Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Instrukcja Użytkownika Pobierz pdf (Strona 58)

100

Using

Comparators

Detect

and

Measure

Discover how to use op -amps

monitor real -world

conditions

OUTPUT

GND

INPUT

INPUT 3

INPUT

13 11

OUTPUT

2 OUTPUT

1 V+ INPUT

INPUT

1+ INPUT 2-

INPUT

Fig.

1 -The LM339

contains

four independent

comparators

and

can operate

from

single

dual -voltage

power supplies.

By Jan Axelson

Comparator circuit

chips offer the experimenter some

of the easiest op -amp circuits

with which

to experi-

ment. Unlike other op -amps,

whose

outputs

vary

line-

arly

with

their inputs, the outputs of comparators

switch

between

just

two

voltage

levels, depending on the relative

voltages at their inputs. You could compare a comparator

to a toggle switch.

When

the toggle, or

lever, is raised slowly

(in a linear fashion) it

will

suddenly snap to the on position -

there's no in- between! Likewise, as the toggle is lowered,

the

inner

switch mechanism flips to the off

position. The

comparison

stops here, because

comparator

circuits are much

flexible

than

ordinary

toggle

switches.

The one -of- two -state characteristic makes comparators

ideal

for

voltage

monitoring in test or alarm circuits. Any condi-

tion, such as temperature or light, that

can be sensed as a

voltage

can be monitored

with

a comparator. Besides using

comparators in simple

voltage-

detecting circuits, you can

use a pair of comparators to detect

whether

an input falls

within a range of

voltages;

or use a series of comparators to

control a bar -graph display, for a good -looking and more

precise indication of signal levels.

The circuits that follow are interesting and typical exam-

ples of

comparators

action,

and

can

used

to guide

you

in choosing and using comparators to fit your own circuit -

crafting needs.

Whats

Special

About

Comparators?

Although

some op -amps are specifically

designated corn -

parators, in many cases a general- purpose

op-amp can also

serve

the same purpose. The dominant limitation

is that op-

amps often include phase and frequency compensation for

better closed

-loop stability. Since comparator circuits oper-

ate open -loop (without negative feedback),

they don't need

this compensation and respond faster without it.

Most of the examples in this article use the low

-cost

and

readily available LM339,

which

contains four independent

comparators on one chip. Figure 1 shows the LM339 chip's

pin out. Conveniently, the chip can be powered from a single

supply

of from 2- to 36 -volts DC or from

dual supplies

(± 1- to

18 -volts DC).

Supply

-current

requirements are

less than one milliampere,

low enough to allow

battery -

powered operation.

Comparator

Basics

Figure

shows a basic comparator circuit using the LM339.

The voltage

to be sensed (Vii) is

connected to the

+ input

(pin 5), and the reference, or trip- point, voltage

(Vref) is at

the input

(pin

4). The

comparator's operation is straightfor-

ward: When

Vin

is greater than Vre1, Voui (at

pin 2) goes high,

and

when

Vin

is less than

Vref, Vou1 goes

low. Figure

illus-

trates the comparator's response (VWd

to a changing input

(Vii)

compared to a fixed

voltage

(Vref).

+9voc

Fig. 2 -In this basic comparator circuit. Vout

goes

high when V,0 is greater than

Vrel

. On the other

hand. the output

voltage

(Vout)

goes low

when V,,,

is less than

Vret

. The reference

voltage

determines

the comparator's toggle point.

For inverting operation

(that

having the

VWi

high and

then go low

when the

Ví

goes

from low to high)

Vref

and

Vin

are swapped at the comparator's pin connections

in Fig.

2. The reference connects to the

+ input,

and

Vin

connects

to the input.

Now

VW1

of the comparator

will

high

when

Vin

less than

Vref.

A light- emitting

diode (LED) provides a simple indicator

of a comparator's

output state. The circuits

in Fig. 4 show

several ways of interfacing an LED to a comparator's

output.

1 2 ... 53 54 55 56 57 58 59 60 61 62 63 ... 99 100

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