+VCC

Vout

TRIGGER

C

1

2 3

8 7

6

R

5k

5k

5k

(pin 3) 0

0

+VCC

+2VCC/3

+VCC

(pin 2)

(pin 6 & 7)

0


(a)

(b)

Figure 3: (a) Astable operation; (b) ideal waveforms

Figure 3b illustrates the waveforms; the timing capacitor has an exponentially rising and

falling voltage and the output of Q is a rectangular wave. Since the charging time constant is

longer than the discharging time constant, the output is not symmetrical; the high state lasts

longer than the low state. To specify how unsymmetrical the output is, we can define duty

cycle as

%100T

WD (2)

Depending on the resistances AR and BR , the duty cycle is between 50 and 100 percent. The

mathematical solutions of the charging and discharging equations give the following

formulas. The output frequency is

CRR

fBA )2(

44.1

(3)

and the duty cycle is

%1002

BA

BA

RR

RRD (4)

If AR is much smaller than BR , the duty cycle approaches 50 percent.

+

_

+

_ S

R Q _

Q

+VCC

Vout C

1

2 3

8

7

6

RA

5k

5k

5k

RB

0

+2VCC/3

+VCC/3

W

T

+VCC

(pin 2 & pin6)

(pin 3)


Voltage-Controlled Oscillator (Pulse Position Modulator)

The free-running multivibrator can be modified to become a voltage-controlled oscillator

(VCO). Recall that pin 5 (control) is connected to the inverting input of the upper Op-amp.

Normally, the control voltage is +2VCC/3 because of the internal voltage divider. In VCO,

however, the voltage from an external potentiometer overrides the internal voltage. In other

words, by adjusting the potentiometer, we can change the control voltage level. If we increase

Vcontrol, the capacitor will take a longer time to charge and discharge; therefore, the frequency

decreases. As a result, we can change the frequency of the circuit by varying the control

voltage.

4.0 Procedures

A. Astable 555 Timer

1. Refer to the 555 timer circuit shown in Figure 4-1. The schematic diagram does not show

the op-amps, flip-flop, and other components inside the 555 timer, but only the pins and

external components.

2. Notice that pin 5 (control) is bypassed to ground through a small capacitor, typically

0.01F. This provides some noise filtering for the control voltage.

3. Based on equations (3) and (4), calculate and record the frequencies (fcal) and duty cycles

(Dcal) for the resistances listed in Table 4-1 in Appendix D2.

4. Connect the circuit of Figure 4-1 on a breadboard with RA = 10k and RB = 10k.

Measure and record the supply voltage VCC(meas) with a multimeter.

5. Using an oscilloscope (set CH1 and CH2 to DC coupling and trigger source to CH1), and

connect the probes at pin 3 (CH1) and pin 6 (CH2), measure the waveforms Vout (at CH1)

and Vpin-6 (at CH2). If the circuit is functioning properly, these waveforms will be similar

to those in Figure 3(b).

6. Align the ground levels of CH1 and CH2 as indicated on Graph 4-1 in Appendix D2.

Adjust Volts/div and Time/div to display the waveforms on the screen as big as possible

with one to two cycles. Sketch Vout and Vpin-6 waveforms on Graph 4-1.

7. Measure and record the period, T and the high portion of the pulse width, W. Determine

the frequency f and duty cycle D from the measurement results.

8. Repeat steps 5 through 7 for the other resistances of Table 4-1.

9. For RA = 100 k and RB = 10 k case, measure and record the maximum and minimum

voltage levels of Vout and Vpin 6 waveforms.

10. Ask the instructor to check all of your results. You must show the last oscilloscope

waveforms to the instructor.

SE/NE555

Figure 4-1: Astable 555 Timer Circuit (TOP VIEW)

VCC=+5V

3

4 8

6

7 555

Timer

2 1

RA

RB 5

C1=0.01F C2=0.01F

Vout

1

2

4

3

8

7

5

6


B. Monostable 555 Timer

1. IC U2 of Figure 4-2 is a 555 timer connected for monostable operation. Calculate the

pulse width for each resistance value R listed in Table 4-2. Record the results under Wcal.

2. IC U1 of Figure 4-2 is the astable multivibrator circuit of Part A. It is used here to provide

the trigger input to the monostable circuit (U2).

3. Connect the circuit of Figure 4-2 with resistance R = 33k. Measure and record VCC(meas).

4. Using an oscilloscope, measure the waveforms at pin 2, Vpin2 (at CH1) and pin 3, Vout (at

CH2) of the monostable circuit (U2). Set CH1 and CH2 to DC input coupling and trigger

source to CH1. Align the ground levels of CH1 and CH2 as indicated on Graph 4-4. Set

Time/div to display the waveforms with one to two cycles on the screen. Set Volt/div to

display the waveforms as big as possible but not overlapping. Sketch the waveforms.

5. Measure and record the pulse width, W, at the output of U2.

6. Repeat steps 4 to 5 for the other resistances R in Table 4-2.

7. For R = 68 k case, connect CH2 to pin 6, measure and record the maximum and

minimum voltage levels of pin 6 waveform.



Figure 4-2: Monostable 555 Timer Circuit

VCC=+5V

3

4 8

6

7 U2

555

Timer

2 1

R

5

0.01F

0.01F

Vout 3

4 8

6

7 U1

555

Timer

2 1

100k

10k 5

0.01F 0.01F


C. Voltage-Controlled Oscillator

1. Connect the voltage-controlled oscillator (VCO) of Figure 4-3 (refer to Appendix A for

the potentiometer legs). Measure and record VCC(meas).

2. Measure the output, Vout (CH1) and pin 6, Vpin 6 (CH2) waveforms with an oscilloscope.

Set CH1 and CH2 to DC input coupling and trigger source to CH1. Align the channel

ground levels as indicated on Graph 4-5. Adjust Volts/div to display the waveforms on

the screen as big as possible.

3. Vary the 1-k potentiometer and notice the changes in the waveforms. Adjust Time/div if

necessary.

4. Turn the potentiometer to get the minimum frequency. Adjust Time/div to display the

waveforms with one to two cycles on the screen. Sketch Vout and Vpin 6 waveforms.

5. Measure and record T, W, Vpin 6 (max) and Vpin 6 (min). Measure and record the DC voltage at

pin 5, Vpin 5 with a multimeter. Calculate the frequency and duty cycle.

6. Turn the potentiometer to get the maximum frequency. Repeat steps 4 and 5.



Figure 4-3: Voltage-Controlled Oscillator Circuit

Report Submission

Submit your report on the same day immediately after the experiment.

VCC=+5V

1k

3

4 8

6

7 555

Timer

2 1

10k

100k 5

C1=0.01F

Vout 1k

1k

Var A

B


APPENDIX A

Log Scale The distance in a decade of the log scale in the figure below is x mm. Since log101 = 0, it is

used as a refernce point (0 mm) in the linear scale. Then, the reading 10 is located at x mm

and the reading 0.1 is located at –x mm. For a reading F, it is located at [1og10(F)]*x mm.

E.g.:

Reading 0.25 is located at [1og10(0.25)]*x mm = -0.602x mm

Reading 2.5 is loacted at [1og10(2.5)]*x mm = 0.398x mm

Reading 25 is located at [1og10(25)]*x mm = 1.398x mm (not shown in the figure)

Reading 250 is located at [1og10(250)]*x mm = 2.398x mm (not shown)

Conversely, a point at z mm location is read as xz /10 .

E.g.:

-0.3x mm is read as 10(-0.3x/x)

= 0.501

0.6x mm is read as 10(0.6x/x)

= 3.98


= 31.6 (not shown)


= 501 (not shown)

9 0.1 0.2 0.3 0.5 1 2 3 5 10

-x 0 x

Linear scale

(mm)

Log scale

(unit)

0.25 2.5

0.398x -0.602x 0.6x

3.98 0.501

-0.3x

0.4 0.6 0.7

0.8 0.9

4 6 7 8

The Resistor color code chart

ABC

AB x 10C pF

.abc

0.abc F

Capacitance

Potentiometer

A Var B


Appendix B: Breadboard Internal Connections

Vertically

connected

Vertically

connected

Horizontally connected Horizontally connected

+VCC

0V

GND

555

1 2 3 4

8 7 6 5

General mistakes: The legs of the resistors and the transistor are shorted

by the breadboard internal connections.

Multimedia University FOE

0.1 F

Internal

connections

Internal

connections

EEE1026 Electronics II

Appendix D2

Experiment EB2: IC Multivibrator Circuits

Lab Report

(Submit your report on the same day immediately after the experiment)

Name: ________________________ Student I.D.: _______________ Date: __________

Majoring: ____________________ Group: ____________ Table No.: ____________

4. Astable 555 Timer

VCC(meas) = _________V [1 mark]

Table 4-1: Astable Operation for various RA and RB

RA (k) RB (k) calf calD T W f D

10 10

10 100

100 10

[3 marks]

For RA = 100 k, RB = 10 k case (Step 9):

Vout (max) = ______ V VCC(meas) – Vout (max) = ______ V

Vout (min) = ______ V

Vpin 6 (max) = ______ V Vpin 6 (max) / VCC(meas) = ______

Vpin 6 (min) = ______ V Vpin 6 (min) / VCC(meas) = ______

[7 marks]

Graph 4-1: Astable Operation for RA = 10 k, RB = 10 k

Time base : ______ s/div, CH1 (Vout) : ______ V/div, CH2 (Vpin 6) : ______ V/div

CH1 & CH2

ground

[5 marks]

EEN1026 Electronics II Experiment EB2



[5 marks]



[5 marks]

* Note: Ask your instructor to verify your results before you proceed to Part B.

Signature: ______________ Time: ___________ Remarks _________________

CH1 & CH2

ground

CH1 & CH2

ground



VCC(meas) = _________V [1 mark]

Table 4-2: Monostable Operation for various R

R (k) Wcal W

33

47

68

[3 marks]

For R = 68 k case (Step 7):

Vpin 6 (max) = ______ V Vpin 6 (max) / VCC(meas) = ______

Vpin 6 (min) = ______ V

[3 marks]

Graph 4-4: Monostable Operation for R = 33 k

Time base : ______ s/div, CH1 (Vpin 2) : ______ V/div, CH2 (Vout) : ______ V/div

[5 marks]

* Note: Ask your instructor to verify your results before you proceed to Part C.


CH2 ground

CH1 ground



VCC(meas) = _________V [1 mark]

Graph 4-5: Voltage-Controlled Oscillator at minimum frequency


[5 + 7 marks]

Voltage-Controlled Oscillator at maximum frequency

[7 marks]

* Note: Ask your instructor to verify your results.


T = ________s

W = ________s

Vpin 6 (max) = ________V

Vpin 6 (min) = ________V

Vpin 5 = ________V

f = ________Hz

D = ________%

T = ________s

W = ________s

Vpin 6 (max) = ________V

Vpin 6 (min) = ________V

Vpin 5 = ________V

f = ________Hz

D = ________%

CH1 & CH2

ground


Discussion

A. Astable 555 Timer

1. Explain the difference between the calculated fcal and the measured f.

________________________________________________________________________

________________________________________________________________________

2. Compare the calculated Dcal to the measured D, and justify their difference.

________________________________________________________________________

________________________________________________________________________

3. Identify how the voltages Vout and Vpin 6 are related in the three graphs.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4. Compare between voltages VCC(meas) and Vout (max) and explain their difference.

________________________________________________________________________

5. Evaluate how WL, f and D changes when RA and/or RB are varied. Propose the expected

minimum and maximum duty cycle values.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

[15 marks]


1. Identify how the voltages Vout and Vpin 3 are related in the three graphs.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. Describe how W changes when R is varied.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

[6 marks]



1. With the help of Figure 1 and Figure 3, compare and evaluate the relationships between

voltages Vpin 6 (max) and Vpin 5, as well as Vpin 6 (min) and Vpin 5. Include numerical calculations

in your answer.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. Describe how W, f and D changes when the value of Vpin 5 is varied.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

[6 marks]

Conclusion

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________.

[15 marks]

ID NO:

Criteria 1 (Need Improvement) 2 (Satisfactory) 3 (Good) 4 (Excellent)

1 Ability in constructing the IC

Multivibrator Circuits: Monostable,

Astable and Voltage-Controlled

Oscillator

Unable to demonstrate

proper steps to construct the

IC Multivibrator Circuits and

not asking for help.

Able to demonstrate basic

steps required to construct

the IC Multivibrator Circuits

with some help

Able to demonstrate good

knowledge on the construction

of the IC Multivibrator Circuits

with minimum help

Able to demonstrate full

knowledge on the

construction of the IC

Multivibrator Circuits without

help.

2 Ability in performing data collection

using lab equipment such as DC power

supply, digital multimeter, oscillocope

and function generator.

Unable to record data, and

no effort is shown

Able to provide adequate

data, and show some

efforts in getting the data

Able to record most of the data

correctly

Recorded all data neatly and

correctly

3 Ability to determine and draw the time

domain waveform for input and output

voltages of the amplifier circuits.

Not able to determine, and

draw the time domain

waveform for input and

output voltages (no effort

was also shown)

Able to at least determine

and draw the time domain

waveform for input and

output voltages and show

some efforts in the

calculations

Able to determine and draw

the time domain waveform for

input and output voltagesand

solve the calculations partially

Completed all the drawing

and calculations correctly

5 Ability to answer the questions in by

Oral Assesment

Not able to answer the

question, no attempt was

made to answer

Able to answer questions

with some basics answers

and demonstrate some

attempts to refer to the

text books, notes, lab sheet

Able to answer most part of

the questions, with some

explanations and elaborations

and demonstrate some

attempts to refer to text books,

notes or lab sheet

Answered all correctly with

proper explanations and

elaborations, without a need

to refer to any references.

Rating Awarded by Assessor

STUDENT'S NAME:

SUBJECT CODE AND TITLE: EEE1026 Electronics 2

EXPERIMENT DATE: TIME:

Able to present results very

clearly and excellent

summary of final outcomes

which answer the objectives

of the lab

EXPERIMENT TITLE: EB2: IC Multivibrator Circuits

Constructing the circuits and performing data collection using lab equipment

4 Able to present results and

summarise mostly to the final

outcomes and answered most

of the objectives of the lab

Conclusions

Unable to present results

clearly and no attempt was

made to summarise final

outcomes

Able to present results and

summarise adequate final

outcomes and reasonably

relating them to the

objectives

The ability to present results and

summarise final outcomes which

answers the objectives of the lab.

RECEIVED BY (LAB'S STAMP):

ID NO:

Late Submission (penalty) or Late-Comer

AVERAGE MARK (total/number of criteria)

STUDENT'S NAME:

SUBMISSION DATE: LATE SUBMISSION: YES/NO

STUDENT'S SIGNATURE:

EXPERIMENTAL TITLE:

STUDENT'S NAME:

ID NO:

SUBMISSION DATE: LATE SUBMISSION: YES/NO

REPORT SUBMISSION RECEIPT (STUDENT'S COPY)



experiment eb2: ic multivibrator circuitsfoe.mmu.edu.my/lab/lab sheet/labsheet trim2 13-14... ·...

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