555 TimerCh. Srividhya

Asst Prof

ECE

JITS

Pin configuration

The 555 timer consists of; Two voltage comparators

A bi-stable flip flop

A discharge transistor

and a resistor divider network.

Internal Circuit Of 555 Timer

VA

VB

Output

Threshold

Control

Voltage

Trigger

Discharge

ResetVCC

Voltage Divider

Comparator

RS-FF

Discharge

TransistorGND

+ +C1

+ +C2

Q

QRD

SD

5KΩ

5KΩ

5KΩ

T

2

4

5

6

7

8

3

1

<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

comparing

1/3 VCCNot permitted

2/3 VCC

+ +

C1

+ +C2

.

.

5KΩ

5KΩ

5KΩ

VA

VB

VCC

RD

SD

5

6

2

V6 V2

<2/3 UCC <1/3 UCC

>2/3 UCC >1/3 UCC

<2/3 UCC >1/3 UCC

Q T

1

0

保持

on

off

保持

Q

QRD

SD

Toutp

ut

RD SD

1 0

10

1 1

Q T

1

0

remain

on

off

remain

remain remain

Monostable Multivibrator & Waveforms

Monostable Multivibrator (One-shot) Operation

(grand)

uC

R1

+VCC

ui

VCC

+ +C1

+ +C2

Q

QRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (Re)

uO

6

5

2

7

>2/3 VCC

0

1 1 0

1

Q=0

on

1 Steady state

(grand)

uC

R1

+VCC

ui

VCC

+ +C1

+ +C2

Q

QRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (reset)

uO

6

5

2

7

1

1 0

1

Q=1

off

Transiet state

0

< 1/3 UCC

0

0

1

Monostable Multivibrator (One-shot) Operation

Monostable Multivibrator(One-shot) Operation

(grand)

uc

R1

+VCC

ui

VCC

+ +C1

+ +C2

Q

QRD

SD

5KΩ

5KΩ

5KΩ

VA

VB

T

1

3

48 (reset)

uO

6

5

2

7

>2/3 VCC

1

0 1

0

Q=1

0

1 01

1

0

Steady state

Q=0

ui

t

uC

tuO

t

tp =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

Q

QRD

SD

.

.

.

5KΩ

5KΩ

5KΩ

VA

VB

T

1

32

4

56

7

8

(grand)

uO

2) Astable Multivibrator(Oscillator)

first

uC=0

0

1

11

0

0

>2/3 UCC

RD=1

SD=0

.

.

uC

R1

R2

.+

–

charge

discharge

1

<1/3 UCC

4856

2

7 1

3

+VCC

uO

.

uC..

C

R1

R2

tp1=(R1+R2)C ln2=0.7(R1+R2)C

tp2 =R2C ln2=0.7R2C

T=tp1+tp2 =0.7(R1+2R2)C

tp1 tp2

2/3UCC

1/3UCC

uC

t

uO

t

What is an astable

multivibrator?Duty Cycle = tp1/T

Applications using 555 timer

A light dimmer ;

A car tachometer

Traffic Lights ;

Infra Red (IR) remote control ;