its all about astable multivibrators

1. *IC555 ARCHITECTURE
AND WORKING
GROUP 03 DIVISION P

2. *
1. WHAT IS A MULTIVIBRATOR?
2. ASTABLE MULTIVIBRATOR?
3. ASTABLE MULTIVIBRATOR CIRCUIT?
4. WORKING OF ASTABLE MULTIVIBRATOR?
5. ADVANTAGES OF ASTABLE MULTIVIBRATOR?
6. DISADVANTAGES OF ASTABLE MULTIVIBRATOR?
7. PRIMARY USES OF ASTABLE MULTIVIBRATOR?
8. APPLICATIONS OF ASTABLE MULTIVIBRATOR?

3. *WHAT IS A
MULTIVIBRATOR?
• A multivibrator is an electronic circuit used
to implement a variety of simple two-state
devices such as relaxation oscillators,
timers, and flip-flops.
• The first multivibrator circuit, the astable
multivibrator oscillator, was invented by
Henri Abraham and Eugene Bloch in World
War 1.

4. *ASTABLE MULTIVIBRATOR
 Astable Multivibrators are free running oscillators
which oscillate between two states continually
producing two square wave output waveforms.
 An Astable Multivibrator, unlike the Monostable
Multivibrator or the Bistable Multivibrator, has
automatic built in triggering which switches it
continuously between its two unstable states both
set and reset.

5. *
Here you can see the simple astable multivibrator circuit using Bipolar Junction Transistor(BJT)

6. *
Here you can see the astable multivibrator circuit using IC 555. Here the duty cycle of the output pulses
can be varied by varying the resistance R1

7. *Astable multivibrator
circuit using ic 555
555 timer IC in an Astable mode can be used to produce a very stable 555
Oscillator circuit for generating highly accurate free running waveforms whose
output frequency can be adjusted by means of an externally connected RC tank
circuit consisting of just two resistors and a capacitor.
Whereas the 555 monostable circuit stopped after a preset time waiting for the
next trigger pulse to start over again, in order to get the 555 Oscillator to operate
as an astable multivibrator it is necessary to continuously re-trigger the 555 IC
after each and every timing cycle.
This re-triggering is basically achieved by connecting the trigger input (pin 2) and
the threshold input (pin 6) together, thereby allowing the device to act as an
astable oscillator. Then the 555 Oscillator has no stable states as it continuously
switches from one state to the other. Also the single timing resistor of the previous
monostable multivibrator circuit has been split into two separate resistors
PRESENTATION TITLE 7

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PRESENTATION TITLE 8
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10. *Astable multivibrator
circuit using ic 555
When connected as an astable multivibrator,
the output from the 555 Oscillator will
continue indefinitely charging and discharging
between 2/3Vcc and 1/3Vcc until the power
supply is removed. As with the monostable
multivibrator these charge and discharge times
and therefore the frequency are independent
on the supply voltage.
The duration of one full timing cycle is
therefore equal to the sum of the two
individual times that the capacitor charges and
discharges added together and is given as:
555 Oscillator Cycle Time
PRESENTATION TITLE 10
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By altering the time constant of just one of the RC combinations, the Duty
Cycle better known as the “Mark-to-Space” ratio of the output waveform can
be accurately set and is given as the ratio of resistor R2 to resistor R1. The
Duty Cycle for the 555 Oscillator, which is the ratio of the “ON” time divided
by the “OFF” time is given by:
555 Oscillator Duty Cycle

12. *Astable multivibrator
circuit using ic 555
The duty cycle has no units as it is a
ratio but can be expressed as a
percentage ( % ). If both timing
resistors, R1 and R2 are equal in value,
then the output duty cycle will be 2:1
that is, 66% ON time and 33% OFF time
with respect to the period.
PRESENTATION TITLE 12
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13. *Working of an astable
multivibrator
• Multivibrators are another form of oscillators.
Oscillator is an electronic circuit that is capable of
maintaining AC signal at the output. It can generate a
square wave, ramp or pulse waveforms.
* 1. The loop phase shift
must be 0 degrees or 360
degree
• It can generate a square wave, ramp or pulse waveforms. For
oscillation, the oscillator must fulfill two Barkhausen
conditions:
*2. The loop gain must be
slightly greater than unity

14. *WORKING OF AN ASTABLE
MULTIVIBRATOR
• To fulfill both the conditions
the oscillator must have some
form of amplifier and a
portion of its output must be
fed back re-generatively to
the input.
• A simple oscillator can
produce sine wave but
it cannot generate a
square wave. The
square wave can be
generated by use of
multivibrator.
• If the gain of the amplifier
is less than unity, the
circuit will not
oscillate and if it is greater
than unity, the circuit will
be over driven and
produce a distorted
waveform.
• A multivibrator is a
form of the oscillator
which has two stages
due to which we can
get output by either of
the states. It is
basically two amplifier
circuits arranged with
regenerative feedback.

15. *WORKING OF AN ASTABLE
MULTIVIBRATOR
• A multivibrator is a form of the oscillator which has
two stages due to which we can get output by either
of the states. It is basically two amplifier circuits
arranged with regenerative feedback. In this, neither
of the transistors conducts simultaneously.
• At a time, only one transistor
conducts and another one is
in off state. Some circuits
have certain states; the state
which has fast transition is
called switching
processes, where there is
fast change of current as well
as voltage. This switching is
termed as triggering. Hence,
we can trigger the circuit
internally or externally.

16. 1. They are very simple in
design.
*3. They are very easy to
construct with a very low cost.
2. They are very reliable.
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4. They do not need any
external triggering.
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5. They can run continuously for a very large amount of time without
any problem.

17. *Disadvantages of astable
multivibrator
The continuous pulse generation by this
multivibrator circuit depends upon the positive
feedback. So due to the resistance in the
circuit, this multivibrator cannot transfer the
whole output signal to the input. So a small
amplifier circuit is used with this multivibrator
circuit which restores the lost energy.
PRESENTATION TITLE 17
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18. * PRIMARY USES OF ASTABLE
MULTIVIBRATOR
• Astable multivibrators are used to generate
square waves.
• In a computer system, it serves as a timing
oscillator or clock.
• It can also be used for circuits that
flashlights, switch, and supply electricity.

19. * PRIMARY USES
OF ASTABLE MULTIVIBRATOR
• They are used as a Frequency dividers
• Used as a saw tooth generators
• They are used as wave and pulse generators
• They are used as standard frequency source
• They are used in radar and tv circuits
• They are also used as a memory elements in
computer

20. *APPLICATIONS OF AN
ASTABLE MULTIVIBRATOR
• The monostable multivibrator is used
as delay and timing circuits.
• It is also used for temporary
memories.
• It is often used to trigger another
pulse generator.
• It is used for regenerating and worn
out pulses.
• It is a source of production of a
harmonic frequency of higher order.

21. *THANK YOU
Shlok Purohit
Viahal Purohit
Pushkar Kamble
Pushpinder
Purva Sarvade
Amey pusegaonkar