1. To Design and Implement Astable Multivibrator Circuit Using IC555
LAB:- CEII (SEM V EXTC)

2. AIM
To Design and Study the operation of astable multivibrator using IC555 Timer
LAB:- CEII (SEM V EXTC)

3. Apparatus 555 Timer IC Resistor ( Values as per design)
Capacitors (Values as per Design)
Function Generator
CRO
Bread Board
Connecting Wires and Probes
LAB:- CEII (SEM V EXTC)

4. Functional Block Diagram of IC555:-
Theory
Functional Block Diagram of IC555:-
Reference:-
LAB:- CEII (SEM V EXTC)

5. Functional Block Diagram of IC555
Timer IC consists of two comparators
A resistive voltage divider generating the required reference voltage for the two comparator
SR flip-flop
Output Buffer
Discharging Transistor T1
LAB:- CEII (SEM V EXTC)

6. Multivibrators
It is a circuit which can have no, one or two stable states of operation.
Types :-
Astable Multivibrator:- No stable state
Monostable Multivibrator:- One stable state
Bistable Multivibrator :- Two stable states
LAB:- CEII (SEM V EXTC)

7. Astable Multivibrator using IC555
Fig 1. :Astable Multivibrator using IC555
Reference:-
LAB:- CEII (SEM V EXTC)

8. Astable Multivibrator Working
The astable multivibrator is also called as free running multivibrator and does not require an external trigger for changing the state
The time constant of the externally connected two resistors(R1 and R2) and a capacitor C to IC 555 determines the ON and OFF time of output waveform.
LAB:- CEII (SEM V EXTC)

9. Astable Multivibrator Working
When the circuit is switched ON, the capacitor (C) voltage will be less than 1/3 Vcc. So the output of the lower comparator will be HIGH and of the higher comparator will be LOW. This SETs the output of the SR Flip-flop.
Thus the discharging transistor will be OFF and the capacitor C starts charging from Vcc through resistor R1 & R2.
When the capacitor voltage will become greater than 1/3 Vcc ( less than 2/3 Vcc ), the output of both comparators will be LOW and the output of SR Flip-flop will be same as the previous condition. Thus the capacitor continues to charge.
LAB:- CEII (SEM V EXTC)

10. Waveforms
Reference:-
LAB:- CEII (SEM V EXTC)

11. Design Equations Ton = 0.69(R1+R2) C Toff = 0.69R2C
Ttotal= Ton + Toff = 0.69(R1+2R2)C
Frequency of oscillation = f= 1 / Ttotal
= 1.44 / (R1+2R2)C
Duty Cycle= Ton / Ttotal
LAB:- CEII (SEM V EXTC)

12. Design Statement for Implementation
Design an astable multivibrator using IC555 for f=5KHz and duty cycle= 50% Soln:- f= 1 / Ttotal = 1.44 / (R1+2R2)C Let C= 0.047uF Hence R1+R2 = 6128Ὠ D= R1/ (R1 + R2) Hence R1 = 0.5 *6128 = 3064Ὠ R2= = 3064Ὠ Thus the components values are:- C= 0.047uF, R1= =R2 = 3064Ὠ
LAB:- CEII (SEM V EXTC)

13. Procedure
1. Mount the circuit as per design on bread board. 2. Apply Vcc= 5V 3. Observe the output and capacitor voltage waveforms on CRO. 4. Compare the obtained results with designed values.
LAB:- CEII (SEM V EXTC)

14. Observation
Map out the time period of the output wave form and compare it with the given one.
Parameter
Designed
Observed
Ton
Toff
LAB:- CEII (SEM V EXTC)

15. Results and Discussion
The waveform is observed and verified with required condition.
LAB:- CEII (SEM V EXTC)

16. Questions What is the working principle of astable Multivibrator?
Why the name is given as astable?
LAB:- CEII (SEM V EXTC)