1. Students should be able to :
Objective
Students should be able to :
1 Describe the characteristics of the 3 basic types of multivibrator.
Astable (Free-running)
Monostable (One-shot)
Bistable (Flip-flop)
2 Explain the function of IC 555 in a timer circuit.
3 Identify the following pin functions of the IC 555.
4 Describe the circuit operation and applications of a monostable multivibrator.
5 Calculate the mark and space timing of the astable multivibrator.
6 Calculate the oscillator frequency using the formula:
1.44
(R1 + 2R2)C
fosc =
7 Calculate the time delay of a monostable using the formula t = 1.1 RC

2. Multivibrator A multivibrator is a circuit whose outputs are bistate.
The output may be either a Low or High.
Multivibrators can be divided into three groups:
- Astable multivibrator (Free-running)
- Monostable multivibrator (One shot)
- Bistable multivibrator (Flip-Flop)

3. Types of Multivibrator
Astable
Monostable
Bistable
Click to select
Main Menu

4. 555 pin connection 1 8 2 7 3 6 4 5 Trigger (Pin 2) Threshold (Pin 6)
Control voltage (Pin 5)
Discharge (Pin 7)
Output (Pin 3)
Reset (Pin 4) R 2 7 Q1 R S FF 3 6 R R 4 5 Q2
Ref

5. TIMER 555 1 2 Internal circuitry of a 555 in the monostable mode Vcc
OUTPUT 1 + _ 1 2 3
S Q R 2 3
INPUT 1 3 1 + _ 2
Internal circuitry of a 555 in the monostable mode
When power is applied initially
Comparator 2 output is driven LOW by positive HIGH at the trigger input.
Comparator 1 output is driven HIGH by the voltage charged across the
external capacitor.
S = 1 , R = 0, Q = 1 and output is LOW.

6. 1 2 Vcc S Q R When the transistor is ON
OUTPUT 1 + _ 1 2
S Q R 2 3
INPUT 1 3 1 + _ 2
When the transistor is ON
This set the flip-flop, Q=1 and the transistor is ON.
The capacitor then discharges to the ground through the transistor and causes the output of comparator 1 to go LOW.
The flip-flop will now remain set because both inputs are LOW (unchanged state).
The output will maintain at this stable state until a LOW trigger inputs is received.

7. 1 2 Vcc S Q R When a Low trigger input is applied at pin 2
OUTPUT 1 + _ 1 2 3
S Q R 2 3
INPUT 1 3 1 + _ 2
When a Low trigger input is applied at pin 2
Inverting input of comparator 2 from pin 2 will go LOW.
This is lower than the non inverting input which is 1/3Vcc.
The output of comparator 2 is driven High.
S = 0, R = 1, Q = 0, and it turns OFF the transistor.
The circuit is now in its unstable state, output is logic ‘1’.

8. 1 2 Vcc S Q R When the transistor is OFF
OUTPUT 1 + _ 1 2 3
S Q R 2 3
INPUT 1 3 1 + _ 2
When the transistor is OFF
Input has returned to HIGH, Logic “1” .
The external capacitor will charge through resistor R.
When the voltage across the capacitor exceeds 2/3Vcc, the
output of comparator 1 is driven High.
Therefore S = 1, R = 0 and this sets the flip-flop. Q = 1.
The circuit returns to its stable state, output is logic ‘0’.

9. The Astable MultivibratorC 6 2 1 5 3 8 7
555
Output
Vcc
t t2
t1 = ( R1 + R2 )C
t2 = R2C
Formula for calculating the oscillator frequency :
1.44
(R1 + 2R2)C
fosc =

10. = 0.693 (4 K + 100K) 0.001µf Output t2 t1 = 0.693 (100K) 0.001µf
Example : To calculate the Mark , Space and the frequency of oscillations of the
output waveform. Given that C = 0.001µf, R1 = 4 K and R2 = 100K.
Solution :
Mark, t1 = (R1 + R2)C
= (4 K + 100K) 0.001µf
= ( ) x 10-6
= s
= 72.1µs
Vcc
Output t1 t2
Space, t2 = R2C
= (100K) 0.001µf
= ( ) x 10-6
= s
= 69.3µs
fosc
Frequency, fosc = =
= 7059Hz )C 2R (R
1.44 2 1 +
) x 10-6
(4000

11. Monostable Multivibrator
Example : To calculate the duration of the quasi-stable state of the monostable multivibrator. Given that C = 0.001µf and R = 10K.
Solution :
Monostable
Multivibrator
Circuit
Input
Output t
T = RC
= 1.1(10 K x 0.001µf )
= 1.1(10 x 103 ) ( x 10-6f )
= s
= 11µs

12. THE END 6

13. The Astable Multivibrator
It is a free-running oscillator.
It output a rectangular wave and does not requires any trigger input.
• Application as oscillators, square-wave generators etc.
Output
Input
Astable
Multivibrator
Circuit
None 4

14. The Astable Multivibrator
It is a free-running oscillator.
It output a rectangular wave and does not requires any trigger input.
• Application as oscillators, square-wave generators etc.
Output
Input
Astable
Multivibrator
Circuit
None 4

15. The Astable Multivibrator
It is a free-running oscillator.
It output a rectangular wave and does not requires any trigger input.
• Application as oscillators, square-wave generators etc.
Output
Input
Astable
Multivibrator
Circuit
None
Click to return
to Main menu 4

16. Monostable Multivibrator
It is also known as one-shot multivibrator.
Is stable in only one of two voltage levels.
When a trigger is received it switches to the other state for a period of time (t ) and then returns to the stable state.
Application as a timer.
Input
Monostable
Multivibrator
Circuit
Output t
Click to return
to Main menu 5

17. Bistable Multivibrator
It is generally known as flip-flop.
It has two inputs.
It has two stables output states and will stay in either one indefinitely.
When a trigger input is received it will switch to the other state and remain in that state until the next trigger input is received.
Application as a digital storage register.
Input
Bistable
Multivibrator
Circuit
Output T1 T1 6

18. Bistable Multivibrator
It is generally known as flip-flop.
It has two inputs.
It has two stables output states and will stay in either one indefinitely.
When a trigger input is received it will switch to the other state and remain in that state until the next trigger input is received.
Application as a digital storage register.
Input
Bistable
Multivibrator
Circuit
Output T2 T2
Click to return
to Main menu 6