EET 157 - New Jersey Institute of Technologyecelabs.njit.edu/fed101/resources/555_Timer.pdf ·...

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Purdue University EET 157 - 05 Electronics Circuit Analysis

Electrical Engineering Technology

EET 157

Electronics Circuit Analysis # 05

Revised by Dr. Athula Kulatunga

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Overview

♦Multivibrators

♦ 555 Timer

♦ 555 Timer Monostatble Multivibrators

♦555 Timer Astable Multivibrators

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Waveforms

sine square rectangle

A dc source can be used to generate all the above shapes

triangle exponentialsawtooth

4


Multivibrators

Pulse generatorsTypes Characteristics

Astable or free-running no stable stateMonostable or one shot one stable stateBistable two stable states

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Astable or Free-running

Astable Multivibrator

Vout

unstableVH

VL unstable

TL

T

TH

W

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Duty Cycle

Defined for rectangular waveforms

%100×=TW

D

Square wave has 50% duty cycle.

Why?

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Duty Cycle Example 1

A rectangular signal is high for 4 ms and low for 6 ms. Find the signal’s period, pulse width, duty cycle, and frequency.

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Duty Cycle Example 2

A rectangular waveform has a frequency of 1k Hz and a duty cycle of 60%. Find the signal’s repetition rate, period, and pulse width.

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Monostable Operation

unstable

VL stable

VH

W

T1

Monostable Multivibrator

VoutVin

T1 T1

The monostable multivibrator produces a well-defined pulse out for an input signal that may not be well defined (a narrow pulse or a spike)

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Bistable OperationThe bistable multivibrator has two stable states and utilizes one input pulse to take the output high and another input pulse to take the output low.

VH

VL stable

stable

Bistable Multivibrator

VoutVin_1

T1

Vin_2 T1 T2

T2

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555 Timer IC - Schematic

SET

RESET

Q

Q

RS Flip-FlopUTP

LTP

To +Esupply

VCC

8

1COMMON

2TRIGGER

THRESHOLD6

5CONTROL

4RESET

3OUTPUT

DISCHARGE

7

R

R

R

U1

U2

Q1

Inverter

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555 Timer IC - Features• Q FF output−high or low output• Q Complement (opposite) of Q−low or high

output• SET FF input−high signal sets high and low• RESET FF input−high signal in resets low and high• U1 Noninverting comparator driving the FF set input• U2 Inverting comparator driving the FF reset input• UTP Upper trip point for U1 set by 2R/3R of Esupply

• LTP Lower trip point for U2 set by R/3R of Esupply

• Q1 BJT used to discharge external capacitor when Q high• Pin 7 Open or tied to common through BJT switched on

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555 Timer - Symbol & Chip

1

2

3

4

5

6

7

8

555

LM

555

1

2

4

3

7

5

6

8

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555 Timer – Monostable Connection

12

3

4

5

6

78

555

+5 V

vout

R1 k

C1 µ

0.01 µ

0 V

3.33 V

T1

W

+5 V

less than 1.67 V

T1

W

T1

+5 V

0 V

input

RCcharge

capacitordischargethrough BJT

Output stays at 0 Vuntil input dipsbelow 1.67 V.

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555 Timer – Monostable Operation

SET

RESET

Q

Q

RS Flip-FlopUTP

LTP

To +E supply

VC C

8

1COMMON

2TRIGGER

THRESHOLD6

5CONTROL

4RESET

3OUTPUT

DISCHARGE7

R

R

R

U1

U2

Q1

Inverter

+5Vdc

1k

1uL

H ON

0V

L

5VL

Remains SET

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555 Timer – Monostable Operation

SET

RESET

Q

Q

RS Flip-FlopUTP

LTP

To +E supply

VC C

8

1COMMON

2TRIGGER

THRESHOLD6

5CONTROL

4RESET

3OUTPUT

DISCHARGE7

R

R

R

U1

U2

Q1

Inverter

+5Vdc

1k

1uH

L OFFStart

Charging

H

H

RESET

Drop below

1.67V

3.3V

SET

H

L

ONStart

Discharging

0V

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Monostable Pulse Width Calculation

ms1F1k1 =µ×Ω==τ RC( ) τ/

C eV5V0V5)( ttv −−+=ms1/

C eV5V5)( ttv −−=ms1/eV5V5V33.3 t−−=

ms10.1=t ms10.1=W

The pulse width W is the time required to charge the capacitor; therefore,

vout

T1

W

+5 V

0 V

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555 Timer – Astable Connection

12

3

4

56

7

8

555

+5 V

vout

R21 k

C1 µ

0.01 µ

R11 k

(R1+R2) Cchargetowardsupply

(R2) Cdischarge

through pin 7and BJT short to

common

W

T

TLTH

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555 Timer – Astable Operation

SET

RESET

Q

Q

RS Flip-FlopUTP

LTP

To +E supply

VC C

8

1COMMON

2TRIGGER

THRESHOLD6

5CONTROL

4RESET

3OUTPUT

DISCHARGE7

R

R

R

U1

U2

Q1

Inverter

+5Vdc

R1: 1k

1u L

ON

R2: 1k

H

Discharge

towards 0V

<1.67V

H

RESET

L

H

OFF

Charge

Towards 5V

from 1.67V

<3.3V

L

H

SET

H

L

ON

Discharge Towards

0V through R2

Length Depend on (R1+R2)C

Depend on (R2)C

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Astable Pulse Width CalculationThe pulse width W depends upon the (R1 + R2) C

time constant τH and the capacitor charging from 1.67 V to 3.33 V while the output is in the high (or 5 V) state. The capacitor must follow the universal dc transient expression

τ/ssinitssC e)()( tVVVtv −−+=

( ) ms2F1k2C2R1R =µ×Ω=+=

( ) ms2/C eV5V67.1V5)( ttv −−+=

ms2/C eV33.3V5)( ttv −−=

τ

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Astable Pulse Width Calculation

The capacitor charges to 3.33 V (or 2/3 of VCC) when it transitions back to its stable state. Set the capacitor voltage expression equal to 3.33 V and solve the expression inversely for the time t required to charge the capacitor to 3.33 V.

ms2/eV33.3V5V33.3 t−−= ms386.1=t

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Astable Pulse Width Calculation

The pulse width W is the time required to charge the capacitor, so

ms386.1=W

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Astable Pulse Space Calculation

uThe time TL of the pulse depends upon the (R2) C time constant τL and discharging the capacitor from 3.33 V to 1.67 V while the output is in the low (0 V) state. The capacitor must follow the universal dc transient expression

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The capacitor starts with a value of 3.33 V (initial value) and attempts to fall to 0 V

( ) ms1F1k1C2RL =µ×Ω==τ

( ) ms1/C eV0V33.3V0)( ttv −−+=

ms1/C eV33.3)( ttv −=

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u The capacitor discharges to 1.67 V (or 1/3 of VCC), when it transitions back to its stable state. Set the capacitor voltage expression equal to 1.67 V and solve the expression inversely for the time t required to discharge the capacitor to 1.67 V.

ms1/eV33.3V67.1 t−=

ms693.0=t

ms693.0=t

ms693.0L =T

ms693.0L =T

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T = TH + TLT = 1.386ms +0.693 msT = 2.079 ms

f = 1/T f = 481 Hz

W

T

TL

TH

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Overview

♦Multivibrators

♦ 555 Timer

♦ 555 Timer Monostatble Multivibrators

♦555 Timer Astable Multivibrators

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