Name of Experiment:

dehisceforkElectronics - Devices

Nov 2, 2013 (3 years and 11 months ago)

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Name of Experiment
:

To
Construct
and Test Astable Multivibrator

Using 555 timer IC
.


Objectives:



After completing this experim
ent we would able to learn
:

1.

What is a 555 timer IC.

2.

What is
a
n

a
stable Multivibrator
.

3.

How 555 timer IC works as an astable multivibrator.

4.

How

it produces
square wave
.


5.

What is the role of capa
citor in an astable

multivibrator circuit.



History
:


The IC design was proposed in
1970

by
Hans R. Camenzind

and Jim Ball. After prototyping, the
design was ported to the Monochip analogue array,

incorporating detailed design by Wayne
Foletta and others from Qualidyne Semiconductors.
Signetics

(later acquired by
P
hilips
) took
over the design and production, and released the first 555s in
1971
.


Theory:


A multivibrator is an
electronic circuit

used to implement a variety of simple two
-
state systems
such as
oscillators
,
timers

and
flip
-
flops
. It is characterized by two amplifying devices
(transistors, electron tubes or other devices) cross
-
coupled by resistors or capacitors. The name
"multivibrator" was initially applied to the free
-
running oscillator version of the circuit because
its output waveform was rich in harmonics. There are three types of multivibrator circuits
depending on the circuit operation:




Astable
, in which the circui
t is not stable in either state

it continually switches from
one state to the other. It does not require an input such as a clock pulse.



M
ono
-
stable
, in which one of the states is stable, but the other state is unstable
(transient). A trigger causes the c
ircuit to enter the unstable state. After entering the
unstable state, the circuit will return to the stable state after a set time. Such a circuit is
useful for creating a timing period of fixed duration in response to some external event.
This circuit is

also known as a
one shot
.



B
i
-
stable, in which the circuit is stable in either state. The circuit can be flipped from one
state to the other by an external event or
trigger

stable

Multivibrator
:



555 timer IC
:





(a)
NE555.
T
imer

(b)

P
inout diagram



Fig:(01)
-
555 timer IC






Fig(02): internal diagram of 555 timer


Pin description:


Depending on the manufacturer, the standard 555 package includes over 20
transistors
, 2
diodes

and 15
resistors

on a
silicon

chip installed in an 8
-
pin mini dual
-
in
-
line package (
DIP
-
8
).


The connection of the pins for a DIP package is as follows:


Pin

Name


Purpose

1

GND

Ground, low level (0 V)

2

TRIG

OUT rises, and interval starts, when this input falls below 1/3
V
CC
.

3

OUT

This output is driven to
+
V
CC

or GND.

4

RESET

A timing interval may be interrupted by driving this input to GND.

5

CTRL

"Control" access to the internal voltage divider (by default, 2/3
V
CC
).

6

THR

The interval ends when
the voltage at THR is greater than at CTRL.

7

DIS

Open collector

output; may discharge a capacitor between intervals.

8

V
+,
V
CC

Positive supply voltage is usually between 3
and 15 V.


F
or each module the discharge and threshold are internally wired together and called
timing.


Specifications
:


These specifications apply to the NE555. Other 555 timers can have different specifications
depending on the grade (military,
medical, etc).


Supply voltage (
V
CC
)

4.5 to 15 V

Supply current (
V
CC

= +5 V)

3 to 6 mA

Supply current (
V
CC

= +15 V)

10 to 15 mA

Output current (maximum)

200 mA

Maximum Power dissipation

600

mW

Power Consumption (minimum operating)

30

mW@5V, 225

mW@15V

Operating temperature

0 to 70 °C




Derivatives
:


Many pin
-
compatible variants

with two or four timers on the same chip
, including
CMOS

versions, have been built by various companies. The 555 is also known under the following type
numbers:

Manufacturer

Model

Remark

Custom Silicon Solutions

CSS555/CSS555C

CMOS from 1.2

V, IDD

<

5

µA

Fairchild Semiconductor

NE555/KA555


IK Semicon

ILC555

CMOS from 2

V

Maxim

ICM7555

CMOS from 2

V

National Semiconductor

LMC555

CMOS from 1.5

V


Texas Instruments

TLC555

CMOS from 2

V

Zetex

ZSCT1555

down to 0.9

V


The 555 has three operating modes:




Monostable

mode: in this mode, the 555 functions as a "one
-
shot" pulse generator.
Applications include timers, missing pulse detection, bouncefree switches, touch
switches, frequency divider, capacitance measurement,
pulse
-
width modulation

(PWM)
and so on.



Astable



free running mode: the 555 can operate as an
oscillator
. Uses include
LED

and
lamp flashers, pulse generation, logic clocks, tone generation, security alarms,
pulse
position modulation

and so on. Selecting a
thermistor

as timing resistor allows the use of
the 555 in a temperature sensor: the period of the output pulse is determined by the
temperature. The use of a microprocessor based circuit can then convert the pulse period
to temperature, linearize it and even provid
e calibration means.



Bistable

mode or
Schmitt trigger
: the 555 can operate as a
flip
-
flop
, if the DIS pin is not
connected and no capacitor is used. Uses include bounce free latched switches.


Astable Multivibrator Operation
:



F
ig
(01)
: astable multivibrator



The circuit diagram for the astable multivibrator using IC 555 is shown here. The astable
multivibrator generates a square wave, the period of

which is determined by the circuit external
to IC 555. The astable multivibrator does not require any external tr
igger to change the state of
the


output. Hence the name free running oscillator.


The time during which the output is either high or low is determined by the two resistors and

a

capacitor which are externally connected to the 555 timer.

The above figure
shows the 555
timer connected as an astable multivibrator. Initially when the output is high capacitor C starts
charging towards

V
cc

through
R
A

and
R
B
.

However as soon as the voltage across the capacitor
equals 2/3 V
cc

, comparator1 triggers the flip
-
flop
and the output switches to low
state.

Now

capacitor C discharges through
R
B

and the transistor Q1. When voltage across C
equals 1/3
V
cc
, comparator 2’s output triggers the flip
-
flop

and the output goes high. Then the
cycle repeats.


Astabl
e Multivibrator
-
Design method

using 555 IC
:


The time during which the capacitor C charges from 1
/3 V
CC

to 2/3 V
CC

is equal to the time the
output is high and is given as

t
c

or T
HIGH

= 0.693 (R
A

+ R
B
) C
, which is proved below.


Voltage across the capacitor at any instant

during charging period is given as,

v
c

=
V
CC

(
1
-
e
-
t/RC
)

The time

(t
1
)

taken by the capacitor to charge from 0 to +1/3 V
CC




1/3V
cc
=V
cc

(1
-
e
-
t
/RC
)

or


e
-
t/RC
=
(
1
-
1/3
)


or

e
-
t/RC
=2/3

or

e
t/RC
=3/2

or

t
1
=ln(3/2
)RC

where t=t
1

& v
c
=1/3V
cc


or

t
1
=0.405RC


The time
(t
2
)
taken by the capacitor to charge from 0 to +2/3 V
CC





2/3V
cc
=V
cc

(1
-
e
-
t
/RC
)

or

e
-
t/RC
=(1
-
2/3)


or

e
-
t/RC
=1/3

or

e
t/RC
=3

or

t
2
=ln(
3
)
RC where t=t
2
& v
c
=2/3V
cc

or


t
2

= log
e
3

RC

= 1.0986 RC


So the time taken by the
capacitor to charge from +1/3 V
CC
to +2/3 V
CC

t
c

= (t
2



t
1
) =
ln(3)
-
ln(3/2)=

ln(3*2/3)
RC =
ln(2)
RC
=
0.693 RC


Substituting R = (R
A

+ R
B
) in above equation we have

T
HIGH
= t
c
= 0.693 (R
A

+ R
B
) C


The time during which the capacitor discharges from +2/3 V
CC

to +1/3 V
CC
is equal to

the time
the output is low and is given as

t
d

or

T
L0W

= 0.693 R
B

C
,


The above equation is worked out as follows: Voltage across the capacitor at any instant during
discharging period is given as

v
c

= 2/3 V
CC
e
-

t
/

R
B
C


Substituting v
c

= 1/3 V
CC

in above equation we have




+1/3 V
CC
= +2/3 V
CC
e
-

t
d/

R
B
C

or

e
-
t/R
B
C
=1/2

or

e
t/R
B
C
=2

or

t
d
=ln(2)R
B
C

where

t = t
d

or


t
d

= 0.693 R
B
C




Overall period of oscillations,
T = T
c

+ T
d

=
ln(2)*

(R
A
+ 2R
B
)C

= 0.693 (R
A
+ 2R
B
)
C
, The
frequency of oscillations being the re
ciprocal of the overall period

of oscillations T is given as


f = 1/T


=
1/{ln(2)
*

(R
A
+ 2R
B
)C
}


=1/
0.693 (R
A
+ 2R
B
) C


=
1.44/ (R
A
+ 2R
B
) C


Where

R
A

and
R
B

are in ohms and C is in Farads.


Note 1:
The output frequency,
f

is independent of the supply voltage
V
cc
.
The
power capability of
R
1

must be greater than.

V
2
/R
A


The duty cycle, the ratio of the time t
c

during which the output is high to the total

time period T
is given as



% duty cycle, D = t
c
/ T * 100 = (R
A

+ R
B
) / (R
A

+ 2R
B
) * 100


From the above equation it is obvious that square wave (50 % duty cycle) output
cannot

be
obtained unless R
A

is made zero. However, there is a danger in shorting resistance R
A

to zero.


Apparatus:


(1)

Two

transistor (C282)

(2)

Capacitors(22,4.7,
3.9,
10nF)

(3)

DC
power supply

(4)

Breadboard

(5)

Resistors(
1,2.2,3.3,
4.7,
5.6,6.8 kΩ)

(6)

Oscilloscope

(7)

Multimeter

(10) Connecting wires


Practical circuit:





Fig 01
: practical

circuit of an astable multivibrator.



An
Improved Practical

Circuit For Astable Multivibrator:




Calculation:


ON time for
TR
1

(
OFF time for T
R
2
)

Here




R
3
=R
4
=R=2.2
KΩ




C
1
=C
2
=C=10nF





t
1
=0.69C
1
R
3



=0.69*10
*10
-
9
*
2.2
*10
3


=15
.18
µs


O
FF time for
TR
1

(
ON time for T
R
2
)







t
2
=0.69C
2
R
4


=0.69*10*10
-
9
*2.2
*10
3


=15.18
µs



T=t
1
+t
2


=0.69(C
1
R
3
+

C
2

R
4
)


=0.69(RC+RC)


=1.38RC


=1.38*2.2*10
3
*10*10
-
9


=30.36
µs



f =1/T=1/1.38RC=1/
30.36

µs
=32.93
kHz


OR




T=t
1
+t
2



=
15.18
µs+
15.18
µs



=30.36
µs





f=1/T

=1/30.36
µs

=
32.93 kHz


Procedure
:


(1)

Firstly we Cheek the transistor, power button of the trainer, calibrate the oscilloscope.

(2)

Biasing voltage is fixed at 12 V.

(3)

Arrange the practi
cal circuit as shown
in fig
-

(01
).

(4)

Then vary

the capacitor when the resistor

is kept fixed and the values are tabulated.

(5)

Then vary the resistor

when the capacitor is kept fixed and the values are tabulated.


Dat
a table
-
01
: Data

for astable
multivibrator when

resistor kept fix
ed(R=
5.6kΩ
)


No
of
obs.

Capacitor

nF

Resistor

kΩ
=
mulse=wi摴h
=

=
T潴al=time
=

=
cre煵ency
=
歨z
=
%⁤=ty=cycle,
=
a=㴠t
c
/ T *
100


=
(R
A
+
R
B
)/
(R
A
+
2R
B
)
* 100



C

R
A



R
B

T
c
(cal)

T
d
(mea)

T
(cal)

T
(mea)

F
(cal)

F
(mea)

c

m


4.7

10

10
0









1

4.7

10
0

10

18.16

15

36.32

32.23





2

34

10

10
0

38.64

30

77.28

64





3

10

10

10
0


85

75.5

170

155







Dat
a table
-
02
: Data

for astable
multivibrator when

capacitor
kept fixed
(C
=
22 nF
)


No
of
obs.

Resistor

kΩ
=
mulse=wi摴h
=

=
T潴al=time
=

=
%⁅rr潲
=
㵻1
-
=

mea/
f
cal
)}*100

={1
-
(T
cal/
T
mea
)}*100

R

t
1(cal)

t
1(mea)

t
2(cal)

t
2(mea)

T
(cal)

T
(mea)

1

4.7

71.34

40

71.34

64

142.68

114

2
4.56

2

2.2

34

28

34

29

68

57

1
9.29

3

5.6

85

81.5

85

79.5

170

161

5.6


Dat
a table
-
03
: Data

for

improved

astable
multivibrator when

resistor kept fixed(R=
3.3
kΩ)


No
of
obs.

Capacitor

nF

Pulse width

µs

Total time

µs

% Error

={1
-

(f
mea/
f
cal
)}*100

={1
-
(T
cal/
T
mea
)}*100

C

t
1(cal)

t
1(mea)

t
2(cal)

t
2(mea)

T
(cal)

T
(mea)

1

4.7

10.7

7

10.7

10

21.4

17

2
5.88

2

10

22.77

17

22.77

18

45.54

35

30.11

3

3.9

8.9

6

8.9

6
.5

17.8

14
.5

22.75


Dat
a table
-
04
: Data

for

improved

astable multivibrator when
capacitor
kept fixed
(C
=
10 nF
)


No
of
obs.

Resistor

kΩ
=
mulse=wi摴h
=

=
T潴al=time
=

=
%⁤=ty=cycle,
=
a=㴠=
c
/ T * 100


=
(R
A
+
R
B
)/
(R
A
+
2R
B
)
* 100

R

t
1(cal)

t
1(mea)

t
2(cal)

t
2(mea)

T
(cal)

T
(mea)

1

5.6

38.64

30

38.64

32

77.28

62

24.64

2

2.2

15.
18

13

15.
18

15.6

30
.
36

28.6

8.42

3

3.3

22.77

18

22.77

17

45.54

3
7.8

20.47


Result:




From the above data

tables we
have found the square wave

according to calculation.

We can see
in the data table
-
01;

where
capacitor, C1
=C
2
=C=10nF, resis
tor,

R
3
=R
4
=R=2.2KΩ
, the calculated
frequency,

f
c
al
=
32
.93 kHz, Total time, T
cal
=30.36µs
but measured

frequency,

f
mea
=
28.6 kHz
,
Total Time

T
mea
=28.6 µs
.This little variation between the calculated and measured values is
occurred
as %

error of
8.42
.

In the experiment the % error is minimum 5.6 and maximum 30.11

and other

outputs are included in data table no
-
01
,

02
, 03

& 04
.


Discussion:


From
the data

table

no. 01 & 03
,

we have seen that when resistor

is fixed and the capacitor is
varied, the measured values of the signals are approximately near to the true values. The
variations is occurred due to many reasons such as any kinds of problem in the instru
ments, eye
estimation problem, di
-
electric loss in capacitor, heating Problem, tolerances of resistors etc.


From
the data

table no. 02 & 04, w
e have also seen that the measured values and true values of
the signals are approximately same. In this t
ime cap
acitor is fixed but resis
tor is varied. These
variations are occurred due to many reasons such as any kinds of problem in the instruments,
eye estimation problem, di
-
electric loss in capacitor, thermal heating, tolerance of resistors etc.


Any how our tabu
lated values are so good. If these variations will be removed

we will get error
less
astable

multivibrator

which is practically impossible.


Precaution
s
:


1.

All parameters and circuit were checked firstly

2.

Whole circuit was arranged tightly and carefully.

3.

Calibrate the oscilloscope

more accurately.

4.

Supply voltage is fixed

at a point and not more than 15
V.

5.

Readings were taken very carefully.















Prepared by








Bhajan Saha


Roll
:
0715012





Sess
: 2007
-
2008







Dept. of AECE.






Islamic University,Kushtia,






Bangladesh
.




Prepared

by






Md.Mushiur Rahman



Roll
-
0715022



Sess:2007
-
2008



Dept. of AEC
E.


Islamic University,Kushtia,




Bangladesh






Prepared by






Sourove Kumar Ray


Roll
-
07150
07


Sess:2007
-
2008



Dept. of AECE.


Islamic University,Kushtia,


Bangladesh






Prepared by


Sharat Chandra barman


Roll
-
0715029


Sess:2007
-
2008



Dept. of AECE.


Islamic University,Kushtia,


Bangladesh


Reference: Websites.