SC_2.4GHZ_Link.pdf

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Here's an easy-to-build project which will

provide very reliable video and audio links

over several hundred metres or more. With

O.5W output, it operates on one of four

channels way up in the 2.4GHz band.

By ROSS TESTER

OU WILL HAVE SEEN

adverts for devices of this

type - they've become quite

popular in recent years.

Operating on a frequency of

2.4GHz (that's 2,400,000,000Hz for the

uninitiated!), most have about 10mW

or so output and while they work well

over a short range, the range is limited

by the low power.

This design has much higher power

- around 0.5W output, in fact. So as

you might expect, the range is very

significantly extended. With the sim

ple coax cable "whip" antennas shown

here, the range is reliably 200m or

more. But if you use a simple dipole

an tenna, you could expect much more

range - maybe 10 times or more.

Gain antennas

Perhaps a word or two about how

and why this is possible is in order.

is sometimes difficult for people to

understand how changing antennas

can gi ve longer range.

SILICON CHIP

The simplest analogy I can think of

is using your own voice. You can talk

at a certain level and you'll be heard

up to a certain range. You can shout,

and of course you'll be heard by peo

ple further away. You're increasing

the power of your voice.

Or you could cup your hands

around your mouth and project your

voice in a certain direction. Those off

to the side WOll't hear as much (if at

all) but those in the direction you're

projecting will hear much more.

That's the equivalent of using a di

rectional antenna. You're concentrat

ing power in one particular direction

at the expense of other (unwanted)

directions.

you replaced your hands with a

long length of pipe, those to the side

would hear little, if anything. But those

www.siliconchip.com.au

at the other end of the pipe, even over

a very much longer distance, could

possibly hear you.

That's the equivalent of using a

highly directional antenna. Very little

energy is radiated in any direction

except the one you want.

OK, now that we know how to get

longer range by increasing power and/

or using directional antennas, let's get

back to the AudiolVideo Link.

ANTENNA

lN4004

7.5-9VDC

AUDIO

VlPN

Modular construction

One of the biggest difficulties for

the hobbyist working at ultra-high fre

quencies is the precision necessary in

construction. As the wavelengths be

come shorter and shorter (and at

2.4GHz the wavelength is only a cou

ple of centimetres), even resistor leads

become effective little antennas - but

probably in areas of the circuit you

don't want radiation.

Surface mount devices (SMDs) have

to a large extent solved that problem

but they are rather difficult devices to

work with given the normal range of

hobbyist tools - and experience.

The beauty of this design is that it

uses pre-built and pre-aligned mod

ules from Oatley Electronics for both

the transmitter and receiver. All you

have to do is solder them to the PC

board, add a few power supply com

ponents, input and output sockets and

an antenna - and the project is largely

completed.

Now before you say "too easy" there

are a couple of wrinkles.

The first is the precision necessary

in soldering the modules to the PC

boards.

you think that soldering nor

mal ICs and multi-pin sockets to PC

boards is difficult, wait 'til you see

this one!

The 12-way connector occupies a

space of just 5mm x 1.5mm. And you

have to solder everyone of those pins

in without any solder bridges. You'll

need a steady hand and a very fine

tipped iron to do it. We'll take a closer

look at this later on.

Second, you have to accurately cut

the antenna to length. As we said be

fore, at 2.4GHz, a few millimetres make

a difference, so you're also going to

have to be pretty careful with this.

Apart from that, construction

should be quite simple.

(0:

':r'

LED

(0:

UNK'

-o--o-

'THESE UNKS

REPLACE C2 & C3

LVI_D_EO

...

2.4GHZ AUDIO·VIDEO TRANSMITTER

Fig.I: using the modules is easy - just add a

regulated power supply circuit,

an antenna and the audio/video sockets. The operating channel is selected using

a wire link.

ANTENNA

l00IlF

VIDEO 1

+ .-

7805

VIDEO

OUT

~AUDIO

OUT

REGl

7805

lN4004

FIT ONE

UNKONlY,

220llF

AUDIO

VIDEO

RECEIVER

MODULE

8 CH1

9 CH2

10 CH3

+ .-

TOSruCT

CHANNEl

+51/

CH4

GND 3

lOOIlF

2.4GHZ AUDIO-VIDEO RECEIVER

Fig.2: the receiver circuit is just as simple as the transmitter but note that

different pin numbers are used to select the operating channel.

The modules

There are two different modules,

one for the transmitter and one for the

www.siliconchip.com.au

receiver (as you might expect!). The

transmitter is the smaller of the two,

measuring 43 x 30 x 8mm. The re

ceiver is 53 x 35 x 10mm.

Apart from the multi-way connec

tion socket on the back which we men

tioned before, the only other connec

tion you need to make is the antennas,

which solder directly to the modules.

Just a word of advice: don't attempt

to open the module cans to see what's

inside. You're highly likely tp damage

them and there's nothing you can re

pair anyway.

The modules solder to identical PC

boards but there are a few more com

ponents on the receiver board than

the transmitter board. Both have on-

FEBRUARY 2002

• K171 2,AGHzA/V RECEIVER

CH4

CHl

CH2

CH3

CHI

CH2

CH3

CH4

SOlDER

SHIBD

TO METAl

CASE

UNKS

UNDER

BOARD

UNKS

UNDER

BOARD

NOTE:

PlNSIN

UPPER

HOlfS

SOLDER.!

SOLDER

INNER

WIRE TO

END OF

PC BOARD

INNER

WIRETO

END OF

PC BOARD

Fjg.3: build

th~

transmitter board by installing the

PiU'ts as shown here. The 3·terminllJ regulator

(REGl) is l!1stalled on the copper side of the PC

bOard - see photo.

Fig.4: an identical PC board is used for the receiver

but note that the parts layout is slightly different to

that used for the transmitter. The channel selection

link is on the copper side of the board.

board RCA sockets for audio and video

input or output, a diode, resistor, LED

and capali:itor (three capacitors in the

case of thk receiver).

On the back of both boards is a 5V 3

terminal regulator.

On the prototypes (as photographed)

there is another small electrolytic ca

pacitor soldered across the regulator

pins (mainly 'cos it was forgotten ... )

However, on production boards this

electro will be transferred to the front,

as shown in the component layout

diagrams.

Construction

We suggest you leave the modules

until last. Assemble the rest of the

components on the PC boards - front

side first, then the 3-terminal regula

tor (REGl)'on the back.

The regulators screw to the PC board

with a 3mrp. x 10mm machine screw

and nut. M;ountingthem hard down

on the board

ass~sts

with keeping them

cool - no further heatsinking is re

quired.

Before soldering the modules to the

PC boards, you have to cut and solder

the antennas (assuming you're using

the simple coax cable type). Fig.5

shows the coax'stripping details.

Solder the aqtenna to the receiver

or transmitter module with the inner

conductor going to the antenna termi

nal and the braid, or shield, soldering

direct to the module case as close as

possible. Next, solder a loop of hookup

wire from the module case around the

SILICON CHIP

antenna (coax insulation) and back to

the case. This holds the antenna in

place.

Now it's time for the difficult bit:

soldering the module onto the PC

board. We used the word "bit" to re

mind us of step 1: fit the finest-possi

ble tip/bit to your soldering iron and

make sure it is very clean and nicely

tinned.

There is no easy way to solder the

module in place and it's easy to acci

dentally bridge adjacent contacts. For

this reason, it would be wise to have a

roll of solder wick on hand to imme

diately remove any bridges you do

make.

You'll also need a high power mag

nifying glass (a jeweller's loupe is bet

ter) and a bright light to visually in

spect the board during and after sol

dering the module.

One possible tip for soldering this

module: solder all the contacts as best

you can and then use the solder wick

to quickly remove the solder you've

just placed. This should ensure that

the pins pads are all nicely "tinned"

and just need the tiniest of touch-ups

with a hot soldering iron and some

very fine solder.

Again, though, we would strongly

advise a lit, magnified visual inspec

tion of this section of the board before

moving on.

And just in case you were wonder

ing - yes, the transmitter only uses 10

of the 12 pads.

(REMOVE

OUTER

SLEEVE

BRAID

ONLY)

31mm

Channel selection

Alongside the 12-pin contacts there

are eight closely-spaced pads which

are used to select the frequency on

which the system works. This can be

changed to avoid interference from

other 2.4GHz systems. The same pair

of pads must be linked on both the

transmitter and receiver boards.

50fl

CO-AXIAL

CABLE

Alignment

Here's the quickest alignment of a

transmitter and receiver in history.

You don't have to do it - it's done.

Fig.5: each antenna is made by

removing exactly 3lmm of the

outer sleeve and braid from one

end of some

son

coaxial cable.

Power supply

A 9V battery is not the best solution

for this project - the input power is

www.siliconchip.com.au

around 1.2W so you'll be dragging

about 130mA or so.

won't last long

at all.

you are using the system inside a

building, a 9-12V, 300mA plugpack

would seem the way to go. Outside,

(or away from mains power), recharge

able nicads or NiMH cells would be a

much better proposition. Six cells will

give about 7.2V,leaving enough head

room for the 7805 regulator.

long-term battery-powered use is

contemplated, another possibility is

to do away with the 7805 regulator

completely and run the circuit (with

appropriate track links) direct from 4

x 1200mA or higher nicads.

At 1.2V each, four cells will give

4.8V when charged - a tad under the

5V from the regulator but within the

modules' spec. You would have to

watch out for low voltage as the nicads

drop their bundle but as a rule they do

that rather quickly. You might need to

also remove diode

to avoid its 0.6V

loss but

you do, remember you have

no protection against "oopses" with

the supply connections.

1200mAH nicads are quite com

monly available these days as are

higher power "C" and "D" cells.

Another option would be a 6V gell

cell. There's 0.6V drop across

Dl,

bringing the supply to about 5.4Y.

you think that's sailing a bit too close

to the wind put another diode in se

ries with the first for a 1.2V drop.

Testing

Once you have the power supply

dilemma solved, hook up appropriate

sources of video and audio to the trans

mitter. This done, connect a video

monitor and amplifier to the receiv

er's video and audio output sockets

respectively and apply power. You

should have the modules separated

by at least several metres for this check.

Assuming no mistakes, you should

find that they work first up. There are

no adjustments to make, with the pos

sible exception of antenna length (but

without specialised testing equipment

even this is quite difficult).

Now you can experiment with the

modules to see just what sort ofrange

you can achieve. We'd be very sur

prised if it is less than a couple of

hundred metres but remember, at

2.4GHz objects in the way can make a

lot of difference - walls, trees, power

lines, etc could be problems. You

might even find that what works on a

dry day is hopeless on a wet day (es

pecially if your path is through foli

age).

Incidentally, the maximum distance

over which we have actually tested

this link.is 50 metres (yes, the length

of my yard!).

worked beautifully

rock solid picture, great audio, etc.

This was in the week prior to Christ

mas but over the break I'm going to

really put it through its paces.

Oatley Electronics report a number

of these units have already been sold

to people who have installed them on

such things as hang gliders and bal

loons, with line-of-sight (air to ground)

ranges in the several kilometres range.

Pity I don't have a hang glider or bal

loon!

Parts List

2 PC boards, 55 x 48mm, coded

K171 (Oatley Electronics)

4 mono PC-mount RCA sockets

1 2.4GHz video transmitter

module (Oatley Electronics)

1 2.4GHz video receiver module

(Oatley Electronics)

2 1N4004 power diodes (01)

27805 5V regulators (REG1)

2 3mm red LEOs (LE01)

2 120mm lengths 50-ohm coax

Hookup wire for power supply

connection, etc

2 M3 x 10mm machine screws,

nuts and washers

Capacitors

1 220JlF 16VW electrolytic

5 100JlF 16VW electrolytics

Resistors (0.25W, 1%)

22.2kQ (red red red brown or

red red' black brown brown)

WHERE TO BUY THE KIT

A kit with all the above-listed parts

is available from Oatley Elect

ronics, PO Box 89, Oatley, 2223.

Phone (02) 9584 3563 or email

sales@oatleyelectronics.com. The

price is $159 plus $7 for postage.

Data transmission?

While we haven't tried

and there

fore cannot comment on success or

failure, Oatley have also had reports

of users putting these links in data

applications, feeding in via the video

input.

anyone has any ideas (or better

still experience) on this, we'd love to

hear from you!

This view shows the completed trans

mitter unit. The antenna is secured

with a wire loop soldered to the back

of the module.

www.siliconchip.com.au

The matching receiver unit is similar

to the transmitter. Don't forget to

install matching channel selection

links on the back of both boards.

Here's how the 3-terminal regulator is

mounted. Ignore the

lOOf.lF

capacitor

the board has been modified so that

it's now mounted on the front.

FEBRUARY 2002

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