control lab exp 7

8/3/2019 Control Lab Exp 7

1/20


2/20

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


3/20



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.


4/20



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.


5/20



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


6/20



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


7/20



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


8/20



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


9/20



Figure 6. Output

waveform of Astable

Multivabrator using op-

amp with potentiometer

at 75%

Figure 7. Output

waveform of Astable



at 50%


10/20



Figure 8. Output

waveform of Astable



at 25%

Figure 9. Output

waveform of Astable



at minimum


11/20



Figure 10. Output

waveform of Astable



at maximum

*If period T is equal to 20 ms,

Resistance of the potentiometer is 0 hms

For computed value:


12/20



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.


13/20



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


14/20



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)


15/20



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:


16/20





If Rpotentiometer is maximum:


17/20



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


18/20



c. If Rpotentiometer is minimum

d. If Rpotentiometer is minimum

e. If Rpotentiometer is minimum


19/20



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


20/20



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.

control lab exp 7

Documents