Elenco Electronics MO-1251 Uživatelský manuál stáhnout pdf (Strana 31)

100

opened.

Since

the alarm

bell, BZ1,

connected

across

RYI's solenoid

coil,

it sounds

off.

Because

some

towns and

cities

frown

alarms that sound

for hours,

an optional

timer is often

connected in

series with

the alarm circuit so

that

BZI is silenced

after 10 or

15 minutes.

Other than

the timer

drop

-out,

BZI

can only be silenced

by pressing

RESET switch

PBI,

or by opening

MASTER

switch

SI.

The loop's

battery is placed

at the

end of the line

to prevent

easy bypass-

ing

of the system

by an intruder.

For

example,

if the intruder

forces

open a

window

that's protected

by switch

SI,

the loop will

be opened

at points A

and B.

Current will

cease flowing

through RY2

and the alarm will

sound.

If the intruder

should

be able,

somehow,

to cut any part

of the loop

wiring,

again the loop

current ceases

and the alarm will

sound.

Even if the

intruder

tries

to maintain the series

circuit by short-

circuiting points

and

C, the short will occur

in front

the battery, so

the current

through

RY2 will

be interrupted

and the

alarm

will

sound.

That is why

EOL prote--

tion is provided

modern high -tech

alarms

even if

a battery isn't

used.

We'll show later

how it's

done

wi:h

resistors.

Notice

that LOOP

2 is essentially

identical

to LOOP

I. In

early

systems,

entire house was

protected

by a

single loop.

Today, we

usually use

multiple

loops,

one reason

being that

it's

easier to find

an open switch.

For

example, if LOOP

I is

used to protect

the basement windows

while

LOOP 2

is only for

the front

and rear

doors,

you

don't have

to run down

to the

basement

to check

all the window

switches

if the supervisory

meter

shows

that LOOP

2 is open.

Sim-

ilarly, if

the meter shows

that LOOP

is open, you

had better

check the

basement windows.

t17VAC

POWER

SUPPLY

4 12VDC

ALARM

'RIGGER

LED 1 `r

LOOP INDICATOR

BELL

OR DIALER

FIG. 2 -IN MANY MODERN

alarm systems:

the loop

power comes from the center'::

power supply.

LED

used as the

closed

-loop

indicator.

BELL

SIREN

117

VAC

KEYSWITCH

TEST

SWITCHES

AND

INDICATOR

LAMPS

H1)©0

POWER

SUPPLY

lCHARGER

-1

MICROPROCESSOR

lBATTERY

TIME

DELAY

CENTRAL PANEL

INSTANTANEOUS

LOOP

DELAYED LOOP

PRE ALARM

DRY

CONTACTS

FIG. 3-A MICROPROCESSOR

simplifies the

operation

of a basic control center that

provides various

time delays and

signals.

Actually, with the relay

-type sys-

tem you

know that something's wrong

because an open loop

will

cause the

bell to sound as soon as the alarm is

turned on ( "armed

").

The

modern

computer -type system,

however, can

or will

arm even if a loop is open; it

simply bypasses

the open loop. That

means

that if you have a multi -loop

system, and say, for

example, that a

basement

window

has

been

left

open,

the alarm

can/will automatically lock

out the

basement

window

loop. The

lock -out

allows you to set the alarm

and leave,

but your home is really

unprotected

against basement entry. It

is precisely

to avoid

unknown -

lockout

problem that all multi -loop

alarms have individual

indicators -

either a meter

or an LED -for each

loop. And

remember, each loop repre-

sents a separate

protected circuit, and

it doesn't matter

how many protective

devices

are on the circuit; if they are

all series-

connected they are on the

same

supervised loop.

(Take note that

high -tech

alarms now include

nor-

mally -open

switches in

loop. Al-

though

the N.O. switches

are

not

supervised,

they are

considered

loop

switches

because the term loop is now

accepted

to mean all

the

switches

and

detectors on the same

control circuit.)

A final note

before

move on to

the high -tech

stuff. To ensure mini-

mum maintenance,

battery Bl was

re-

placed by a line-

powered supply that

automatically

switched

to a battery

when

the linepower

failed -a not un-

common occurrence

until recent

years. Later

still, the loop

battery

(B2, B3, etc.) was

also replaced by

power from the line-

powered supply,

which

created a reliability problem

that can plague

even microprocessor -

based

alarms.

The problem is

shown in Fig. 2, a

simplified

supervised

loop that is

powered

by the main power supply.

Light-

emitting diode LEDI lights

when switches

SI and

Sn are closed -

showing

that the loop

is armed. But

what

if an intruder

can reach through

a broken window,

or had

previously

short-

circuited

points A and

The

loop

indicator still shows

a closed su-

pervised loop,

but SI and Sn

can

opened without

triggering the

alarm

because they are located

after the

short-

circuit.

Several

early solid -state

alarms

used the loop

powering shown in Fig.

2. Many "home-

type" alarms still

use the Fig. 2 loop

powering, al-

though they

often have

the

option of

EOL (End

Line)

resistor termina-

tion, which

provides

the same securi-

as the EOL battery. We'll

explain

the EOL

resistor termination later.

The control

center

The

control center -which is

also

called a control panel,

a control box,

an alarm control, or whatever

-usu-

ally contains

everything except for the

alarm

bell or siren, and the protective

devices.

Early solid -state control

cen-

ters simply

replaced the relays with

transistors

and/or

SCR's. Most

were

disasters

because an electric distur-

bance

or RF radiation

could trigger

the solid -state

devices, and a light-

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