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Apr 11, 2023
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IT2001PAEngineering Essentials (2/2)
Chapter 15 - Multivibrator
2
Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Lesson Objectives
Upon completion of this topic, you should be able to: Explain and demonstrate the function of IC555 in a
timer circuit.
Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Specific Objectives
Students should be able to : Describe the characteristics of the 3 basic types of multivibrator :
Astable (free running) Monostable (one shot) and Bistable (flip flop)
State their applications. Identify the IC 555 package. Identify the following pin functions of the IC 555 :
Trigger (Pin 2), Threshold (Pin 6), Control voltage (Pin 5), Discharge (Pin 7), Output (Pin 3) and Reset (Pin 4)
4
Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
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)
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Astable Multivibrator
ASTABLEO/P
INPUT NO INPUT
OUTPUT
Free-running oscillator that
outputs a rectangular wave and
requires no trigger input.
E.g.: as oscillators,
square-wave generators.
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Bistable Multivibrator
BISTABLEO/PI/P
INPUT
OUTPUT
Has two stable states and will
• stay in either one indefinitely.
E.g.: as digital storage register.
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Mono Stable Multivibrator
MONO STABLE
O/PI/P
Stable in only one of two voltage levels.
E.g.: as a timer. INPUT
OUTPUT
8
Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
555 IC Timer
Two modes of operation; i.e. :
• Astable (free-running) multivibrator
• Monostable (one-shot) multivibrator.
9
Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
Free-running multivibrator
Output oscillators between high and low.
The frequency of which is determined by
• an external resistor-capacitor network.
555 Free-
running mode
Vo
Vo
Vcc
Output High
Output wave form
Output Low
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
One-shot Multivibrator
Output voltage is low until a
• negative-going trigger pulse is applied to the timer.
• Then output switches to high.
555 One-shot
mode Vo
Vo
Vcc
Time output is high is determined by the circuit, not input pulse
Output wave form
Ei Negative-going trigger pulse
A
Ei
t0A0 t
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
555 Pin Connection
1
2
3
4
8
7
6
5
R
SFF
Ref
Q2
R
R
R
Q1
Trigger (Pin 2)
Threshold (Pin 6)
Control voltage (Pin 5)
Discharge (Pin 7)
Output (Pin 3)
Reset (Pin 4)
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Chapter 15 - Multivibrator
IT2001PA Engineering Essentials (2/2)
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