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555 TimerIntroduction:

The 555 Timer is one of the most popular and versatileintegrated circuits ever produced!

SigneticsCorporation first introduced this device as the

SE/NE 555 in early 1970.

It is a combination of digital and analog circuits.

It is known as the time machineas it performs a wide

variety of timing tasks.

Applications for the 555 Timer include: Ramp and Square wave generator

Frequency dividers

Voltage-controlled oscillators

Pulse generators and LED flashers 2

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555 timer- Pin Diagram

The 555 timer is an 8-Pin D.I.L. Integrated Circuit or chip

Notch

Pin 1

3

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555 timer- Pin Description

Pin Name Purpose

1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.

3 OUT This output is driven to approximately 1.7V below +VCCor GND.

4 RESET

A timing interval may be reset by driving this input to GND, but the

timing does not begin again until RESET rises above approximately

0.7 volts. Overrides TRIG which overrides THR.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3 VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

7 DISOpen collectoroutput; may discharge a capacitor between intervals.

In phase with output.

8 V+, VCC Positive supply voltage is usually between 3 and 15 V. 4
http://en.wikipedia.org/wiki/Vcchttp://en.wikipedia.org/wiki/Vcchttp://en.wikipedia.org/wiki/Vcchttp://en.wikipedia.org/wiki/Open_collectorhttp://en.wikipedia.org/wiki/Open_collectorhttp://en.wikipedia.org/wiki/Vcchttp://en.wikipedia.org/wiki/Vcchttp://en.wikipedia.org/wiki/Vcc

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555 Timer

Description:Contains 25 transistors, 2 diodes and 16 resistors

Maximum operating voltage 16V

Maximum output current 200mA

If you input certain signals they will be processed / controlled in a

certain manner and will produce a known output.

INPUT PROCESS OUTPUT

Best treated as a single component with required

input and output

5

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Inside the 555 Timer

S R Q Q

0 0 No Change

0 1 0 1

1 0 1 0

1 1 X X

Threshold

Control Voltage

Trigger

Discharge

Vref

+

R

S Q

Q

Truth Table

Fig: Functional Diagram of 555 Timer

-

6

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Inside the 555 Timer

Operation: The voltage divider has three equal 5K resistors. It

divides the input voltage (Vcc) into three equal

parts.

The two comparators are op-amps that compare

the voltages at their inputs and saturate depending

upon which is greater.

The Threshold Comparator saturates when the voltageat the Threshold pin (pin 6) is greater than (2/3)Vcc.

The Trigger Comparator saturates when the voltage at

the Trigger pin (pin 2) is less than (1/3)Vcc

7

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I nside the 555 Timer The flip-flop is a bi-stable device. It generates two

values, a high value equal to Vccand a low valueequal to 0V.

When the Threshold comparator saturates, the flip flop isReset (R) and it outputs a low signal at pin 3.

When the Trigger comparator saturates, the flip flop is Set(S) and it outputs a high signal at pin 3.

The transistor is being used as a switch, it connectspin 7 (discharge) to ground when it is closed.

When Q is low, Q bar is high. This closes the transistorswitch and attaches pin 7 to ground.

When Q is high, Q bar is low. This open the switch andpin 7 is no longer grounded

8

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Uses of 555 timer

What the 555 timer is used for:

To switch on or off an output after a certain time delay i.e.

Games timer, Childs mobile, Exercise timer.

To continually switch on and off an output i.e.

warning lights, Bicycle indicators.

As a pulse generator i.e.

To provide a series of clock pulses for a counter.

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Schematic Diagram of 555 Timer

10

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555 Timer operating modes

The 555 has three operating modes:1. Monostable Multivibrator

2.Astable Multivibrator

3. Bistable Multivibratior

11

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555 Timer as Monostable Multivibrator

Description:

In the standby state, FF holds

transistor Q1ON, thus

clamping the external timing

capacitor C to ground. The

output remains at ground

potential. i.e. Low.

As the trigger passes through VCC/3, the FF is set, i.e. Q bar=0, then

the transistor Q1OFF and the short circuit across the timing

capacitor C is released. As Q bar is low , output goes HIGH. 12

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555 Timer as Monostable Multivibrator

Fig (a): Timer in Monostable Operation with Functional Diagram

Fig (b): Output wave Form of Monostable 13

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Monostable Multivibrator- Description Voltage across it rises exponentially through R towards

Vccwith a time constant RC.

After Time Period T, the capacitor voltage is just greater

than 2Vcc/3 and the upper comparator resets the FF, i.e.

R=1, S=0. This makes Q bar =1, C rapidly to groundpotential.

The voltage across the capacitor as given by,

sec1.1)3

1ln(

)1(3

2

3

2,

)1(

RCTRCT

e RCT

VccVcc

VccvcTt

e RCt

Vccvc

at

Ifve going reset pulse terminal (pin

4) is applied, then transistor Q2-> OFF,

Q1-> ON & the external timing

capacitor C is immediately discharged.

14

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Behavior of the Monostable

Multivibrator

The monostable multivibrator is constructed by

adding an external capacitor and resistor to a 555

timer.

The circuit generates a single pulse of desiredduration when it receives a trigger signal, hence it

is also called a one-shot.

The time constant of the resistor-capacitorcombination determines the length of the pulse.

15

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Uses of the Monostable Multivibrator

Used to generate a clean pulse of the correct

height and duration for a digital system

Used to turn circuits or external components on or

off for a specific length of time.

Used to generate delays.

Can be cascaded to create a variety of sequential

timing pulses. These pulses can allow you to timeand sequence a number of related operations.

16

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Monostable Multivibrator

17

Fxx

x

R

T

C 9.01001.1

100

1.1 10

103

3

Problem:

In the monostable multivibrator of fig, R=100k

and the time delay T=100ms. Calculate the value of C ?

Solution:

T=1.1RC

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Applications in Monostable Mode

1. Missing Pulse Detector.

2. Linear Ramp Generator.

3. Frequency Divider.

4. Pulse Width Modulation.

18

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1.Missing Pulse Detector

Fig (a) : A missing Pulse Detector Monostable Circuit

Fig (b) : Output of Missing Pulse Detector19

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Missing Pulse Detector- Description

When input trigger is Low, emitter-base diode of Q is

forwarded biased capacitor is clamped to 0.7v(ofdiode), output of timer is HIGH width of T o/p of

timer > trigger pulse width.

T=1.1RC select R & C such that T > trigger pulse. Output will be high during successive coming of

input trigger pulse. If one of the input trigger pulse

missing trigger i/p is HIGH, Q is cut off, timer acts as

normal monostable state.

It can be used for speed control and measurement.

20

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2.Linear Ramp Generator

at pin 2 > Vcc/3

Capacitor voltage

at pin 6

21

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Linear Ramp Generator- Description

iQ3

Applying KVL around base-emitter loop of Q3

)(

)1()()(21

1

i

i

I

RRIRIRIRIIRIIRIVVRR

R

C

EECEBEBEBBEBCEEBECC

))()(

21

211

21

211

( RRRRRVVR

RRRRVVRR

E

BECCBECC

E ii

t

C

dt

C

idt

C RRR

RRVVR

RRR

RRVVRv

E

BECC

t

E

BECC

t

c }

)

)({

1}

)

)({

11

21

211

0 21

211

0 ((

TRRCR

RRVVRV

E

BECC

CC

)

)(

3

2

21

211

(

)(

)3

2

211

21(

RRVVR

RRCRV

BECC

ECC

T

When becomes at T,vc VCC3

2

Voltage Capacitor,

Ic

Analysis:

22

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3.Frequency Divider

Fig: Diagram of Frequency Divider

Description:

A continuously triggered

monostable circuit when triggered by a

square wave generator can be used as a

frequency divider, if the timing interval is

adjusted to be longer than the period of the

triggering square wave input signal.

The monostable multivibrator will

be triggered by the first negative going edge

of the square wave input but the output will

remain HIGH(because of greater timing

interval) for next negative going edge of the

input square wave as shown fig.

23

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4.Pulse Width Modulation

Fig a: Pulse Width Modulation Fig b: PWM Wave Forms

24

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Pulse Width Modulation- Description

The charging time of capacitor is entirely depend upon 2Vcc/3.

When capacitor voltage just reaches about 2Vcc/3 output of the timer

is coming from HIGH to Low level.

We can control this charging time of the capacitor by adding

continuously varying signal at the pin-5 of the 555 timer which is

denoted as control voltage point. Now each time the capacitor voltage

is compared control voltage according to the o/p pulse width change.So o/p pulse width is changing according to the signal applied to

control voltage point. So the output is pulse width modulated form.

25

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Pulse Width Modulation

Practical Representation

Fig: PWM & Wave forms

26

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Astable Multivibrator

27

1Ground 5FM Input (Tie to gnd via bypass cap)

2Trigger 6Threshold

3Output 7Discharge

4Reset (Set HIGH for normal operation) 8Voltage Supply (+5 to +15 V)

Fig (a): Diagram of Astable Multvibrator

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28

Fig (b): Functional Diagram of Astable Multivibrator using 555 Timer

A1

A2

V1

V2

VTVC

Vo

VA

R2

R1

R3

A1

A2

Q1

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Astable Multivibrator- Description

29

Connect external timing capacitor between trigger point

(pin 2) and Ground.

Split external timing resistor R into RA& RB, and connect

their junction to discharge terminal (pin 7). Remove trigger input, monostable is converted to Astable

multivibrator.

This circuit has no stable state. The circuits changes its

state alternately. Hence the operation is also called free

running oscillator.

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30

Resistive voltage divider (equal resistors) sets threshold

voltages for comparators

V1= VTH = 2/3 VCC V2= VTL = 1/3 VCC

Two Voltage Comparators

- For A1, if V+> VTH then R =HIGH

- For A2, if V-< VTL then S = HIGH

RS FF

- If S = HIGH, then FF is SET, = LOW, Q1OFF, VOUT= HIGH

- If R = HIGH, then FF is RESET, = HIGH, Q1ON, VOUT = LOW

Transistor Q1is used as a Switch

Astable 555 Timer Block Diagram Contents

Q

Q

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31

Operation of a 555 Astable

VCC

VC(t)

RA RB

1) Assume initially that the capacitor is discharged.a) For A1, V+= VC = 0V and for A2, V-= VC= 0V, so R=LOW,

S=HIGH, = LOW , Q1 OFF, VOUT = VCC

b) Now as the capacitor charges through RA& RB,eventually VC> VTLso R=LOW & S=LOW.

FF does not change state.

Q

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32

Operation of a 555 AstableContinued

VC(t)

RB

Q1

2) Once VCVTHa) R=HIGH, S=LOW, = HIGH ,Q1 ON, VOUT = 0

b) Capacitor is now discharging through RBand Q1toground.

c) Meanwhile at FF, R=LOW & S=LOW since

VC< VTH.

Q

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33

Operation of a 555 AstableContinued..

3) Once VC< VTL

a) R=LOW, S=HIGH, = LOW , Q1 OFF, VOUT = VCC

b) Capacitor is now charging through RA& RBagain.

VCC

VC(t)

RA RB

Q

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Timing Diagram of a 555 Astable

34

VC(t)VTH

VTL

VOUT(t) TL

TH

t = 0 t = 0't

t

1 2 3

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Astable Multivibrator- Analysis

35Contd.

The capacitor voltage for a low pass RC circuit subjected to a step input of Vccvolts is

given by,

The time t1taken by the circuit to change from 0 to 2Vcc/3 is,

)1( eVv RCt

CCc

RCteVV RC

CC

CCt

09.1)1(3

21

1

VV CCC3

2

The time t2to charge from 0 to vcc/3 is VV CCC 3

1

RCteVV RC

CC

CCt

405.0)1(3

2

2

So the time to change from Vcc/3 to 2Vcc/3 is, RCRCRCtttHIGH 69.0405.009.121

So, for the given circuit, C

RRt BAHIGH )(69.0

The output is low while the capacitor discharges from 2Vcc/3 to Vcc/3 and the

voltage across the capacitor is given by,

eVV RCt

CC

CC

3

2

3

Charging time

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Astable Multivibrator- Analysis

36

CRt BLOW 69.0

After solving, we get, t=0.69RC

For the given circuit,

Both RAand RBare in the charge path, but only RBis in the discharge path.

The total time period,

CCT RRRtt BBLOWHIGH A 69.0)69.0 (

CCCCT RRRRRRRR AAA BBBBB )69.0)69.0])69.0 2(([(

CCTf

RRRR AA BB )

45.1

)69.0

11

2(2( Frequency,

Duty Cycle,100

)

)100

)69.0

)69.0100%

2(

(

2(

(XX

C

CX

TD

RR

RRRR

RRt

A

A

A

A

B

B

B

BHIGH

100)

100)69.0

69.0100%

2(2(XX

C

CX

TD

RR

RRR

Rt

AA B

B

B

BLOW

Discharging time

.1.45 is Error Constant

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Behavior of the Astable Multivibrator

The astable multivibrator is simply an oscillator. The

astable multivibrator generates a continuous stream of

rectangular off-on pulses that switch between two

voltage levels.

The frequency of the pulses and their duty cycle are

dependent upon the RC network values.

The capacitor C charges through the series resistors RA

and RB with a time constant (RA+ RB)C.

The capacitor discharges through RBwith a time

constant of RBC

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Uses of the Astable Multivibrator

Flashing LEDs

Pulse Width Modulation

Pulse Position Modulation

Periodic Timers Uses include LEDs, pulse generation, logic

clocks, security alarms and so on.

38
http://en.wikipedia.org/wiki/Light-emitting_diodehttp://en.wikipedia.org/wiki/Light-emitting_diode

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Applications in Astable Mode

39

1.Square Generator

2.FSK Generator

3.Pulse Position Modulator

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1.Square Generator

40

0

%50100)

)

1

2

2

2((

1

1

R

RR

RR

Here

XDutyCycle

To avoid excessive discharge current through Q1when R1=0

connect a diode across R2, place a variable R in place of R1.

Charging path R1& D; Discharging path R2& pin 7.

10F

C1

3

Fig: Square Wave Generator

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2. FSK Generator

41

Description:

In digital data communication,

binary code is transmitted by

shifting a carrier frequency

between two preset

frequencies. This type of

transmission is called Frequency

Shift Keying (FSK) technique.

Fig: FSK Generator

Contd..

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FSK Generator

42

The frequency of the output wave form given by,

CRRf

O )

45.1

2( 1 2

When input digital is LOW, Q1is ON then R3parallel R1

CRRRf

O )

45.1

2||( 3 1 2

A 555 timer is astable mode can be used to generate FSK signal.

When input digital data is HIGH, T1is OFF & 555 timer works as

normal astable multivibrator.

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2. Pulse Position Modulator

43

Fig (a): Pulse position Modulator

Fig (b): Output Wave Form of PPM

Description:

The pulse position modulator can be

constructed by applying a modulating

signal to pin 5 of a 555 timer connected

for astable operation.

The output pulse position varies with

the modulating signal, since the

threshold voltage and hence the time

delay is varied.

The output waveform that the

frequency is varying leading to pulse

position modulation.

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44

Problem:

In the astable multivibrator of fig, RA=2.2K, RB=3.9K and C=0.1F. Determine

the positive pulse width tH, negative pulse width tLow, and free-running frequency fo.

Solution:

msXKCRt BLOW 269.0)1.0)(9.3(69.069.0 10 6

?)

45.11

2(

CT RRf

A Bo

msXKKC

RRt BAHIGH

421.0)1.0)(9.32.2(69.0)(69.0

10

6

?1009.322.2

9.32.2100

)

)100%

2(

(

XKXK

KXX

TD

RR

RRt

A

A

B

BHIGH

Duty Cycle,

?1009.322.2

9.3100

)100%

2(

X

KXKXX

TD

RR

Rt

A B

BLOW

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45

One PossibleSolution:

One Possible

Solution:

Example: A 555 oscillator can be combined with a J-K FF to

produce a 50% duty-cycle signal. Modify the above

circuit to achieve a 50% duty-cycle, 40 KHz signal.

Example: Design a 555 Oscillator to produce an approximate

square-wave at 40 KHz. Let C > 470 pF.

F=40KHz; T=25s; t1=t2=12.5sFor a square-wave R

A

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Comparison of Multivibrator Circuits

46

Monostable Multivibrator Astable Multivibrator

1. It has only one stable state 1. There is no stable state.2. Trigger is required for the operation

to change the state.

2. Trigger is not required to change the

state hence called free running.

3. Two comparators R and C are

necessary with IC 555 to obtain the

circuit.

3. Three components RA, RBand C are

necessary with IC 555 to obtain the

circuit.

4. The pulse width is given by T=1.1RC

Seconds

4. The frequency is given by,

5. The frequency of operation is

controlled by frequency of triggerpulses applied.

5. The frequency of operation is

controlled by RA, RB& C.

6. The applications are timer, frequency

divider, pulse width modulation etc

6. The applications are square wave

generator, flasher, voltage controlled

oscillator, FSK Generator etc..

CT RRf

A Bo )

45.11

2(

S h itt T i

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Schmitt Trigger

47

The use of 555 timer as a Schmitt trigger is shown in fig.

Here the two internal comparators are tied together and externally

biased at Vcc/2 through R1 and R2. Since the upper comparator will

trip at 2Vcc/3 and lower comparator at Vcc/3, the bias provided by R1

and R2is centered within these two thresholds.

Fig (b): Output Wave FormFig (a): Circuit Diagram of Schmitt Trigger

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Features of IC 555 TimerThe Features of IC 555 Timer are:

1. The 555 is a monolithic timer device which can be used

to produce accurate and highly stable time delays or oscillation. It

can be used to produce time delays ranging from few

microseconds to several hours.

2. It has two basic operating modes: monostable and

astable.

3. It is available in three packages: 8-pin metal can, 8-pin

mini DIP or a 14-pin. A 14-pin package is IC 556 which consists of

two 555 times.

48

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Features of IC 555 Timer

4. The NE 555( signetics ) can operate with a supplyvoltage in the range of 4.5v to 18v and output currents of

200mA.

5. It has a very high temperature stability, as it is

designed to operate in the temperature range of -55c to

125oc.

6. Its output is compatible with TTL, CMOS and Op-

Amp circuits.

49

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PHASE-LOCKED LOOPS

50

PLL

PHASE LOCKED LOOPS Introduction

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PHASE-LOCKED LOOPS- Introduction

51

The phase-locked loop is a negative feedback system in

which the frequency of an internal oscillator (vco) is

matched to the frequency of an external waveform with

some Pre-defined phase difference.

Vd(t)

PHASE

COMPARATOR

(PC)

LOW PASS

FILTER

(LPF)

VCO

AMPLIFIER

(A)

Vi(t)

Vo(t)

Vp(t)

(EXTERNAL R & C DETERMINES

VCO FREQUENCY)

v

fKo

Contd..

PHASE LOCKED LOOPS

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PHASE-LOCKED LOOPS

52Contd..

The phase comparator (phase detector) can be as simple asan exclusive-or gate (digital signals) or is a mixer (non-linear

device - frequency multiplier) for analog signals.

The phase comparator generates an output voltage Vp(t)(relates

to the phase difference between external signal Vi(t)and vcooutput Vo(t)).

If the two frequencies are the same (with a pre-defined phase

difference) then Vp(t) = 0.

If the two frequencies are not equal (with various phase

differences), then Vp(t) = 0and with frequency components

about twice the input frequency.

Phase Comparator:

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PHASE-LOCKED LOOPS

53Contd..

The low pass filterremoves these high frequency components and

Vd(t)is a variable dc voltage which is a function of the phase

difference.

Voltage Controlled Oscillator: The vcohas a free-running frequency, fo, approximately equal to

the input frequency. the vco frequency varies as a function of Vd(t)

The feedback loop tries to adjust the vco frequency so that:

Vi(t) FREQUENCY = Vo(t) FREQUENCY

THE VCO IS SYNCHRONIZED, OR LOCKED TO V i(t)

Low pass filter:

PLL LOCK RANGE

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PLL LOCK RANGE

54

Lock range is defined as the range of frequencies in the vicinity of

the vcosNatural frequency (free-running frequency) for which the

pll can maintain lockwith the input signal. The lock range is also

called the tracking Range.

The lock range is a function of the transfer functions of the pc,

amplifier, and vco.

Hold-in range:

The hold-in range is equal to half the lock range

The lowest frequency that the pll will track is called the lower lock

limit. The highest frequency that the pll will track is called the upper

lock limit Contd..

Lock range:

PLL LOCK RANGE

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PLL LOCK RANGE

55

PLL CAPTURE RANGE

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PLL CAPTURE RANGE

56Contd.

Capture range is defined as the band of frequencies in the vicinity

of fowhere the pll can establish or acquire lock with an input range(also called the acquisition range).

Capture range is a function of the BW of the lpf ( lpf BW capture

range).

Capture range is between 1.1 and 1.7 times the natural frequency

of the vco.

The pull-in range:

The pull-in range is equal to half the capture range

The lowest frequency that the pll can lock onto is called the lower

capture limit

CAPTURE RANGE:

PLL CAPTURE RANGE

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PLL CAPTURE RANGE

The highest frequency that the pll can lock onto is called

the upper capture limit

57

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58

PLL LOCK/CAPTURE RANGE

LOCK RANGE > CAPTURE RANGE

PLL Basic Components

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PLL-Basic Components

59

Phase detector:

Transfer function: K[V/radians].

Implemented as: four quad

multiplier, XOR gate, state

machine.

Voltage controlled oscillator (VCO):

Frequency is the first derivative ofphase.

Transfer function: KVCO/s

[radians/(Vs)]

Low pass filter:

Removes high frequency components coming from the phase detector.

Determines loop order and loop dynamics.

PLL OPERATION Putting All Together

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PLL OPERATION-Putting All Together

60

e

dd

VK

edd KV dafout VKKV

af

outd

KK

VV

d

de

K

V

v

fKo

o

out

K

fV

out

o

V

fK

ooutKVf

nin fff

oafdL KKKKK

OPEN-LOOP GAIN:

PLL OPERATION

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PLL OPERATION

61

KdKf Ka

Ko

dded KKV2

maxmax

Loafd KKKKKf22

max

HOLD-IN RANGE

LKfRangeLock

max2

PLL 565 Pin Configuration

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PLL 565 Pin Configuration

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PLL Example

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PLL- Example

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Problem:

fn= 200 kHz, fi= 210 kHz, Kd= 0.2 V/rad, Kf= 1, Ka= 5, Ko= 20 kHz/V

radkHzKL /20)20)(5)(1)(2(.

PLL OPEN-LOOP GAIN:

VCO FREQUENCY CHANGE for LOCK:

kHzfff nin 10200210

PLL OUTPUT VOLTAGE:

VVkHz

kHz

K

fV

o

out 5./20

10

Solution:

Contd..

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PLL-Example

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STATIC PHASE ERROR:

65.285./2.

1.

radradV

V

K

V

e d

d

HOLD-IN RANGE:

LOCK RANGE:

kHzKf L 4.312

max

kHzfRangeLock 8.622 max

PHASE DETECTOR OUTPUT VOLTAGE:

VKK

VV

af

outd 1.

)5(1

5.

Salient Features of 565 PLL

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Salient Features of 565 PLL1. Operating frequency range =0.01Hz to 500KHz

2. Operating voltage range = 6v to 12v

3. Input level required for tracking:

10mv rms min to 3v peak to peak max

4. Input impedance = 10ktypically.

5. Output sink current : 1mA typically.6. Output source current: 10mA typically

7. Drift in VCO Centre frequency: 300 PPM/ c

8. Drift in VCO Centre frequency with supply voltage: 1.5

percent/Vmax

9. Triangle wave amplitude: 2.4 Vppat 6v supply voltage.

10. Square wave amplitude: 5.4 Vppat 6v supply voltage.

11. Bandwidth adjustment range: < 1 to 60%65

PLL APPLICATIONS

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PLL APPLICATIONS

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Analog and digital modulation

Frequency shift keying (fsk) decoders

Am modulation / demodulation

Fm modulation / demodulation

Frequency synthesis

Frequency generation

PLL APPLICATIONS

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PLL APPLICATIONS

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1.FM Demodulator:

2.FM Modulator:

Voltage Controlled Oscillator (VCO)

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Voltage Controlled Oscillator (VCO)

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A voltage controlled oscillator is an oscillator circuit

in which the frequency of oscillations can be

controlled by an externally applied voltage

i

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VCO Operation

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VCO A l i

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VCO Analysis

70Contd..

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VCO Analysis

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Features of VCO

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li i f

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Applications of VCO

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The various applications of VCO are:

1. Frequency Modulation.

2. Signal Generation (Triangular or Square Wave)

3. Function Generation.

4. Frequency Shift Keying i.e. FSK demodulator.

5. In frequency multipliers.

6. Tone Generation.

VCO

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VCO

74Contd.

VCO

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VCO

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Thank You

Documents

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