Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Instrukcja Użytkownika Pobierz pdf (Strona 69)

100

BUILD

THE

Route out

interferance,

evesdropping,

and

with

handheld

detector

Charles

D. Rakes

ELECTRONIC

"BUG"

SWATTER

IS IT

SAFE? HAS YOUR LUCK

SUDDENL

gone sour?

your

competition always

one jump ahead? Could

it be that some-

one

is listening in on your

private con-

versations?

Is there an electronic

bug hidden

in your home or office? Does paranoia

touch

you at every turn?

Is some elusive equipment

noise

interfering

with

your

favorite radio program?

If the answer to one

or more of those perplexing

questions is yes, then

might

be to your advantage

take a closer look at the

Electronic Bug Swatter

-an

RF sniffer,

which we

jocularly

refer to as Mr. McSniff.

Mr. McSniff can sniff

out minuscule RF signals

from

just about any source including

radiation from a hidden

bug to the

garbage

that cometh from our

microwave

ovens.

adjusting

the Bug Swatter's

antenna length and

sensitivity control, signals as

small as a few milliwatts

antenna meltdown can be detected

and their

locations pinpointed. RF radiation

with frequencies as

low as 10 kHz to

wiggles as fast as 1 GHz can be picked

up by the Bug Swatter.

If you decide to take

the challenge and build the

Bug

Swatter,

for no other reason

than to ferret out RF

interference, it could prove

to be the most

valuable little

gadget in your electronic

detectives bag of tricks.

which is connected in

Ula's

feedback loop.

With

no RF present

at the

antenna,

the out-

put of Ula (pin 1) is near

ground potential,

and LEDI and

LED2 are dark. But as soon

as a signal

is detected, the op -amp's output

goes positive,

lighting one or both

the

LED's.

LED2

lights first for low

-level RF signals;

LEDI comes

the signal increases

strength. By aiming

the antenna

and adjust-

ing the gain control

(R18),

while watching the two

LED

indicators, the

RF source can be located

in short order.

The remaining

three op -amps

add the coup

de maitre

to the detector's

output circuit, by supplying

an audible

tone that

varies

in frequency

step

with

the

strength of

the detected RF

signal.

Op -amps UIb

and Ulc, along

with their associated

components, comprise

a voltage -controlled

oscillator

(VCO)

that operates

in the low

audio -frequency

range.

When

very

weak

signal

is detected, the oscillator

responds

with

a low-

frequency growl

that increases to a

more pleasant tone of

several hundred

Hertz as the RF

signal increases

in level.

Op -amp

Uld

adds isolation

between

the

oscillator

circuit and

the speaker's driver

transistor, Q2.

Modus Operand!

Figure 1 shows a schematic diagram

of the Bug

Swatter. Its front end is designed

pass

a broad

band of

RF signals to

a sensitive germanium

diode detector.

Dl.

energy

induced into the pull

-up antenna is

coupled

to the -detector diode through Cl

. The two RF

chokes (LI and L2)

are connected in series to

extend the

maximum detectable

frequency range. LI

(a 10 -µH

choke)

offers circuit isolation for

the higher

frequen-

cies, and L2 (a 2.2 -mH

choke) accommodates

the

lower frequencies.

When an RF signal is detected,

DI produces a nega-

tive

voltage, which is

fed through

the two chokes

to the

high end of the gain

control, R18. The detected

output

is tapped off at the

wiper

and

fed to the negative input

of Ula (pin 2) through

R17. The op -amp's

gain is set

near its

maximum

value by R5 (a 5- megohm resistor),

The

Bug Swatter's printed-

circuit

board is

shown

here

fully

populated just prior

to being tucked away

in its

enclosure.

Remember to

keep the leads from

the

board

tc the off-board

components

as short as possible

to prevent

the leads

from

picking up

stray

signals, and giving

false

indications.

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