2. Chapter 17 - Monostable Operation of the 555 IC Timer
Lesson Objectives
Upon completion of this topic, you should be able to:
Students should be able to describe and verify the
circuit operation and applications of a monostable
multivibrator.
Calculate the pulse width of a monostable multivibrator
circuit.
IT2001PA Engineering Essentials (2/2) 2
3. Chapter 17 - Monostable Operation of the 555 IC Timer
Specific Objectives
Students should be able to :
Draw the circuit of a monostable multivibrator using IC
555.
Explain the operation of the monostable multivibrator.
State the applications of monostable multivibrator.
IT2001PA Engineering Essentials (2/2)
4. Chapter 17 - Monostable Operation of the 555 IC Timer
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 Circuit Output
Monostable
Multivibrator t
IT2001PA Engineering Essentials (2/2) 4
5. Chapter 17 - Monostable Operation of the 555 IC Timer
555 as Monostable Multivibrator
+V Trigger
input
R Vcc
8 Output 1
Vcc
7 3 3
C 6
555 Vo
tp
Vcc Output
Trigger 5 Vout
input 2
1 0.01µF
0 t
t = 1.1RC
Duration of output pulse at HIGH state, t = 1.1 RC
IT2001PA Engineering Essentials (2/2) 5
6. Chapter 17 - Monostable Operation of the 555 IC Timer
Example : To calculate the duration of the quasi-stable state of the
monostable multivibrator. Given that C = 0.001µf and R = 10KΩ.
Solution :
Input Circuit Output
Monostable
Multivibrator t
T = 1.1 RC
= 1.1(10 KΩ x 0.001µf )
= 1.1(10 x 103Ω ) ( 0.001 x 10-6f )
= 0.000 011s
= 11µs
IT2001PA Engineering Essentials (2/2) 6
7. Chapter 17 - Monostable Operation of the 555 IC Timer
Internal Circuitry of a 555 in the Monostable Mode
Vcc
R
C OUTPUT
1
+
3
_
1 S Q 0
2
3
INPUT
1
3
1 +
Vcc 2 R
0
2 _
Vcc
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.
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8. Chapter 17 - Monostable Operation of the 555 IC Timer
Vcc
R
C OUTPUT
1
+
_
1 S Q 0
2
3
INPUT
1
3 +
1
Vcc 2 R
0
2 _
When the transistor is ON Vcc
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 remain at this stable state until a Low trigger inputs is
received.
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9. Chapter 17 - Monostable Operation of the 555 IC Timer
Vcc
R
C OUTPUT
1
+
3
_
1 S Q 0
2
3
INPUT
1
3
1 Vcc +
0
2 _ 2 R
Vcc
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’.
IT2001PA Engineering Essentials (2/2) 9
10. Chapter 17 - Monostable Operation of the 555 IC Timer
Vcc
R
C OUTPUT
1
+
3
_
1 S Q 0
2
3
INPUT
1
3
1 +
0
2 Vcc
_ 2 R
Vcc
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’.
IT2001PA Engineering Essentials (2/2) 10
