Elenco Electronics MO-1251 Manual do Utilizador descarregar pdf (Página 30)

100

central

monitoring

facility. A comput-

at the central

station tells the

duty

operator

precisely

what is

wrong

the

home: a fire, a burglary,

a hold -up,

invalid needing

medical attention,

even

freezing cold

or a leaking

water

pipe. The central

station's

operator

usually

-not always

-phones the

house

to find out

what is

wrong and to

get a

verbal all -clear

password in the

event

that the alarm

was accidental.

the operator

does not get

an abso-

lutely precise

password,

he or she im-

mediately notifies

the proper

au-

thorities, such

as the police

or fire

departments,

an emergency

rescue

service,

or a neighbor (if

your alarm

sends

a "freezing

temperature"

"water leak" signal).

Why

an absolutely

precise pass-

word?

Well, assume that an

intruder

has forced you

at gunpoint

to silence

the alarm, and

then he listens

in on an

extension

telephone -still

pointing

the gun

-as you answer

the central

station's call.

Assume that your pass-

word is the numbers

5678. You reply

8956. The

operator

will say

"Thank

you," and the

intruder

will

feel

safe.

Meanwhile,

the

operator

calls

for the

cops.

With

high -tech alarm there

might

no call -back from

the central

sta-

tion.

A keypad code that

is used to

silence the alarm also

tells the central

station's computer that

all is

well.

an armed

intruder forces you to

turn

off the alarm,

entering an extra

number tells

the central

station that

you

are being held at gunpoint.

For

example,

if the code 5678

turns off

the alarm and

automatically

sends an

"all safe" signal to

the central

station,

the

code 56789

will

turn

off the alarm

but send a

signal to the central

station

that

a hold -up is in progress.

Perimeter defense

The

earliest

home -security

system

was

the "perimeter

alarm,"

which in

refined or

"supervised" form is

still

the end result

for many -usually

the

best

-high -tech

home alarms.

Basically,

it consists of

a battery in

series

with a normally -open

switch

and an alarm

bell. The switch

might,

for example, be a

mat switch, the

kind

used by

supermarkets to

trigger a

door

-opening mechanism.

Anything,

or anyone causing

the switch to close

completes

the electric

circuit,

which

causes

the alarm bell to

sound.

As you can

well figure out, there

are

two things

wrong with that kind

P81

RESET

t--

LOOP

I 6V

RY1

OFF

MASTER

LOOP 1

FIG.

1 -IN

SIMPLE

SUPERVISED

SYSTEM

the protective

loop's power

is provided

by an

end

-of -line (EOL)

battery.

meter indicates

if the

loop is closed.

alarm. First, the

alarm

stops

when

the

switch

is opened -when

the person

steps

off the mat.

But more impor-

tant,

it does not tell

if the circuit

working. Suppose

the

switch

be-

comes

defective. How do

you

know

that it's defective?

An intruder can

step

defective mat -switch

and the

alarm

will not sound.

was

to overcome those

limita-

tions that the

latching alarm

with

supervised

loop was developed.

Al-

though initially designed

to use corn-

monly available relays,

it remains the

basic alarm circuit;

simply use

high -tech components

to do the

same

thing. Once

you understand

the super-

vised loop

you

can understand

just

about everything,

including

wireless

and a

microprocessor -based alarm.

Supervision

Figure 1

shows a simple latching

supervised alarm.

was

probably de-

signed by Methuselah, although

it is

still in common

use today because

it is

reliable,

and it is absolutely

free from

electrical disturbances

-which can-

not be

said

solid

-state

alarms.

The reason

Fig. l is called a super-

vised alarm is because the

user knows

for certain

that the protective

de-

vices- normally -closed

switches -

are

not only

working, but are properly

set.

Trace the series -circuit

labeled

LOOP!,

which

consists

of relay

RY2,

battery

B2, meter Ml,

switch S2, and

switch Sn.

Sn represents

any number

of series -connected

switch

devices:

fine

wires, spring

switches, magnetic

switches

-any kind of device

that

will open

series circuit.

Relay RY2

is an

extremely sensitive

device,

re-

yuiring only 3 -7

mA to pull in. The

reason that the relay

must

so sen-

sitive is because

it is powered by a 1.5-

volt

battery, B2. The battery

is a spe-

cial

type known as a No. 6 telephone,

railroad,

protective alarm cell. It is

humongous,

more than 6- inches high

and 25

/8-

inches in diameter.

(You

probably used one many years ago

your

school's science class.) Since it

supplies just a few milliamperes

current -and

only

when

the loop cir-

cuit is closed -the battery could

last

for years -or at the

very

least,

several

months.

The

battery

called

an EOL (End

Of Line) power

source because it is

literally at the end of all the

wiring in

the loop, usually buried

somewhere

out

sight.

When

all switches

the

loop are

closed,

the current flowing in the loop

causes a

reading on meter MI. If the

user

sees

meter reading he knows

that the circuit is "armed ": All

switches

are closed

and operating. If

there is no meter reading the user

knows that

either

a window, a door, or

some other protected entrance

open, or a switch is defective,

the

battery is pooped out. In

short,

the

current

flowing in

the

loop supervises

the

loop.

When

the loop is armed,

the cur-

rent flowing

in the loop

causes

RY2's

spring - loaded contacts

to be pulled

open.

anything

stops the current

flow in the loop

-we'll show shortly

how

it's done -RY1's

contacts spring

back, closing the series connection

battery B1, relay RYI, momentary

switch PB1, etc. RYI's contacts

self -

latch RYI so that the relay remains

pulled in, even

if RY2's

contacts

are

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