Marantz 5220 Manuel d'utilisateur télécharger pdf (Page 38)

THEFT

ALARM

FOR HANDHELD

CALCULATORS

BY

TOMMY

N. TYLER

HANDHELD

calculators are a

prime target for

thieves

because

they can be quickly picked up

and

tucked out

of sight.

Though

you could

physically fasten the

calculator to your

desk, this defeats the

purpose of

its

portability.

Here's

another ap-

proach-an

audible

alarm that sounds

off

when

the

calculator

is

unplugged

from its

charger.

The

Alarm

Circuit. Simplicity is the

key feature

of this alarm. It is

inserted

between the

charger and

the

cal-

culator, as

shown

in

Fig. 1,

and

draws

a

nominal

amount of power from the

charger. As long

as a

trickle charging

current (at least

100

µA) flows into the

calculator,

silicon diode D1 conducts.

The

forward

voltage drop

across it

keeps

germanium transistor Q1

turned

on.

Transistor

Q2,

which

can

be

almost

any

pnp

device,

is cut

off,

and the

Sonalert

alarm

is

silenced.

However,

if the

calculator

is

un-

plugged, Q1

turns

off, Q2 turns

on,

and the

Sonalert

starts

to

howl. Obvi-

ously, if the charger

is

unplugged, the

alarm will not operate.

So, it's

impor-

tant

either to hide the

charger

or

sec-

ure it in

some

way

so

that

the thief will

not disconnect

it. To

prevent the

alarm

from

becoming

obvious

(when

it is

si-

lenced!),

it's

a good

idea

to build the

alarm

and

the

charger into

one

small

enclosure.

Two

variations

on the circuit

are

shown

in Figures 2

and 3.

The

relay

contacts

can be used to trigger

a

re-

mote

signalling device. Install

diode

D2 to

prevent destruction

of

Q2

by

in-

ductive voltage

spikes

generated

by

Fig.

i.

if'Aea trickle

carrent tlnoagA

I)1 .titups,

Soaalert is

activated.

42

PARTS LIST

C1

* -1

-pF,

15

-volt electrolytic capacitor

DI, D2 *- 1N4001 rectifier

J1-Calculator-type power

jack

K1 * -6-9

volt,

500 -ohm relay (Radio

Shack 275 -004 or equivalent)

P2 -Charger

-type

power plug

QI-

General- purpose

germanium pnp

transistor

Q2-

General- purpose

pnp transistor

RI -1000 -ohm,

r

/s

-W, 10% resistor

R2 -2200 -ohm

or

10,000

-ohm, rh

-W, 10%

resistor (see

text)

Sonalert- Mallory SC628P

Spkr

* -3.2 -ohm dynamic

speaker.

T1* -500-

or

1000- ohm/3.2 -ohm audio

transformer

Misc. Perforated or

pc

board, hookup

wire, solder, suitable enclosure,

machine hardware, etc.

*Optional. See

text.

keying the relay. Figure

3

shows a

small transistor

oscillator which

can

be

used

in place

of the

Sonalert. It can

be assembled

from

junk

box

parts.

(Note

R2 is changed

to

2200

ohms.)

Construction.

Take

care

in

wiring

jack J1 and plug

P2,

observing correct

polarities. Although

D1 will prevent

damage to

the calculator

from

reverse

current, the

unit's batteries

will never

charge! And,

of course, be

sure that P2

and

JI are the

same types

as those

on

the

charger

and

calculator. Either

pc

or

perforated

board may

be used. Both

parts placement

and the

selection

of

semiconductors

are

not

critical.

Just

be

sure

that

D1 is rated

to handle

the

charger's maximum

output (in the

event that P2

is

accidentally

shorted),

and Q1

is

a germanium device.

Q

TO

EXTERNAL

ALARM

I-

F'irt.

?.

Fur'

a IV raote alnrr,

rr>

a

relalt iasterrd

Soaalcr.l.

Fig. .1.

_ladio oscillator

rna

also

he

aserl to provide

aa

aarlible

sigaal.

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