PowerPoint Presentation on using IC 555 Timer as an Astable Multi vibrator. Working of the astable multi vibrator, advantages and disadvantages of an Astable Multi-vibrator,Input and Output Pins of 555 IC, Formulae for calculating the charge and discharge time and cycle time of the astable multi vibrator.

1. IC 555 Timer as Astable
Multivibrator
Ummiya Mohammedi

2. IC Timer 555
• The 555 timer IC is an integrated circuit (chip) used
in a variety of timer, pulse generation,
and oscillator applications.
• The 555 can be used to provide time delays, as
an oscillator, and as a flip-flop element
• Introduced in 1971 by American company Signetics,
the 555 is still in widespread use due to its ease of
use, low price, and stability. It is now made by many
companies in the original bipolar and also in low-
power CMOS types. As of 2003, it was estimated that
1 billion units are manufactured every year.

3. • It is called IC 555 timer as three 5k resistors
are connected to form a voltage divider
network.

4. IC Timer 555

5. 555 Input and Output Pins

6. FREE RUNNING OR ASTABLE MULTIVIBRATOR

7. Circuit Explanation
The circuit consists of:
Comparator A
Comparator B
3 Resistors of 5k ohms each
Capacitor of 0.01µF

8. 555 IC Timer Astable Multivibrator Functional Block
Diagram

9. WORKING
• 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 astable multivibrator generates a square
wave, the period of which is determined by
the circuit external to IC 555.

10. CIRCUIT

11. Let t1 and t2 represent the charge and discharge times of the astable
multivibrator respectively.
were RA and RB are in ohms and C is in Farad.
Astable Multivibrator Charge and Discharge Times

12. Let T be the cycle time of the astable multivibrator.
Astable Multivibrator Cycle Time

13. The duty cycle is defined as the ratio of charge time
constant (t1) to the total cycle time (T) and for the
astable oscillator it is found to be:
Astable Multivibrator Duty Cycle Time

14. ADVANTAGES
• The simple operation and less number of
external components make the astable
multivibrator using 555 IC a popular one.
• The perfect square wave output from the
circuit makes it good for many applications.
• Easy to design, low cost and no external
triggering are the other advantages.

15. DISADVANTAGES
• The disadvantage is that we cannot achieve a
duty cycle less than or equal to 50% using this
circuit.
• This problem can be solved by inverting the
output or by using a diode parallel to RB.

16. THANK YOU FOR YOUR ATTENTION