Epson L90P - TM Two-color Thermal Line Printer Manual de usuario descargar pdf (Pagina 46)

HOW IT

WORKS

Room

sounds

are picked up by

the microphone

and passed

through the LEVEL control

to the

audio amplifier

module. The load for the amplifier

is R3, whose

value

is selected

for a

higher -than-

normal output voltage (not power).

This voltage

is

applied

through current -limiting

resistor

R2

to

the gate and cathode

of

SC

Rl. Diode D1 allow

-

only positive

-going

pukes to reach the gate of

SCR1.

When the room

noise level is sufficiently

high.

SCR1 conducts

and permits current

to now

through the coil of relay

Kl

and

upu transistor

Q1. This transistor is turned

on by

the bias pro -

vided by resistor R-1. When the relay

is

energized,

one set of contacts

supplies

power

to

the

external

alarm and

the

other

set applies d.c. to the timing

circuit composed of

Cl. R5, R6,

and

unijunction

transistor

02.

Capacitor

Cl starts

to

charge through R5 (the

timing

control)

and

when

it reaches

a

certain

level, fires 02. With (22 conducting. C'1 is

dis-

charged, cutting

off

Q1. The series circuit

through

the

relay coil is thus broken and the

alarm

stops.

Because the SCR is operating from

a d.c. source.

its series circuit must be interrupted

to

make it

turn off. The circuit is then ready to operate

again

whenever

the roost sound level reaches

the

pre-

scribed

level. The setting of potentiometer

R5

determines

how long the alarm

operates

after be-

ing set off.

Diode

D2 suppresses toltage

spikes

generated

When

the

relay

is switched

off.

and

connect a

20,000-

ohms /volt

d.c. volt-

meter

between

the

battery negative

and

the

cathode

of SCR1. With

R1

turned

fully

up, tapping the microphone

should

produce an

indication

on the voltmeter.

If

not, make sure

diode

D1 is

wired cor-

rectly, and increase the

value

of R3

while

reducing the

value

of

R2 in small

steps.

Remove the

short on the collector

and

emitter of QI and again tap the

micro-

phone.

If the alarm still doesn't sound

off, QI

may not be turned on. Reduce the

value of R4 in

small

increments

until

the

alarm sounds when the

microphone is

tapped.

If the

alarm sounds

continuously after

it

once

turns

on, Q1 may

not

be

cutting

off

when Q2

fires. To

check

this, connect

a

high- impedance voltmeter between the

battery negative

and the

emitter

of Q2.

The indicated

voltage should

rise slowly

and then

fall rapidly

as

Q2 fires.

The val-

ue of R5 ( with the value

of C1)

deter-

mines

the

rise time.

Calibration. The

LEVEL potentiometer,

R1, can be calibrated

in arbitrary

values

across

its range. As examples of

calibra-

tion,

you can use steps

such as "someone

sick in the

house," "birthday party,"

"Saturday

night," "normal

riot," etc. or

you can calibrate

it in hours of the day,

with the

least

amount

of noise permitted

for the late hours.

Once the

microphone

has

been

placed

in

an

out -of- the -way place,

and

the

LEVEL set as desired,

the alarm will

sound off

if

the

room noise exceeds

that

for

which the

Riot Restrainer

is set.

To use the device as an

intruder de-

tector, place

the

microphone

in the cen-

ter of the

room, and set the

LEVEL

control as desired.

Then tiptoe

out. Un-

fortunately,

the barking

of a nearby dog,

a plane overhead, or the

horn of a pass-

ing car can set

off the alarm.

Front -panel

components.

As in

the

internal

layout, physical

arrange-

ment

is not critical, and

any

pack-

aging approach

will do.

The audio

output jack

can

be

eliminated

if

the chosen

loudspeaker

is wired di-

rectly

to the

correct impedance

tap of

amplifier output transformer

through one

contact

of

switch S1.

50

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