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Audio-Video
Transmitter
Whether you are want to monitor
a surveillance camera or transmit
your VCR’s signal to another TV set,
this Video Transmitter will come in
handy. It avoids the need for difficult
cabling and can send signals over a
20-metre range.
By JOHN CLARKE
I
T IS BECOMING MORE and more
common to install video surveillance
cameras, to improve safety and to deter thieves. You can use them to monitor
the swimming pool, the front or rear
door and the baby sleeping. But
while they provide you with
a clear coverage of the area
under surveillance, they
can be very difficult to install. This is because you
need to run wiring between
the camera and monitor
which can involve drilling
through brick walls and
accessing tight spots in the
ceiling or under the floors.
Using a Video Transmitter removes the need to run
the wiring. It also means that
you can use a television set to receive
the signal. This is because the video signal from the camera is modulated and
transmitted through the air just like a
miniature TV station.
Note that a video monitor cannot
directly be used to receive the signal. If
you want to use a video monitor, you will
need to use a VCR to receive the signal
36 Silicon Chip
first. The video output from the VCR
can then be connected to the monitor.
The Video Transmitter can also be
used to transmit signals from a VCR
to a second or remote TV set in the
home without interconnecting wires.
The Video Transmitter is housed in
a small plastic case. It has a telescopic
antenna to transmit the signal and is
powered from a 12VAC plugpack. It
has a 12VDC output for powering a
video camera and audio board. The
transmitter’s modulator will accept
both video and sound inputs.
The schematic arrangement for the
Video Transmitter circuit is shown in
Fig.1. It comprises a power supply, a
video modulator and two wideband
amplifiers. The video modulator
produces a modulated television
transmission on VHF channel 0 or
channel 1. This radio frequency signal
is then boosted by the two amplifiers
to a sufficient level for transmission
via the telescopic whip antenna.
The transmission range depends
on the transmitting and receiving
antennas. From our experiments you
can expect good reception using a
short antenna over a 15m range in
Fig.1: block diagram of the video transmitter. The video modulator
operates at VHF channel 0 or 1.
a typical single level home, while
reception over a 20m range requires
both antennas to be adjusted in length
to match the transmission frequency.
If you have a 2-storey home, you will
probably be able to obtain satisfactory
results from one level to another but
if your home has reinforced concrete
floors (ie, with steel mesh), the system
will not work from floor to floor.
Circuit description
The circuit for the Video Transmitter is shown in Fig.2. The video signal
Main Features
•
•
•
•
•
•
Transmits over a 20m range.
Provides both video and
mono sound.
Channel 0 or channel 1
selection.
12VAC plugpack operation.
12V DC output for a video
camera.
12V output for a microphone
adaptor.
Fig.2: two cascaded wideband monolithic wideband amplifiers boost the video modulator’s signal so that it has a
range of about 15 metres in a typical single-storey household.
JULY 1999 37
Fig.5: winding details for
the input filter inductor L1.
The signal at the wiper of VR1 is
AC-coupled to the emitter of transistor
Q1 which acts as a DC level shifter
and clamp to provide DC restoration.
In fact, Q1 is connected as an emitter
follower which is biased by trimpot
VR2. So how does Q1 work? What
happens is that the voltage at the emitter is held constant at 0.6V (nominal)
below the base. The video signal is
AC-coupled to the emitter of Q2 via
a 470µF capacitor and while it is able
to drag the emitter up in voltage it
cannot pull it down below the level
set by VR2. This means that the bottom
of the sync pulses are clamped to the
emitter voltage while the video signal
can ride up above this level.
Hence, DC restoration has been
applied to the video signal before it
is fed to the modulator. So what does
all that mean in practice? It allows the
best contrast range in the transmitted
picture; ie, the full range from black to
white in a B&W picture or luminance
in a colour picture.
The audio signal for the modulator
is coupled in via two 10µF capacitors
connected back-to-back.
The RF output from the modulator
is applied to two monolithic amplifiers, IC1 & IC2, connected in cascade.
The specified amplifier, uPC1688G,
is a wideband surface mount device
capable of providing amplification for
frequencies up to 1GHz and the power
gain at 500MHz is typically 21dB.
A .001µF capacitor couples the signal from the video modulator to the
Fig.3: wiring layout for the video transmitter. The two surface-mount wideband amplifiers are mounted on the copper side of the PC board – see photo.
Fig.4: actual size artwork for the PC board.
is AC-coupled via two back-to-back
470µF capacitors to remove any DC
offset and then applied to trimpot VR1
which sets the video level applied to
the modulator. If the video level is set
too high, the modulator will produce a
signal that is received as overly bright
and washed out. Conversely, if the
video signal is too low, the picture
will appear unsaturated (dark) and
may have a tendency to roll due to an
inadequate sync signal.
Table 1: Resistor Colour Codes
No.
1
2
38 Silicon Chip
Value
10kΩ
82Ω
4-Band Code (1%)
brown black orange brown
grey red black brown
5-Band Code (1%)
brown black black red brown
grey red black gold brown
Parts List
Fig.6: this is the full-size artwork for the front-panel label.
input of IC1 while and 82Ω resistor
provides loading for the signal. The
output of IC1 is then AC-coupled to
IC2 via another .001µF capacitor and
loading is provided by an 82Ω resis
tor connected in series with a .001µF
capacitor.
With two of these amplifiers working in cascade you might think that
there would be quite a strong signal
being fed to the whip antenna but the
reality is a little different. Let’s look
at what happens. First, the nominal
output level from the video modulator
is 78dBµV and this means that the
output is 78dB above 1µV. This corresponds to about 8mV of signal into
75Ω or 0.85µW. The two uPC1688G
amplifiers each have a power gain
of about 21dB (at or below 500MHz)
and so we are amplifying by a total
of 42dB. This corresponds to a power
amplification of 15,848 and the total
expected power at the antenna is
13.4mW. That’s in theory.
In practice, the coupling losses
caused by loading mis
matches at
each stage and to the whip antenna
mean that the signal radiated by the
whip antenna is really quite small but
adequate to give a maximum range of
about 20 metres inside a typical home.
Power for the circuit comes from
a 12VAC plugpack which is fed via a
low-pass filter consisting of inductors
Table 2: Capacitor Codes
Value
IEC Code EIA Code
.01µF 103 10n
.001µF 102 1n
L1a & L1b, wound on a balun core,
together with two .01µF capacitors.
These prevent video signals from
being radiated from the mains leads
and also prevents hum modulation in
the video transmission.
The filtered AC voltage is full wave
rectified using diodes D1-D4 and
smoothed with a 470µF capacitor. It is
then fed to two 3-terminal regulators.
REG1 provides the +5V supply for the
modulator and amplifiers while REG2
provides +12V for a video camera.
Construction
The Video Transmitter is constructed on a PC board coded 0240-5991 and
measuring 105 x 60mm. It is housed
in a plastic case measuring 130 x 68
x 41mm.
You can begin construction by
checking the PC board against the
published pattern. Check that the hole
sizes are correct and that there are
no shorts or breaks between tracks.
Check also that the PC board clips
neatly into the integral clips within
the plastic case.
Fig.3 shows the PC board component overlay and wiring layout. Install
the resistors first, using Table 1 as a
guide to the colour codes for each
value. Then insert the PC stakes at the
input terminals, the antenna output,
the RF output from the modulator and
the three anchor points for the vertical
shield between the RF output and the
3-terminal regulators.
Install the video modulator and be
sure to solder its three earth pins to the
PC board groundplane. The RF output
from the modulator requires an RCA
plug to be inserted with a short length
1 PC board, code 02405991,
105 x 60mm
1 single sided blank PC board,
15 x 50mm
1 front panel label, 125 x 65mm
1 VHF video modulator (Jaycar
LM 3850)
1 plastic case, 130 x 68 x 41mm
1 12VAC 300mA plugpack
1 balun former 1050/2/F29, L1
2 RCA panel-mount sockets
2 DC panel sockets
1 TV telescopic antenna
1 RCA line plug
1 150mm length of 0.63mm
enamelled copper wire
1 150mm length of medium duty
hookup wire
2 spade or eyelet connectors
1 6mm spacer
1 M3 x 15mm screw and two
nuts
1 M3 x 10mm screw, star washer
and two nuts
14 PC stakes
Semiconductors
2 uPC1688G wideband
amplifiers (IC1,IC2d) (DSE
Z-6011)
1 7805 5V regulator (REG1)
1 7812 12V regulator (REG2)
1 BC337 NPN transistor (Q1)
4 1N4004 1A diodes (D1-D4)
Capacitors
1 470µF 25VW PC electrolytic
3 470µF 16VW PC electrolytic
5 10µF 16VW PC electrolytic
4 .01µF ceramic
4 .001µF ceramic
Resistors (1%, 0.25W)
1 10kΩ
2 82Ω
1 1kΩ horizontal trimpot, VR2
1 100Ω horizontal trimpot, VR1
of hookup wire soldered to the centre
terminal. This centre terminal wire
connects to the PC stake as shown.
Solder a short length of wire at the
side of the RCA plug and connect it
to the earth PC stake.
The remaining components can
now be mounted. The electroly
tic
capacitors must be inserted with
the correct polarity while the small
ceramic types can be inserted either
JULY 1999 39
The vertical shield piece consists of a piece of blank 15 x 50mm PC board and
is installed by soldering it to three PC stakes, as shown here. A piece of tinplate
could also be used for the shield if you don’t have any blank PC board.
way round. The codes marked on the
side indicate their value and Table 2
shows the possible markings for the
two values used in this circuit. The
ceramic capacitors should be mounted
as close as possible to the PC board.
The 100Ω trimpot (VR1) may be
marked with a 101 code while the 1kΩ
trimpot (VR2) may be marked 102. Install these in the positions shown. Diodes D1-D4 must be oriented as shown
and when installing the regulators be
sure you position the 5V one (REG1)
nearest to diode D2. The regulators are
mounted with a 6mm spacer between
them, with a 15mm M3 screw and nut
clamping them together. This acts as
a form of heatsinking for REG1 and
as an earth point for the supply filter.
The vertical shield is made from a
piece of blank PC board (or tinplate)
measuring 15 x 50mm. It is mounted
as shown by soldering the copper to
all three of the PC stakes.
Amplifiers IC1 & IC2 are tiny
surface-mount devices which are
mounted on the copper side of the PC
board. The surface-mount package is
rectangular with a tab connection at
each corner. You will need a soldering
iron with a very small tip and ideally,
you should use a desk mount magnifier lamp when doing the job. You will
certainly need it to identify pin 1 on
each uPC1688G package. It is slightly
wider than the remaining three pins
and must be positioned as shown on
the PC board.
Inductor L1 is wound as shown in
Fig.4. Use 0.6mm enamelled copper
wire and wind on two coils of five
turns each, in the directions shown,
for L1a and L1b.
Use the panel label as a guide to
drilling the holes for the RCA sockets
and the DC sockets in the side of the
box. A 3mm hole is also required for
the antenna mounting screw which is
positioned adjacent the antenna output PC pin. Drill out these holes and
position the PC board in the box. Wire
up the antenna to an eyelet terminal
and secure it with the 3mm screw, star
washers and two nuts.
If you do not want to use a tele
scopic antenna, you can use a length
of wire instead.
Fit the RCA sockets and wire these
up with hookup wire. The 12V DC
socket can also be connected with
hookup wire. The 12VAC socket is
wired via inductor L1 and the terminals are bypassed with the .01µF
ceramic capacitors. These are earthed
to a solder lug eyelet which is secured
to the screw located on the regulator
tabs.
Testing
With all the wiring complete, check
your work carefully against the dia-
PIN 1
PIN 1
You will need to use a fine-tipped soldering iron, a pair of
tweezers and a magnifying lamp when soldering the two
uPC1688G wideband amplifiers (circled) to the copper
side of PC board. Be sure to correctly identify pin 1 (the
wider pin) of each IC before soldering it into place.
40 Silicon Chip
This close-up view shows how the telescopic antenna
is mounted on the side of the case using an M3 x 10mm
machine screw, two nuts and a star washer. If you don’t
want to use a telescopic antenna, you can use a length of
wire instead.
The two 3-terminal regulators are bolted together with a 6mm spacer between
them, using a 15mm M3 screw and nut. This acts as a form of heatsinking for
REG1 (5V). Be sure to orient these two device correctly; their metal tabs both
face towards the shield piece.
grams of Fig.2 & Fig.3. Then apply
12V from an AC (or DC) plugpack
to the input socket and measure the
voltage at the outputs of REG1 and
REG2. You should obtain +5V and
+12V respectively. The supply pins
to IC1 and IC2 can be measured on
the .01µF capacitor leads and should
be +5V. Similarly, the input and
output DC voltages on IC1 & IC2 can
be measured on the .001µF coupling
capacitors. They should be +0.91V
on the input and about +3.3V at the
output.
You can test the transmitter by
applying a video signal to the input.
This signal can be obtained from the
output of a VCR, a video camera or
a TV pattern generator. Connect an
antenna to the TV set using a set of
telescopic “rabbit ears”, a “spiral”
dipole or a ribbon cable dipole. Alternatively, you may be able to receive
the signal via your roof mounted TV
antenna.
Adjust trimpot VR1 slightly anticlockwise from its fully clockwise
setting and set VR2 fully clockwise.
Check that the TV can receive the
signal transmitted by tuning the TV
set to the transmitted channel. The
channel switch on the video modulator is channel 0 when positioned
towards the outside of the case and on
channel 1 when positioned towards
the RF socket.
Now adjust VR1 for the best picture
contrast. In some cases, you may need
to adjust VR2 slightly anticlockwise
to improve the contrast from a video
camera. You should not need to use
this adjustment when the video signal
is from a good program source such as
from a VCR recording or off-air signal.
Antenna adjustments
If you want the maximum range
from the Video Transmitter you will
need to carefully adjust the element
length of both receiving and transmitting antennas and make sure that both
antennas are oriented identically. For
example, they can be mounted both
upright or both horizontal.
For channel 0, the antenna can be
1/4-wavelength (whip) at 810mm
long or half wavelength (dipole) at
1.62m. The channel 1 antenna length
should be 660mm (whip) or 1.3m
(dipole). Note that these lengths may
need to be made about 5-10% shorter
to compensate for the effect of the
antenna thickness on the radiation
impedance. You can make small adjustments to the antenna lengths to
obtain the best transmission.
In most cases, a nominal 1/4-wavelength whip antenna on the transmitter will give good results but
1/2-wavelength antennas will provide
better distance reception.
Note that we have not provided an
on/off switch for the Video Transmitter. This is because it is envisaged that
the transmitter will mainly be used
for surveillance cameras where the
power will be on all the time.
Alternatively, if the transmitter
is used to send signals to a second
TV set for occasional usage, you can
turn the transmitter off by switching off the plugpack at the power
point, by unplugging the plugpack
from the Video Transmitter socket or
by installing a switch in the Video
SC
Transmitter box.
JULY 1999 41
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