Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 61)

lines in

the silver

coating of two

small

glass

mirrors.

One grid was mounted

fixed position,

the other

attached to

the

diaphragm

of a modified

telephone mi-

crophone. When voice

energy

was

direct-

ed against

the diaphragm,

the grid to

which

it was

attached moved

back and

forth in response

to the

signal. This

caused a representative

fluctuation in

the

sunlight passing

through the

two grids.

The

fluctuating light was

detected

by a

homemade

selenium

light

detector de-

signed by Bell.

Professor

Bell called his

pioneering

lightwave

communicator the

"photo -

phone."

In the first version,

described

above, the detector was

separated

from

the grids

by only a few centimeters.

Soon

afterward, Bell and Tainter were

talking

over beams of

sunlight

at distances of

hundreds

of feet. For many

of these

ex-

periments

they

used a

transmitter

so sim-

ple it

can be assembled

by a child.

Figure

5 shows construction

details

of a

modern

version

of one

of Bell's photo

phones.

Bell's version

used

very

thin

glass

mirror

attached

to the

end of a

speaking tube.

The mirror

became

alter-

nately

convex

and concave in response

the

pressure of

sound

waves,

causing the

divergence

of the

beam of sunlight re-

flected from the

mirror to vary

according-

ly. Consequently,

the

intensity of

the

beam at a distant

point was

amplitude or

intensity

modulated

by the

spoken

words.

There

are

many ways

to fashion

transmitter for

a do

-it- yourself

photo -

phone. The

one shown in Fig.

is made

attaching a sheet of

aluminum

foil

(shiny side

out) to one end of

hollow

tube. The tube

can be

cardboard, plastic

or metal. The foil

can be

attached

with

tape or a rubber

band. Aluminized Mylar

provides

a smoother, flatter

surface than

aluminum foil, projecting

narrower

beam than that reflected from

an alumi-

num

foil transmitter.

Unfortunately,

aluminized Mylar

is partially transparent

and, therefore, has less reflectance

than

aluminum foil.

The receiver

for a homemade

photo -

phone

can use any of the light

sensors

mentioned

above. For

best results, how-

ever, use a silicon

solar cell.

The large

sur-

face

area of

this

detector provides

excel-

lent

collection

efficiency without requir-

ing

an external lens.

Since the photo -

phone

is designed to

be used in daylight,

it's important

increase

the

signal -to-

noise

(S /N) ratio

installing

the cell in

one end of

hollow

tube. For

best

results,

spray the inside of

the

tube with

flat

black

paint or line the tube with

black

construc-

tion

paper.

use the photophone,

simply

reflect

a spot of

sunlight toward the

detector.

While

holding

the transmitter

as still as

possible,

speak into its open

end. The per-

son at the receiver

end should then hear

every

word

you speak.

The quality of

Fig. 5. A

basic

photophone

system.

,OrcE ))l

NOLL

TAPE

ALu

,ÑuM

FOIL

TUES

AWMIWI2ED

IX/LAP.

TRANS

MIT TER

SOLAR

CELL.

SPEAKER

Ara

PURER

RECE

\VER

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- . ..... .

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DESCRAMBLER

PARTS

Adaptor

tronics

#701

Includes

diodes,

#702

Original

#704

Both

#701,

We stock the exact

parts, PC Board and A.C.

for an

article

published in Radio Elec-

Magazine (Feb., 1984) on building

your

own

CABLE

DESCRAMBLER.

PARTS PACKAGE $29.95

all

the original resistors,

capacitors,

transistors,

integrated circuits, coils

and

IF transformers.

PC BOARD $12.95

etched & drilled silk

-screened PC Board

used

in the article.

AC ADAPTOR $12.95

(14

volts

DC @ 285MA)

#701

& #702 $39.00

702

704 $49.00

FREE!! Reprint el Radio Electronics

Article on Building Your Own Cable

Oescrambler with any purchase.

Add $2.50

Shipping

& Handling

$4.50 tor

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Orders

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April 1986 /

MODERN

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/ 71

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