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