555 Timer

Pin configuration The 555 timer consists of;The 555 timer consists of;

Two voltage comparatorsTwo voltage comparators A bi-stable flip flopA bi-stable flip flop A discharge transistorA discharge transistor and a resistor divider network.and a resistor divider network.

Internal Circuit Of 555 Timer

VA

VB

OutputOutput

Threshold Threshold

Control Control VoltageVoltage

TriggerTrigger

DischargeDischarge

ResetResetVCC

Voltage DividerVoltage DividerComparatorComparator

RS-RS-FFFF

DischargeDischargeTransistorTransistor GND

+ +C1

+ +C2

Q

QRD

SD

5KΩ

5KΩ

5KΩ

T

22

44

5

66

77

88

33

11

<2/3 VCC <1/3 VCC 1 0

>2/3 VCC >1/3 VCC 0 1

<2/3 VCC >1/3 VCC 1 1

>2/3 VCC <1/3 VCC 0 0

RD SDV6 V2

The result for The result for comparingcomparing

1/3 VCCNot permitted

2/3 VCC

+ +C1

+ +C2

.

.

5KΩ

5KΩ

5KΩ

VA

VB

VCC

RD

SD

5

6

2

VV66 VV22

<2/3 <2/3 UUCCCC <1/3 <1/3 UUCCCC

>2/3 >2/3 UUCCCC >1/3 >1/3 UUCCCC

<2/3 <2/3 UUCCCC >1/3 >1/3 UUCCCC

QQ TT

11

00

保持保持

onon

offoff

保持保持

Q

QRD

SD

Toutpoutp

utut

RRDD SSDD

11 00

1100

11 11

QQ TT

11

00

remainremain

onon

offoff

remainremain

remainremain remainremain

Monostable Multivibrator & Waveforms

Monostable Multivibrator (One-shot) OperationMonostable Multivibrator (One-shot) Operation

(grand)

uC

R1

+V+VCCCC

ui

VVCCCC

+ +C1

+ +C2

QQRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (Re)

uO

65

2

7

>2/3 VCC

0

1 1 0

1

Q=0

on

1 Steady stateSteady state

(grand)

uC

R1

+V+VCCCC

ui

VVCCCC

+ +C1

+ +C2

QQRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (reset)

uO

65

2

7

1

1 0

1

Q=1

off

Transiet stateTransiet state

0

< 1/3 UCC

0

0

1

Monostable Monostable MultivibratorMultivibrator (One-shot) Operation (One-shot) Operation

Monostable Multivibrator(One-shot) OperationMonostable Multivibrator(One-shot) Operation

(grand)

uc

R1

+V+VCCCC

ui

VVCCCC

+ +C1

+ +C2

QQRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (reset)

uO

65

2

7

>2/3 VCC

1

0 1

0

Q=1

0

1 01

1

0

Steady stateSteady state

Q=0

ui

tuC

tuO

ttp =RC ln3=1.1RC

2/3UCC

What is a monostable multivibrator?

•With one stable state

•Normally in its stable state,to change to its unstable state only when triggered

•Once triggered,to remain in its unstable state for a predetermined length of time and then return to its stable state automatically

Astable Multivibrator & Waveforms

UCC

+ +C1

+ +C2

QQRD

SD

.

.

.

5KΩ

5KΩ

5KΩ

VA

VB

T

1

32

4

56

7

8

(grand)

uO

2) Astable Multivibrator(Oscillator)2) Astable Multivibrator(Oscillator)

firstfirstuuCC=0=0

0

1

11

0

0

>2/3 UCC

RD=1

SD=0

.

.

uC

R1

R2

.+

–

charge

discharge

1

<1/3 UCC

485627 1

3

+VCC

uO

.

uC..

C

R1

R2

ttp1p1 =(=(RR11++RR22))CC ln2=0.7( ln2=0.7(RR11++RR22))CCttp2p2 ==RR22CC ln2=0.7 ln2=0.7RR22CC

TT==ttp1p1++ttp2p2 =0.7(=0.7(RR11+2+2RR22))CC

tp1 tp2

2/3UCC

1/3UCC

uC

t

uO

t

What is an astable multivibrator?Duty Cycle = ttp1p1/T/T

Applications using 555 timer

A light dimmer ; A car tachometer Traffic Lights ; Infra Red (IR) remote control ;