IC 555 Timer - Monostable and Astable Multivibrator Circuits

Prerequisite: Study the operation of Monostable and Astable Multivibrator.

Objective:

1. To design a monostable multivibrator for a required pulse width using 555 timer.
2. To design astable multivibrator for a given frequency using 555 timer.

Apparatus:

Bread board	1
CRO (20MHz)	1
IC 555	1
Resistors
47K	2
2.2K	1
10K	1
100K	1
Capacitors
0.01μF	2
0.1μF	2
PN Diode 1N4007	1
RPS	1
Function generator (10 Hz-20MHz)	1

Design:

Monostable Multivibrator:

T = 1.1R₁C₁

Let C₁ = 0.01µF & R₁ = 47K then T = 0.3ms(approx)

Astable Multivibrator:

For 55% duty cycle choose R_B = 10K

T_HIGH = T_c = 0.693 (R_A + R_B)C

T_LOW = T_d = 0.693 R_BC

 T = T_HIGH + T_LOW = 0.693 (R_A + 2R_B)C 

f = 1/T = 1.44/ (R_A + 2R_B)C

% duty cycle, D = Tc / T * 100 = (R_A + R_B) / (R_A + 2R_B) * 100

Circuit diagrams:

Monostable Multivibrator:

Triggering Circuit

Fig 1. Monostable Multivibrator

Astable Multivibrator:

Fig 2. Astable Multivibrator

Procedure:

Monostable Multivibrator:

1. Connect the circuit as shown in the circuit diagram Fig 1.
2. Apply Negative triggering pulses of frequency 1 KHz at pin 2.
3. Observe the output waveform at pin 3 and measure capacitor voltage across it at pin 6.
4. Theoretically calculate the pulse duration as T = 1.1R₁C₁
5. Compare it with experimental values.
6. Plot the graph for the input and output waveforms.

Astable Multivibrator:

1. Connect the circuit as shown in the circuit diagram Fig 2.
2. Observe the output waveform at pin 3 and measure capacitor voltage across it at pin 6.
3. Theoretically calculate the Time period as T = 0.69 R_BC + 0.69 (R_A + R_B)C.
4. Compare it with experimental values.
5. Plot the graph for the input and output waveforms.

Model Wave Forms:

Monostable Multivibrator:

Waveforms at Capacitor and monostable output

Astable Multivibrator:

Result: Designed and verified the waveforms of monostable multivibrator and Astable multivibrator using 555 Timer.

Outcome: After conducting this experiment, students are able to design the monostable multivibrator and Astable multivibrator using IC 555 Timer.

VIVA Questions:

1. Define duty cycle?

Ans: The ratio of the time of the high output to the time of the total output is the duty cycle

2. Draw the pin diagram of 555 timers?

Ans:

3. What are the applications of 555 timers in monostable mode?

Ans: Missing pulse detector, Frequency divider, Pulse width modulation etc.      

4. Explain capacitor output waveform in monostable mode?

Ans: Tin stable state, FF output is zero and the discharge transistor Qd is ON and the capacitor voltage is zero through Qd. Whenever a trigger is given, FF goes to high level, Qd is OFF and the Capacitor charges through R till it reaches 2/3 V_cc which makes the FF output to zero and the process repeats. The charging time of the capacitor from zero to 2/3 V_cc is the time of the pulse width. 

5. Write down the expression for output pulse width in monostable mode?

Ans: T = 1.1R₁C₁

6. Why the number has come for 555 IC as 555?

Ans: It has three 5K Resistors at the input of the comparators to get 2/3 V_cc and 1/3 V_cc, hence the name came as 555.

7. Write down the expression for output pulse width in Astable mode?

Ans: T = 0.69(R_A + 2R_B)C , T_ON = 0.69(R_A + R_B)C, T_OFF = 0.69R_BC

8. What are the applications of 555 timers in Astable mode?

Ans: Square wave generator, FSK generator, pulse position modulator etc.

9. What is a quasi stable state and what is a steady state?

Ans: A quasi stable state is also known as temporary state, where the output stays in this state only for a fixed time and goes back to the stable state after that time period. A stable state is a state where the output will be in that state until unless an external trigger is given.

10. What are the other names for the Monostable Multivibrator and Astable Multivibrator?

Ans:

Monostable Multivibrator: pulse width generator, one shot, delay generator.

Astable Multivibrator: free running oscillator, clock signal, square wave generator.