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COURSE: DIGITALELECTRONICS

BATCH: 12ELECTRONICSDEPARTMENT OF ELECTRONICS

MEHRAN UNIVERSITY OF ENGINEERING &

TECHNOLOGY, PAKISTAN

SubjectTeacher:KehkashanAsma

AssistantProfessor

DeptofElectronics

MUET,PK

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This lecture

Multivibrators Astable

Monostable

Bistable

555 timer IC

Monostable operation of 555 timer

Problems

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Multivibrators (MV)

These devices are very useful as pulsegenerating, storing & counting circuits

There are three types of multivibrators.

A stable Mono stable

Bi-stable

These multivibrators are distinguished by

the type of stable logic conditions at theiroutput.

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ECE 3450

M. A. Jupina,

VU, 2010

Multivibrator CircuitsAstable Multivibrator (Clocks or Oscillators)

Transistor circuit Schmitt-Trigger Inverter (7414)

555 Timer

Crystal Oscillator

Monostable Multivibrator (One-Shots) Transistor circuit

555 Timer

Non-Retriggerable (74121)

Retriggerable (74123)

VHDL CodingBistable Multivibrator (Flip-Flops) Transistor circuit

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MV using transistor circuit

A MV is basically two stage amplifier withpositive feedback from the output of oneamplifier to the input of the other.

This feedback is supplied in such amanner that one transistor is driven tosaturation & other to cutoff.

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Astable multivibrator An Astable multivibrator produce continuous square wave

signal at its output when power is applied. Its name is derived from its characteristics of having no

stable state.

Astable multivibrator is also known as free runningrelaxation oscillator.

It has no stable state but only two quasi-stable states b/wwhich it keeps oscillating continuously of its own accord

without any external excitation. In this circuit neither of the two transistors reaches a

stable state.

A stable

multi-vibrator5V0V

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When one transistor is ON, the other isOFF & they continuously switch back n

forth at a rate depending on RC timeconstant in the circuit.

It has two energy storing elements i-e 2

capacitors.

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Transistor circuit ofAstable MV Circuit operation:

The circuit operation would be easy tounderstand if it remembered that due tofeed back when Q1 is ON, Q2 is OFF orwhen Q2 is ON and Q1 is OFF.

when the power is switched ON one ofthe transistor will start conductingbefore the other because no twotransistor can be exactly alike.

Suppose transistor Q1 start conductingbefore Q2 does. The feedback system is

such that Q1 will be rapidly driven tosaturation and Q2 cut off . If Q1 is insaturation/ON pointA (output1) has lowvoltage, c1 is discharged hence Q2 goesin cut off because base of Q2 isconnected with it.

As Q2 is OFF, pointB (output2) has highvoltage that will keep the Q1 saturated.

Now slowly C1 charges, and as itreaches >=0.7v it starts putting Q2 insaturation hence now Q2 is ON & Q1 isOFF.

PointA = high, pointB = low; this cyclerepeats.

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Switching times

T1 =0.69 R1C1 T2=0.69 R2C2 T= T1 +T2

T= 0.69 (R1C1 + R2C2) For 50% duty cycle R1=R2=R and

C1=C2=C T= 1.38 RC

f= 1/T =1/1.38RC =0.7/RC f= 0.7/RC

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Monostable Mutivibrators

A Mono stable MV produces a temporary logic level voltageat output after all activity signal is applied to its input.

Its name is derived from its characteristics having anyonestable logic level signal output. The output state of somemonostable MV in their stable state is a logic zero andsome of logic one.

Mono stable MV has one absolutely stable state.

It has one half stable state.

It has one energy storing element that is capacitor

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Transistor circuitof monostable MV

Circuit operation:Initially Q2 is in saturation and Q1is in cut off.

When the trigger is applied to baseof Q1, Q1 will turn ON and Q2 turnsoff. Therefore point B gives high

voltage.

When trigger is removed Q1 goesback to cut off and Q2 will be insaturation.

It is also called one shot MMV

T=0.69C1R2

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Bi-stable Multivibrators

Its another name is a flip-flop. Its name isderived from its characteristics of having two

stable states output that is zero or one. Bi-stable MV can remain in a stable condition and

produces the opposite logic state. Until all inputscauses it to output that is opposite logic leveloutput signal.

There is no any storing element.

It does not oscillate

BMV

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Transistor Circuitof Bi-stable MV

Circuit operation:

If transistor is Q1 conducting,Q2 is off. Therefore the point Bis high.

If trigger is applied Q2 will

turn ON, point B will be low.

Therefore Q1 is turned off.

Now even trigger is removedQ2 remains ON and Q1remains off.

If trigger is applied Q1 will beturned ON and Q2 will be OFFand therefore point A will below and B will be high.

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ECE 3450

M. A. Jupina,VU, 2010

Astable MV using schmitt trigger inverter

(a) If input transition times are too long, a standard logicdevice-output might oscillate or change erratically; (b) alogic device with a Schmitt-trigger type of input will

produce clean, fast output transitions.

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ECE 3450

M. A. Jupina,VU, 2010

Schmitt-Trigger Oscillator

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555 Timer

Paul Godin

Updated February 2008

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A Versatile Device

The 555 Timer is one of the best knownICs. The 555 is part of every experimenter's tool kit

Capable of creating a wide variety of circuits,including:

Oscillators with adjustable frequency and DutyCycle

Monostable Multivibrators

Analog to digital Converters

Frequency Meters

Many other applications.

555.17

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555 Timer Configurations

In this course we will use the 555 timer in 2modes: Astable

With some calculations, we can determine the

values of the capacitor and the 2 resistors (Ra andRb) for astable operations.

Monostable

We can determine the value of the resistor and thecapacitor with a simple formula for one-shot

operations.

555.18

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555 Layout

Also available:556 (two-555s in one DIP package)555 in a metal can configuration

555.19

12

3

4

8

7

6

5

Ground

Trigger

Output

Reset

Vcc

Discharge

Threshold

Control

1

2

34

5

6

78

Ground

Trigger

Output

Reset

Vcc

Discharge

Threshold

Control

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General Configuration

Basic connections: Ground

Vcc

Note: some 555

timers may functionat voltages otherthan 5 volts.

Reset (active low)

Output

555.20

1

2

3

4

8

7

6

5

Ground

Trigger

Output

Reset

Vcc

Discharge

Threshold

Control

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General Configuration

Specializedconnections: Trigger monitors low voltage

Threshold monitors high voltage

Discharge path to ground, todischarge the capacitor

Control specialized input filtering special applications

555.21

1

2

3

4

8

7

6

5

Ground

Trigger

Output

Reset

Vcc

Discharge

Threshold

Control

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555 Timer Operation

555.22

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Internal Operation of the 555Timer

Understanding the internal operation ofthe 555 timer is important

The device combines various circuit theories.

Understanding its internal function makes iteasier to create new designs with the device.

555.23

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Comparator

The comparator is an operational amplifier(op-amp) configuration.

The comparator compares 2 analog voltages

and provides a digital output.

+

-If V+ > V-, the output is a digital 1

If V- > V+, the output is a digital 0

555.24

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Reference and Comparators

The Comparatorsprovide digital logic toan SR Latch

Comparators compare voltagelevels.

If + is higher, output = 1If - is higher, output = 0

A 3-resistor voltagedivider providesreference voltages

555.25

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Reference and Comparators

Pin 8: Vccconnection

Pin 5: Controlconnection, usedwith filter cap.

Pin 1: Gndconnection

Pin 6: Thresholdconnection (highComparator)

Pin 2: Triggerconnection (lowComparator)

555.26

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Latch and Output

The Q output controls the

transistor (on or off).The SR Latch holdsits output states

The transistor actslike a switch.

The reset input can beused to enable or disablethe timer.

The buffer has importantelectrical functions.

555.27

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Latch and OutputPin 4: Resetconnection (activelow reset)

Pin 7:

Dischargeconnection

Pin 3: DigitalOutput

555.28

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555.29

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+ < -

0

- < +

1

Vc

0

1

Animated charge cycle

+ > -

1

- > +

01

0

Capacitor Charges via Ra and RbLatch in a set stateQ is low; Q output is highCapacitor continues to chargeLower comparator provides logiclow. Latch in hold state.Upper comparator + input isgreater than 2/3 Vcc referenceLatch receives a reset stateQ is high; Q output is lowTransistor is on and a connectionto ground is made.Capacitor begins to discharge

555.30

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Vc

1

01

0+ < -

- < +

Animated discharge cycle

01

+ > -

0

- > +

1

Capacitor is discharging. Q outputis low.Upper comparator + voltage lessthan reference voltage.Latch in a hold state.Lower comparator + voltage isgreater than voltage.Latch is set. Q is low.Q output is high.Transistor is off. Capacitorbegins to charge.

555.31

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Function of the Control Input (Pin5)

Pin 5 is the control voltage connection,and is used to access the 2/3 Vcc point ofthe voltage divider. Normally, a 0.01F capacitor is connected to

ground to provide output voltage stability. Used as an input for some applications.

AC to pulse modulation

voltage controlled oscillator

555.32

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Asta

bleMultivibrator

555.33

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Oscillator Configuration

Externally, the 555 requires an RC circuitto create the time delays required for the

time high and the time low.

Standard configuration requires common capacitor

a resistor for the charge cycle

a resistor for the discharge cycle

555.34

A t bl C fi ti #1

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Astable Configuration #1(Standard Configuration)

1

23

4

8

76

5

Ground

Trigger

Output

Reset

Vcc

Discharge

Threshold

Control

Vcc

Ra

Rb

C

Vcc

555.35

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AstableFilter Cap0.01F

CalculatedValues

Note: Duty Cyclemust be > 50%

555.36

A t bl C fi ti #1

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Astable Configuration #1(Standard Configuration)

Rb2Ra

RbRa

tt

t

DC21

1

C)Rb2Ra(693.

1

tt

1f

21

CRb693.t

C)RbRa(693.t

2

1

Minimum duty cycle > 50%555.

37

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Calculations: Astable

C)RR(693.0T

CR693.0T

BAH

BL

TH TL

Notes:The value 0.693 is a factor associated with thecharge/discharge cycle of the 555 timer.

Duty Cycle must be > 50%

Time High, Time Low Set

555.38

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555.39

Sample Calculation

Design an oscillator with a frequency of200Hz with a duty cycle of 78%.1. Determine Period (T):

2. Determine TH and TL:

s005.0Hz200

1

F

1T

ms1.1s0011.0s005.0%22T

ms9.3s0039.0s005.0%78T

L

H


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555.40

3. Since there are 2 variables in the TL equation, selectC:

4. Determine RB by using the TL equation:

7.158R

F10R693.0ms1.1

CR693.0T

B

B

BL

C=10F

Sample Calculation


S l C l l i

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555.41

5. Determine the value for RA:

1.404R

7.158R8.562F10)7.158R(693.0ms9.3

C)RR(693.0T

A

A

A

BAH

Sample Calculation


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Calculations: Astable

C)R2R(693.0

1F

)R2R(

)RR(DC

BA

BA

BA

Notes:

The value 0.693 is a factor associated with thecharge/discharge cycle of the 555 timer.

Duty Cycle must be > 50%

Frequency, Duty Cycle Set

555.42

S l D i

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Sample Design

Build an oscillator using a 555 timerwith a frequency of 72kHz at 75% D.C.Use a 100F capacitor.


555.43

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Design Solution

1- Determine the ratio of the resistors Ra and Rb:

2- Use the ratio in the frequency equation (substitution):

Rb2Ra

RbRb5.1Ra75.0RaRb5.1Ra75.0)Rb2Ra(75.0RbRa

75.0Rb2Ra

RbRaDC

RbC4

44.1

C)Rb2Rb2(

44.1

C)Rb2Ra(

44.1f


555.44

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Design Solution

3- Solve for Rb:

4-Solve for Ra:

5-Use standard values (optional step):

Ra=100k

Rb=47k

k50100k724

44.1

RbC4

44.1kHz72

k100Rb2Ra


555.45

i l i

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Design Solution

6- Calculate actual frequency and DC:

%1Error

757.0))k472(k100(

)k47k100(DC

%1.3Error

kHz2.74

F100))k472(k100(

44.1f


555.46

D i S l ti

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Design Solution

7- Create the circuit diagram using EWB:

555 Timer, 74.2kHz @ 75.7% D.C.


555.47

Mi i V l f R

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Minimum Value for Ra

The dischargetransistor causesthe capacitor todischarge toground.

Ra must have aminimum value of25 to prevent ashort circuit of thepower supplythrough thedischarge transistor.

Minimum Value

555.48

I Cl P ti P bl

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In-Class Practice Problem

Design an oscillator with a frequency of500Hz and a duty cycle of 80%. Use a 10FCapacitor. Calculate using each of theequation sets.

555.49

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Other Astable Configurations

555.50

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Astable Configuration#2Rb must be < .5 Ra 555.

51

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Astable Configuration #3555.

52

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Design Exercises

555.53

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Design Exercises

1. Using a 555 timer, design an oscillatorwith an output of 6Hz with a duty cycle

of 60%. Use a 100F capacitor. Build inEWB.

2. Using a 555 timer, design an oscillatorwith an output of 10KHz with a dutycycle of 50%. Use a 3.3F Capacitor.Build in EWB.

555.54

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Monostable operation of 555Timer IC

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Operation: Initially (before the trigger is applied, Vout is low,

shorting pin7 to ground and discharging capacitor.

Pin2 is normally held high by the R resistor (10 k) pull-up resistor. To trigger the one shot, a negative goingpulse less than Vcc is applied to pin2.

The trigger forces the lower comparator high, which setsa flip-flop, making Vout high and opening the dischargepin pin7.

Now the capacitor is free to charge from 0v up to Vcc viaRA.

When Vc crosses the threshold of Vcc, the uppercomparator goes high, representing the flip-flop, makingVout low, and shorting the discharge transistor.

The capacitor discharges rapidly to zero volt and the oneshot is held in the stable (Vout=low) until another trigger

is applied.tw = 1.10 RAC

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Homework: related exercises from williamklietz, floyd, tocci.

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