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Apr 11, 2023
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IT2001PAEngineering Essentials (2/2)
Chapter 17 - Monostable Operation of the 555 IC Timer
2
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
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.
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
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.
4
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
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.
Output
MonostableMultivibrator
CircuitInput
t
5
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
555 as Monostable Multivibrator
555
R
C
7
6
2
83
5
1 0.01F
Output
+V
t = 1.1RC
Duration of output pulse at HIGH state, t = 1.1 RC
Trigger input
Vout
Vo
Vcc
tp
0 t
Vcc
Vcc13
Trigger input
Output
6
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
Example : To calculate the duration of the quasi-stable state of the monostable multivibrator. Given that C = 0.001µf and R = 10K.
T = 1.1 RC = 1.1(10 K x 0.001µf ) = 1.1(10 x 103 ) ( 0.001 x 10-6f ) = 0.000 011s = 11µs
Solution :
Output
MonostableMultivibrator
CircuitInput
t
7
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
Internal Circuitry of a 555 in the Monostable Mode
When power is applied initiallyComparator 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.
+
_S Q
R+
_
R
C
3
2
1
2
Vcc
Vcc
Vcc
13
23
1
0
1
0
OUTPUT
INPUT
8
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
+
_S Q
R+
_
R
C
2
1
2
Vcc
Vcc
Vcc
13
23
1
0
1
0
OUTPUT
INPUT
When the transistor is ONThis 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.
9
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
+
_S Q
R+
_
R
C
3
2
1
2
Vcc
Vcc
Vcc
13
23
1
0
1
0
OUTPUT
INPUT
When a Low trigger input is applied at pin 2Inverting 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’.
10
Chapter 17 - Monostable Operation of the 555 IC Timer
IT2001PA Engineering Essentials (2/2)
+
_S Q
R+
_
R
C
3
2
1
2
Vcc
Vcc
Vcc
13
23
1
0
1
0
OUTPUT
INPUT
When the transistor is OFFInput 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’.
