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8/3/2019 Control Lab Exp 7
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8/3/2019 Control Lab Exp 7
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Astable Multivibrator Experiment 7
Control SystemLaboratory
INTRODUCTION
This circuit diagram shows how a 555 timer IC is configured
to function as an astable multivibrator. An astable multivibrator is a
timing circuit whose 'low' and 'high' states are both unstable. As
such, the output of an astable multivibrator toggles between 'low'
and 'high' continuously, in effect generating a train of pulses. This
circuit is therefore also known as a 'pulse generator' circuit.
In this circuit, capacitor C1 charges through R1 and R2,
eventually building up enough voltage to trigger an internalcomparator to toggle the output flip-flop. Once toggled, the flip-flop
discharges C1 through R2 into pin 7, which is the discharge pin.
When C1's voltage becomes low enough, another internal
comparator is triggered to toggle the output flip-flop. This once
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again allows C1 to charge up through R1 and R2 and the cycle
starts all over again.
C1's charge-up time t1 is given by: t1 = 0.693(R1+R2)C1.
C1's discharge time t2 is given by: t2 = 0.693(R2)C1. Thus, the
total period of one cycle is t1+t2 = 0.693 C1(R1+2R2). The
frequency f of the output wave is the reciprocal of this period, and is
therefore given by: f = 1.44/(C1(R1+2R2)), wherein f is in Hz if R1
and R2 are in megaohms and C1 is in microfarads.
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DATA
Part I
Figure 1. 555 IC Astable Multivibrator with switch 1 connected to 0.01 uF
capacitor using LED as an indicator.
Observation: The LED indicator blinks fast.
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Figure 2. 555 IC Astable Multivibrator with switch 1 connected to 0.01 uF
capacitor and output pulse
Period: 220 microseconds
Frequency: 4.55 kHz
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Figure 3. 555 IC Astable Multivibrator with switch 1 connected to 1 uF capacitor
using LED as an indicator.
Observation: The LED indicator blinks slower than the previous
configuration
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Figure 2. 555 IC Astable Multivibrator with switch 1 connected to 1 uF capacitor
and output pulse
Period: 20.7 milliseconds
Frequency: 48.31 Hz
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Part II
Figure 5. Asatable Multivibrator using op-amp
Position Min Max
T(ms) 22.40 76.80 131.60 186.20 240.90F(Hz) 44.64 13.02 7.60 5.37 4.15
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Figure 6. Output
waveform of Astable
Multivabrator using op-
amp with potentiometer
at 75%
Figure 7. Output
waveform of Astable
Multivabrator using op-
amp with potentiometer
at 50%
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Figure 8. Output
waveform of Astable
Multivabrator using op-
amp with potentiometer
at 25%
Figure 9. Output
waveform of Astable
Multivabrator using op-
amp with potentiometer
at minimum
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Figure 10. Output
waveform of Astable
Multivabrator using op-
amp with potentiometer
at maximum
*If period T is equal to 20 ms,
Resistance of the potentiometer is 0 hms
For computed value:
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ANALYSIS
Part I
1. Describe the output waveform of the astable circuit.
***The output waveform of an astable circuit is a series of
digital pulses.
2. Draw the block diagram of the major operations contained on
the 555 timer chip.
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3. Compute for the output frequency when (a) SW1 is connected
to C1 and (b) SW1 is connected to C2.
()( )a. If C1 is connected
()( )
()()( )
b. If C2 is connected
()( ) ()()( ) 4. Compare the measured value with the computed value of the
frequency for C1 and C2.
a. If C1 is connected
b. If C2 is connected
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Part II
1. What is the effect of the feedback resistance to the period of the
output waveform?
***As the feedback resistance increases the period of the
output waveform also increases.
2. Plot the graph of the potentiometer position versus the
frequency and versus the period. Describe the graph drawn.
***by looking at the graph, I can say that the feedback
resistance is directly proportional with the period (T) and the
line of their relationship is linear. The graph of therelationship of frequency and feedback resistance is
inversely proportional as the feedback resistance increase
the frequency decrease; and is asymptotic to the y-axis.
0
50
100
150
200
250
300
Minimum 1/4 1/2 3/4 Maximum
Period(ms)
Frequency(Hz)
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3. With the use of the stated equation compute for the frequency
(f) and period (T).
If Rpotentiometer is minimum:
If Rpotentiometer is 1/4:
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If Rpotentiometer is 1/2:
If Rpotentiometer is 3/4:
If Rpotentiometer is maximum:
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4. Compute for the percent error between the measured value
recorded in the table and the computed value of f and T.
Measured Values:
Position Min Max
T(ms) 22.40 76.80 131.60 186.20 240.90F(Hz) 44.64 13.02 7.60 5.37 4.15
Computed Values:
Position Min Max
T(ms) 21.97 76.90 131.83 186.87 241.69F(Hz) 45.51 13.00 7.59 5.35 4.13
a. If Rpotentiometer is minimum
b. If Rpotentiometer is minimum
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c. If Rpotentiometer is minimum
d. If Rpotentiometer is minimum
e. If Rpotentiometer is minimum
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DISCUSSION
For this experiment, entitled Astable Multivibrator, we deal
with the function of an Astable Multivibrator circuit. This experiment
is divided into two parts; the first one is an astable multivibrator
using a 555 timer IC, and for the second part it is with a use of
operational amplifier.
We construct the circuit using a computer-based program
called Multisim. With the use of Multisim, we will be able to simulate
a response of a circuit with an ideal condition with in a click of amouse.
At first, we construct the given circuit. It is an astable
multivibrator with a 555 timer on its circuitry. It has also a single
pole double throw switch with two capacitors of different values
connected on its throw. For the first observation, we try to analyze
the response of the circuit with a 0.01 uF capacitor connected on its
trigger pin. As I observe its response, I noticed that the LEDindicator blinks fast. And with that, I try to analyze the output
waveform using an oscilloscope connected on the output of the
circuit. And I record the frequency and the period of the output
signal.
After that, the experiment says that I must change or
connect the switch to the 1 uF capacitor. And by repeating the
procedure on the first configuration, I noticed that the LED indicator
blinks slower than the latter condition. And also when I analyze its
output waveform, I have seen that the period and frequency of the
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output signal alter. The period is much greater and the frequency is
lower.
For the second part, I construct a circuit of astable
multivibrator but this time its with the use of an operational
amplifier. And with the varying factor, the experiment said to use a
potentiometer on the feedback of the op-amp.