Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Bedienungsanleitung PDF herunterladen (Seite 60)

64

How

Much Feedback?

he feedback

resistor

(R6 in Fig.

5) is usually

chosen

to

be much

larger than

the input resistor

(R5).

Its precise value

isn't critical,

but the smaller

it is,

the greater will

be

the

difference

between the

turn -on and

turn -off

trip

voltages.

Although

you

can calculate

the effects

of the feedback

mathematically,

for

basic alarm

circuits like

this, it's

often

just

as easy to set

the trip point

by experimentation.

Simply

bring

R1 to the desired

alarm

temperature

and adjust

R4 so

the buzzer

just turns on.

As shown,

the difference

between

the trip points

in Fig. 5 is

around one millivolt.

Positive

feedback

is also

useful in relay-

control

circuits.

Figure

6 shows

a

light-

sensing

circuit that

controls

a relay.

The light sensor,

R2,

is a cadmium -sulfide

photoresistor

whose

resistance

decreases

as the light hitting

it increases.

When

an increase

in light

level causes pin

5 to be

greater

than pin

4, pin 2 goes

high and turns

on Q I ,

activating relay

K I . Resistor R6

makes sure

that K I

turns on and stays

on

until the light level

has fallen

a certain

amount (determined

by the

value

of

R6).

CdS

PHOTO

RESISTOR

+9v

01

2N2222

NPN

Fig.

6- Photoresistor

R2

senses the light

level. At

the light -

intensity

level

set by R4, the

relay

solenoid is

energized.

Monitoring

Large

Voltages

An

important

characteristic

of comparator

devices

is their

common -mode

input -voltage

rating.

That is

the maximum

voltage

difference allowed

between

the +

and inputs

of

the device

for proper

operation.

Many

comparators,

includ-

ing the

LM339,

can handle

input

differences nearly

as large

as the

difference

between

their

+

and supply

pins.

If you

need

to monitor voltages

larger

than

the input

rating

allows,

a

voltage

divider

can be

used to input

a smaller,

proportional

part of

the

total

voltage.

Or,

a

single

-ended

comparator

like the one

shown

in Fig. 7

can be

used. Here

both

Vin

and Vret

connect

through

resistors R2

and R3

to the

+ input

of

the comparator,

and the input

is grounded

through Rl.

In the single

-ended

configuration,

Vret

is

in

proportion,

but

not equal to,

the trip voltage

of Vii.

If R2

is selected

to be much

larger

than R3,

the

voltage

at pin

5

will

remain well

within

the

common -mode

input

rating,

even with very

large

input voltages.

For instance,

in Fig. 7,

if

Vret

is

set at I volt,

the

trip

voltage

at Vin is

100 volts!

One important

limitation

to the single

-ended configura-

tion is that

Vret

must be

of opposite

polarity

from the

trip

voltage.

In the circuit shown

in

Fig. 7,

Vre('

is always

nega-

tive, so the

trip

voltage

will

always be

positive.

Germanium

diode D1 protects

the comparator

by limiting

negative

-

voltage

inputs

to 0.3 volt.

+9V

Fig.

7 -Use

the

single -ended

comparator

circuit

to measure

volt-

ages that

would

exceed the

comparator's

input

-voltage

rating.

Creating

a

Window

What

if you want

to

determine

if a voltage

falls

between

an

upper and

a lower limit?

A window

detector

is the answer,

and

the LM339,

with its

multiple

comparators

and open

-

collectoroutputs,

is ideal

for that

use. In

Fig. 8, the

thermistor/

resistor voltage

divider of

R4 and

R5 connects

to

the input

of one

comparator

and

the

+

input

of

another.

+9v

Vrethigh

Vrefdow

+9V

NTC

THERMISTOR

R4

10K

AT

25°C

R7

39012

R3

20K

LEDI

01

2N2222

NPN

Fig.

8 -This

window

detector

circuit lets

the

experimenter

know

when the

detected

temperature

is

within

a pre -selected

range.

The trip

points for

the

comparators

are

taken from

another

voltage

divider

made

up of

R1, R2,

and R3.

Because

the

outputs

of

the LM339

have open,

or

uncommitted,

collec-

tors, they

can be

connected

together

as shown,

and

a low

output

on

either one will

pull

their

combined output

low.

So

when

Vi,

falls between

Vrct_high

and

Vrer-hrw

the outputs

of

both comparators

go

high

and turn

on LEDI.

But if

Vin

is greater

than

Vret_hi

h

or less

than

Vrerkn,

the output

goes

low

and LEDI

is off.'

The LED

is on only when

the

tempera-

ture is within

the window

set

by resistor

network

R I ,

R2,

and

R3.

Figure 9 is

similar,

but different

-it's

an out -of- window

detector.

The output

of

the bottom

comparator

goes

high

when

the

temperature is

too high,

lighting

LED2 (red),

and

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