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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