Name of Experiment: To Construct and Test Astable Multivibrator Using 555 timer IC.
Objectives:
After completing this experiment we would able to learn:
1. What is a 555 timer IC.
2. What is an astable Multivibrator.
3. How 555 timer IC works as an astable multivibrator.
4. How it produces square wave.
5. What is the role of capacitor 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 Philips) 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 circuit 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.
Mono-stable, in which one of the states is stable, but the other state is unstable
(transient). A trigger causes the circuit 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.
Bi-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. Timer
(b) Pinout 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 VCC.
3 OUT This output is driven to +VCC 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 VCC).
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+, VCC Positive supply voltage is usually between 3 and 15 V.
For 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 (VCC)
4.5 to 15 V
Supply current (VCC = +5 V)
3 to 6 mA
Supply current (VCC = +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 provide 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:
Fig(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 trigger 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 Vcc through RA and RB. However as soon as the voltage across the capacitor
equals 2/3 Vcc , comparator1 triggers the flip-flop and the output switches to low
state. Now capacitor C discharges through RB and the transistor Q1. When voltage across C
equals 1/3 Vcc, comparator 2’s output triggers the flip-flop and the output goes high. Then the
cycle repeats.
Astable Multivibrator-Design method using 555 IC:
The time during which the capacitor C charges from 1/3 VCC to 2/3 VCC is equal to the time the
output is high and is given as tc or THIGH = 0.693 (RA + RB) C, which is proved below.
Voltage across the capacitor at any instant during charging period is given as, vc =VCC (1-e-t/RC)
The time (t1) taken by the capacitor to charge from 0 to +1/3 VCC
or
or
or
or
or
1/3Vcc=Vcc (1-e-t/RC)
e-t/RC=(1-1/3)
e-t/RC=2/3
et/RC=3/2
t1=ln(3/2)RC
where t=t1 & vc=1/3Vcc
t1=0.405RC
The time (t2) taken by the capacitor to charge from 0 to +2/3 VCC
2/3Vcc=Vcc (1-e-t/RC)
or e-t/RC=(1-2/3)
or e-t/RC=1/3
or et/RC=3
or t2=ln(3)RC
where t=t2 & vc=2/3Vcc
or t2 = loge 3 RC = 1.0986 RC
So the time taken by the capacitor to charge from +1/3 VCC to +2/3 VCC
tc = (t2 – t1) = ln(3)-ln(3/2)= ln(3*2/3)RC = ln(2)RC=0.693 RC
Substituting R = (RA + RB) in above equation we have
THIGH = tc = 0.693 (RA + RB) C
The time during which the capacitor discharges from +2/3 VCC to +1/3 VCC is equal to the time
the output is low and is given as td or TL0W = 0.693 RB C,
The above equation is worked out as follows: Voltage across the capacitor at any instant during
discharging period is given as vc = 2/3 VCC e- t/ RBC
Substituting vc = 1/3 VCC in above equation we have
+1/3 VCC = +2/3 VCC e- td/ RBC
or e-t/RBC=1/2
or et/RBC=2
or td =ln(2)RBC where t = td
or td = 0.693 RBC
Overall period of oscillations, T = Tc + Td = ln(2)* (RA+ 2RB)C = 0.693 (RA+ 2RB)C, The
frequency of oscillations being the reciprocal of the overall period of oscillations T is given as
f = 1/T
= 1/{ln(2)* (RA+ 2RB)C}
=1/0.693 (RA+ 2RB) C
=1.44/ (RA+ 2RB) C
Where RA and RB are in ohms and C is in Farads.
Note 1: The output frequency, f is independent of the supply voltage Vcc.The power capability of
R1 must be greater than. V2 /RA
The duty cycle, the ratio of the time tc during which the output is high to the total time period T
is given as
% duty cycle, D = tc / T * 100 = (RA + RB) / (RA + 2RB) * 100
From the above equation it is obvious that square wave (50 % duty cycle) output cannot be
obtained unless RA 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 TR1 (OFF time for TR2)
Here
R3=R4=R=2.2KΩ
C1=C2=C=10nF
t1=0.69C1R3
=0.69*10*10-9*2.2*103
=15.18µs
OFF time for TR1 (ON time for TR2)
t2=0.69C2R4
=0.69*10*10-9*2.2*103
=15.18µs
T=t1+t2
=0.69(C1R3+ C2 R4)
=0.69(RC+RC)
=1.38RC
=1.38*2.2*103*10*10-9
=30.36µs
f =1/T=1/1.38RC=1/30.36 µs=32.93kHz
OR
T=t1+t2
=15.18µs+15.18µs
=30.36µs
f=1/T =1/30.36µs =32.93 kHz
Procedure:
(1)
(2)
(3)
(4)
(5)
Firstly we Cheek the transistor, power button of the trainer, calibrate the oscilloscope.
Biasing voltage is fixed at 12 V.
Arrange the practical circuit as shown in fig - (01).
Then vary the capacitor when the resistor is kept fixed and the values are tabulated.
Then vary the resistor when the capacitor is kept fixed and the values are tabulated.
Data table-01: Data for astable multivibrator when resistor kept fixed(R=5.6kΩ)
No Capacitor Resistor
of
nF
kΩ
obs.
Pulse width
µs
Total time
µs
Frequency
khz
% duty cycle,
D = tc / T *
100
=(RA+RB)/(RA+2R
B)*
100
C
RA
RB
Tc(cal)
Td(mea)
T(cal)
T(mea)
4.7
10
10
0
1
4.7
2
34
10
0
10
10
18.16
15
36.32
32.23
10
0
10
0
38.64
30
77.28
64
3
10
10
85
75.5
170
155
F(cal)
F(mea)
c
m
Data table-02: Data for astable multivibrator when capacitor kept fixed(C=22 nF)
No
of
obs.
Resistor
kΩ
R
Pulse width
µs
t1(cal)
t1(mea)
t2(mea)
Total time
µs
T(cal)
T(mea)
% Error
={1- (fmea/fcal)}*100
={1-(Tcal/Tmea)}*100
t2(cal)
1
4.7
71.34
40
71.34
64
142.68
114
24.56
2
2.2
34
28
34
29
68
57
19.29
3
5.6
85
81.5
85
79.5
170
161
5.6
Data table-03: Data for improved astable multivibrator when resistor kept fixed(R=3.3kΩ)
No
of
obs.
Capacitor Pulse width
nF
µs
C
t1(cal)
t1(mea)
t2(mea)
Total time
µs
T(cal)
T(mea)
% Error
={1- (fmea/fcal)}*100
={1-(Tcal/Tmea)}*100
t2(cal)
1
4.7
10.7
7
10.7
10
21.4
17
25.88
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
Data table-04: Data for improved astable multivibrator when capacitor kept fixed(C=10 nF)
No
of
obs.
Resistor
kΩ
R
Pulse width
µs
t1(cal)
t1(mea)
t2(cal)
t2(mea)
Total time
µs
T(cal)
T(mea)
% duty cycle,
D = tc / T * 100
1
5.6
38.64
30
38.64
32
77.28
62
=(RA+RB)/(RA+2RB)* 100
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
37.8
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=C2=C=10nF, resistor, R3=R4=R=2.2KΩ, the calculated
frequency, fcal=32.93 kHz, Total time, Tcal=30.36µs but measured frequency, fmea=28.6 kHz,
Total Time Tmea=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 instruments, eye
estimation problem, di-electric loss in capacitor, heating Problem, tolerances of resistors etc.
From the data table no. 02 & 04, we have also seen that the measured values and true values of
the signals are approximately same. In this time capacitor is fixed but resistor 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 tabulated values are so good. If these variations will be removed we will get error
less astable multivibrator which is practically impossible.
Precautions:
1.
2.
3.
4.
5.
All parameters and circuit were checked firstly
Whole circuit was arranged tightly and carefully.
Calibrate the oscilloscope more accurately.
Supply voltage is fixed at a point and not more than 15V.
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 AECE.
Islamic University,Kushtia,
Bangladesh
Prepared by
Sourove Kumar Ray
Roll-0715007
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.