4 Channel Video Amplifier using NJM2582

A very simple 4 channel video amplifier electronic circuit project can be designed using NJM2582 ic suitable for video applications with SCART connector . Design of the circuit is very simple and require few external electronic parts .

Some features of the NJM2582 are : Operating Voltage ±5V, +5V, +11V ; 6input 4output , 2input 1output Video SW , Internal LPF , 6dB Amplifier , Internal 75Ω Driver Circuit , DC output for SCART (FUNCTION SW, BLANKING) .

Signal Tracer and Injector

Description:
A simple test circuit to fault find audio and radio equipment. Can be used to inject a square wave signal, rich in harmonics, or used with headphones as an audio tracer. a single pole double throw sitch is used to switch between inject and trace modes. The diagram is drawn in trace mode, the earpiece being connected to the collector of the last transistor. Both transistors are wired as emitter followers, providing high gain. DC blocking is provided by the 1n capacitor at the probe end, and the two stages are capacitively coupled.

Circuit Diagram

when the switch is thrown the opposite way (to the blue dot) both transistors are wired as an astable square wave generator. This provides enough harmonics from audio up to several hundred kilohertz and is useful for testing AM radio Receivers.

CCFL Tester for LCD Screens

Liquid crystal displays (LCDs) are used in a wide range of products including flat-screen computer monitors, laptop computers,tablet PCs, PDAs, digital cameras and portable instruments, compact cold cathode fluorescent lamp (CCFL)-based backlight (illumination source for the LCD screen) arrangement in these applications enables a wellviewable display in both dim and bright ambient light conditions. LCD backlight mainly consists of a light source, diaphragm, light-leading board and plastic frame. It has high brightness, long lifetime and good uniformity features.

CCFL frequently employs fluorescent lamps, which have a phosphorcoated glass cylinder with cathodes at each end. CCFL tube is powered by a small electronic inverter (CCFL inverter) circuit that glows the screen electronically. The inverter circuit accepts a low-level DC input voltage and provides a high-level AC output to run the backlight CCFL tube(s).

Fig. 1 shows the CCFLs for LCD. Often a need arises to check a faulty LCD backlight circuit for finding the dead components. Absence of backlight means either the CCFL inverter and/or the CCFL tube is in dead state, here is a simple circuit to test the CCFL tube in a LCD backlight unit, which might help the hardware technician to speed up the repair work. This basic circuit (go/no go test) is portable, 6V battery operated and can be used to test almost all types of LCD backlight CCFL tubes.

The working of the CCFL circuit is shown in Fig. 2. A timer IC NE555 (IC1) is wired as an astable multivibrator (AMV) to drive a standard MOSFET T1 (IRF 512/IRF 830). Components R2, R3 and C3 determine the operating frequency of the AMV. MOSFET T1 switches the inverter output transformer (X1) to produce high-voltage AC supply at its output terminals. Here an ordinary step-down transformer is used as the inverter transformer by reversing its primary and secondary windings.

Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Fix the 2-pin connector on front side of the cabinet in such a way that the CCFL under test can easily be connected here. Fix the test switch on top of the frame. Keep the 6V battery in side the cabinet.

To test a CCFL, connect its terminals to the primary leads of transformer X1 and press test switch S1 momentarily. If the CCFL is good, you will notice a dim/bright glow in it (depends on the wattage rating of the CCFL under test). Always try to keep the test time as short as possible.

Warning. Repairing the LCD backlight unit is not an easy task even for experienced technicians. If you do something wrong you may permanently damage the LCD screen and have to buy a new one!

3-Input Video MUX Cable Driver Using LT1399

The circuit diagram shows a low-cost 3-input video MUX cable driver. In this circuit, the amplifier is loaded by the sum of RF and RG of each disabled amplifier. Resistor values have been chosen to keep the total back termination at 75 Ω while maintaining a gain of 1 at the 75-Ω load.

The switching time between any two channels is approximately 32 ns when both enable pins are driven. When designing a circuit board for this cable driver, care should be taken to minimize trace lengths at the inverting input. The ground plane should also be pulled away from RF and RG on both sides of the board to minimize stray capacitance. Current consumption of the cable driver is a modest 8mA.

Motorized Video Camera Mount

Introduction:
This article describes a motorized, waterproof mount for a miniature video camera. If a suitable waterproof camera is used, this unit can be used outdoors.

Theory:
The camera rotator circuit uses a 2716 EPROM to store a table of logic values that control the motor driver (H-bridge) circuit. The EPROM data is shown in the schematic. By using the EPROM, a large number of discrete gates are eliminated. The logic table is designed to allow the motor to turn clockwise until the clockwise limit sensor is activated. The same operation happens with counter clockwise rotation and the counter clockwise limit sensor.

Inputs to the EPROM come from the limit sensors and the two control switch directions. Outputs go to the four H-bridge transistor gates. The control switch signals are buffered through the 7400 quad NAND gate, this allows for a long control wire. All of the input values are active in the low state.The H-bridge array consists of two N-channel MOSFETs and two P-channel MOSFETs. Diagonal pairs of transistors are turned on to move the motor one way or the other. If all of the transistors are off, the motor does not move. Note that the P channel transistors turn on with a 0 logic output level and the N channel transistors turn on with a 1 logic output level.

There are several disallowed output states, if three or four transistors were to be turned on, the transistors would go up in smoke. Don't do this. If the EPROM is programmed correctly, this should never happen.
The voltage regulator produces 5 volts for running the logic ICs and the motor. A better H-bridge driver circuit (or IC) could be used if higher motor currents are needed, this one was sufficient, simple to build, and easy to find parts for.

Note that a much simpler version of this circuit could be made by using a cross-wired center off DPDT direction switch and mechanical limit sensor switches in series with the motor power wires and with diodes across the switches. This circuit has fewer moving parts, and the sensors can fit into a smaller space than switches. The all solid state design should also last longer.

Specifications:

• Degrees of rotation: 350 (depending on hall effect sensor locations)
• Operating Voltage: 12V D.C.
• Operating Current: approx. 500ma when rotating and 30ma idle.
• Remote Control Interface: 4 wire shielded cable
• Height: 16 inches
• Width: 2-1/4 inches

Electrical Construction:
There are two circuit boards in this project, both were hand-wired. The main board houses the logic circuitry and the motor H-driver VMOS FET array. It was constructed on a solderable bread-board.The hall effect sensor array was built on a piece of copper PC board. The board was cut to a C shape and holes were drilled in the ends so that the board could be screwed to the motor mount tabs. The two hall effect sensors were glued to the PC board with epoxy glue. Be sure to secure the wire coming from the sensor board with some form of strain relief. Plastic wire ties are suitable for this job.

Both sensors are located an equal distance from the center of the motor shaft. Other sensor board components were hand-wired to the sensors. The sensor board can be seen in the photo, it is mounted on the top of the motor where the shaft exits the motor case. The D-shaped aluminum block has a small, but powerful magnet glued to the side that passes directly over the hall effect sensors. The magnets pass within about 1/8 inch over the sensors.
Mechanical Construction

This project involves a fair amount of mechanical work. The tube that holds the camera, electronics, and motor was fabricated with Schedule 40 ABS black pipe. The longer portion (top) of the pipe is stationary, and should be secured to an external mount with hose clamps or other mounts. The top portion consists of a cap on top, a long tube with room for the motor and logic board, and an ABS pipe sleeve. Glue the sleeve to the bottom of the long pipe. Don't get any glue on the bottom half of the sleeve. The top cap can be connected to the upper pipe by drilling a small hole through the cap pipe, and installing a small stainless steel screw. A small hole is drilled through the upper pipe, this allows the video camera and motor control wires to exit the assembly.

When the assembly is complete, seal the wires where they exit with Silicone caulk.The small, lower portion of the pipe houses the video camera. The top inch of the lower portion should be evenly filed around the outside so that it can be fitted easily into the top portion's sleeve. Keep filing until the lower portion of the pipe spins easily in the sleeve. A hole needs to be cut in the lower portion to house the camera. This can be done by drilling small holes, then filing the opening until the camera fits snugly. The hole should be large enough to allow the camera to be adjusted up and down. I secured my camera in the lower assembly with blue packing foam. A pipe cap covers the bottom of the lower assembly.

The camera's wire should pass through the inside of the tube, put a few loops of extra wire on the camera side. It is important to verify that the wire does not get hung up on any objects in its path.
Water flow should be considered, if the assembly is built correctly, it should be able to withstand blowing rain without getting the electronics wet. A small hole should be drilled in the lower pipe cap to allow any moisture that condenses inside to escape.Several metal pieces need to be fabricated. A small bracket is needed to connect the motor mount to the side of the upper tube. The size of the bracket depends on the motor that is used. The motor shaft should be exactly centered in the tube when the mount is complete.

The shaft mount piece is a D-shaped chunk of aluminum, a hole was drilled to fit the motor shaft, a side hole was drilled and tapped to hold a set-screw for securing the mount to the shaft. Two mount holes were drilled and tapped into the mount, screws pass through the lower pipe into the mount. The magnet is glued to the bottom of the shaft mount, it should pass right over the hall effect sensors. Test the magnet on the hall effect sensors before gluing them in place, the sensors only respond to one side (pole) of the magnet.

Alignment:
Make sure that the camera wire does not get hung up on the insides of the camera, this is achieved by adjusting the length so that the wire has some slack when it is at either extreme of the movement. It is advisable to round any sharp edges that are in the area where the video wire rotates.
The hall effect sensors should be checked out, make sure that the magnet changes the logic state on both sensor outputs when it passes over them.

Use:
Turn the switch to CCW, the camera should rotate couter clockwise until the limit is sensed. Turn the switch to CW, the camera should rotate clockwise until the other limit is reached. Turn the switch off, the camera should stand still.

Miniature Black and White TV System

Introduction:
Incredibly small video cameras have recently become available at reasonable prices. Small televisions are available for very little money at online auction sites such as eBay. It is now possible to build a miniature short-range wireless video system with off-the-shelf parts. This wireless nature of this circuit is not suitable for long distance operation, the video modulator just provides a simple method for interfacing the video signal to a standard TV. Transmission distance is limited to a few feet.

Theory:
The 12VDC supply provides power for the camera and video modulator circuits. The video from the camera is fed into the video input of the modulator circuit. The modulated RF from the modulator is fed into a small antenna. Use of a dipole antenna that is resonant at the frequency of the modulator can extend the signal range. The RF travels across a short distance to the Sony Watchman TV receiver. A black and white image magically appears on the TV screen.

Construction:
The camera's video signal is connected to the video modulator with an RCA jumper cable. Power to the camera and video modulator is connected to the 12V power supply. Be careful with polarity, reversing the leads may damage the modulator. The camera that was used had reverse polarity protection built in. The modulator's RF output signal is connected to a small antenna, the antenna can be made with two short lengths of #16 gauge solid wire. Alternately, for long distance wired operation, the RF signal can be fed into a length of 75 ohm coax cable with a 75 ohm terminating resistor across the far end of the cable. Connect the remote center conductor of the cable to the TV antenna through a 330 ohm resistor.

Use:
Turn the power for the camera and modulator on, turn on the TV. Tune the TV to the channel of the RF modulator, fine-tune the TV for the best picture. For full-time operation, use the appropriate AC adapter for the watchman. If battery operation is desired, run the TV from its internal batteries and use a 12V battery for powering the camera and modulator. A rechargeable lead acid battery with a series fuse (and a recharging circuit) is recommended.

A fun use for this system would be to create an engineer's view of a model train layout. A loop of wire near the track would make a good receiving antenna. Power could be pulled from the engine motor circuit using a bridge rectifier feeding into an electrolytic capacitor, just make sure not to exceed the camera's 12V maximum supply voltage.

Parts:

• Miniature black and white video camera, 12VDC, Model PC-206XP
• Video modulator block, Jameco 141639CJ or equivalent
• Sony Watchman miniature TV
• 12V DC power supply, beware of wall-warts - they often have inaccurate voltage ratings.
• Miscellaneous wires and cables

NTSC-PAL TV Signal Identifier

This circuit is able to identify PAL and NTSC video signals. Its output is high for an NTSC signal and low if the signal is PAL. This output signal can be used, for example, to automatically switch in a colour subcarrier converter or some other device while an NTSC signal is being received. One application is for the reception from satellites of 'free-to-air' TV signals, which in Australia generally contain a mixture of 625-line PAL and 525-line NTSC programs. Operation of the circuit is as follows. IC1 is an LM1881 video sync separator which takes the video input signal and generates vertical synchronisation pulses.

For an NTSC signal, these pulses are 16.66ms apart, corresponding to the 60Hz field rate, while for a PAL signal they are 20ms apart, corresponding to the 50Hz field rate. The vertical sync pulses are fed into IC2a, the first of two dual retriggerable monostable multivibrators in the 74HC123A. IC2a has a period of very close to 17.9ms, set by the 200kO resistor and 0.22µF capacitor at pins 14 & 15. Because the monostable is retriggerable, NTSC sync pulses arriving every 16.66ms will keep its Q output, at pin 13, high.

Circuit diagram:

NTSC-PAL TV Signal Identifier Circuit Diagram

However PAL sync pulses arriving every 20ms will allow the Q output to go low after 17.9ms, before being triggered high again 2.1ms later. Thus an NTSC signal will give a constant high output while a PAL signal will result in a train of pulses 2.1ms wide. The Q output from IC2a is fed to the inverting input of IC2b, the second monostable, which has a period of about 0.5s, as set by the 270kO resistor and 4.7µF tantalum capacitor at pins 6 & 7. With its input constantly high, resulting from an NTSC signal, IC2b is not triggered and its Q output remains low.

However, the pulse train from a PAL signal will constantly retrigger it, so its Q output will remain high. The period of IC2b also effectively makes it a low-pass filter which removes spurious switching due to any input glitches. The output signal is taken from the Q-bar (inverted) output, so that an NTSC signal gives a high output, while PAL gives low. For the particular application for which the circuit was developed, diode D1 and the resistor network shown drive the base of an NPN switching transistor and relay. A dual-colour 3-lead LED can also be fitted to indicate NTSC (red) or PAL (green). Note that with no video input, the output signal is high and will indicate NTSC.

Slave Flash With Red-Eye Delay

Digital cameras are becoming more and more affordable. At the economy end of the market cameras are usually equipped with a small built-in flash unit that is ideal for close-ups and simple portraiture. The power rating of the built-in flash unit is quite low so that any subject further away than about 2 to 3 metres (maybe 4 m if you are lucky) tends to disappear into the gloom.

You soon become aware of the limitations if you need to photograph a larger group of people say at a function under artificial light in a large hall or outdoors. The majority of these cameras are not fitted with an accessory socket so it is not possible to simply connect a second flash unit to increase the amount of light.



 Single lens reflex cameras also need additional lighting (e.g. fill-in flash) to reduce the harsh contrast produced by a single light source. For all these cases an additional slave flashgun is a useful addition to the equipment bag. Rather than shelling out lots of cash on a professional slave flashgun, the circuit here converts any add-on flashgun into a slave flash unit triggered by light from the camera flash.

Simple slave flash circuits can have problems because most modern cameras use a red eye reduction pre-flash sequence. This pre-flash is useful for portraiture. It is designed to allow time for the subjects pupils to contract so that the red inner surface of the eye is not visible when the picture is taken.


 Some cameras use information gathered at this preflash time to estimate the light power required for the main flash period and some use this time to fine-tune the auto focus. A simple slave flash circuit will be triggered by the pre-flash sequence and will therefore not provide any additional lighting when the main flash occurs and the picture is actually taken.

The circuit shown here is quite simple but neatly solves the pre-flash problem. With switch S1 set to ‘Normal’, the pulse produced by D1 when it detects the camera flash will trigger both monoflops IC1a and IC1b. The output of IC1.A does not perform any useful action in this mode because the logic level on the other side of resistor R4 is pulled high by D3.


 The output of IC1.B will go high for approximately 10 ms switching T1 on and causing the triac to conduct and trigger the slave flash. The use of a triac optocoupler here has the advantage that the circuit can be used on older types of flashgun triggered by switching a voltage of around 100 V as well as newer types that require only a few volts to be switched. With switch S1 in the delay position the first flash will trigger IC1.

A and its output will enable IC1.B but the low pass characteristics of the filter formed by R4 and C5 slow the rising edge of this waveform so that IC1.B will only be enabled 10 ms after the first flash is detected. IC1.B is now enabled for a period of about 1s (governed by R1 and C3).

When the main flash occurs in this time window it will immediately trigger IC1.B and the triac will be switched as described above. The circuit requires a supply of 3 V and draws very little current from the two 1.5 V button cells. It will run continuously for quite a few days, should it be accidentally left on. Switch S1 can be either a three-position toggle or slider type.

Circuit construction is greatly simplified and the finished unit looks much neater if it is built on the available PCB. Space is also provided to fit the PCB mounted battery holders. A suitable flash extension cable or adapter can be found in most photo shops.

Resistors:

• R1, R3 = 100kΩ
• R2 = 100Ω
• R4, R5 = 220kΩ
• R6 = 1kΩ

Capacitors:

• C1, C3 = 10µF 16 V radial
• C2, C4 = 100nF
• C5 = 47nF

Semiconductors:

• D1 = TLRH180P
• D2, D3 = BAT85
• IC1 = 4538P
• IC2 = MOC3020
• T1 = BC547B

Miscellaneous:

• Bt1 = two 1.5V batteries (LR44) with PCB mount holder
• S1 = 3-position slide switch
• Cable or adaptor for external flasher

Video Amplifier

The video amplifier in the diagram is a well-known design. Simple, yet very useful, were it not for the ease with which the transistors can be damaged if the potentiometers (black level and signal amplitude) are in their extreme position. Fortunately, this can be obviated by the addition of two resistors. If in the diagram R3 and R4 were direct connections, as in the original design, and P1 were fully clockwise and P2 fully anticlockwise, such a large base current would flow through T1 that this transistor would give up the ghost.

Moreover, with the wiper of P2 at earth level, the base current of T2 would be dangerously high. Resistors R3 and R4 are sufficient protection against such mishaps, since they limit the base currents to a level of not more than 5 mA. Shunt capacitor C4 prevents R4 having an adverse effect on the amplification.

Turtle Beach Anounces iSeries Media Headsets.

When it comes to audio brands, there is one particular name that stands out above the rest for some people, and Turtle Beach would be one of them. The name Turtle Beach has long been synonymous with high quality audio devices, and this time around the company has announced their spanking new iSeries range of high end media headsets.

The iSeries will arrive in the form of the wireless surround sound i60 and the wireless amplified stereo i30, where these would no doubt be the first headsets that have been specially constructed for use with both Mac and iOS products. The iSeries headsets themselves will arrive with a bunch of innovations that Turtle Beach originally introduced to gaming headsets but so happen to be new for mobile users. Hopefully, with a little bit of something for everyone, folks will be satisfied.

Some of the new features introduced to gaming headsets that mobile users would fall in love with include improved chat and voice call quality as well as an unmatched level of control over audio. Regardless of whether you settle for the i60 or the i30, they will both share the same unique design which blends style, quality, comfort and durability, while boasting premium finishes, where among them include a leather headband and leather-covered memory foam, noise-isolating ear cups.

The two headsets will also be accompanied by a built-in remote which plays nice with the iPhone, iPad and the Mac. They will come with innovative dual boom-less microphones which will hopefully, deliver clear, high-quality audio during chat, voice calling and use with Siri.

Of course, the i60 will be the flagship model, where it features a control unit for use with Mac desktops and laptops that deliver fingertip audio and preset control. Turtle Beach does have some pretty high hopes that the i60 will also be the first wireless headset for the Mac platform that boasts of 7.1 DTS surround sound with adjustable speaker angles and EQ presets, a fully programmable digital signal processor (DSP), dual-band Wi-Fi for uncompressed, interference free wireless audio and dual-pairing Bluetooth 4.0 connectivity. [Via]

The Audio/Video Distribution Amplifier

With the amount of equipment in home entertainment centers today the need to be able to vary the gain of the audio or video signal is needed. I found this particular circuit helpful when used in conjunction with the Universal Descrambler and a Stabilizer circuit I built for making copies of video tapes.

It not only allowed me the ability to fine tune the video strength it also helped me increase the recorded audio which typically becomes poor when making tape copies. Circuit operation is straight forward for amplifier circuits. The second channel for the audio amplifier is made up of the same components except the other half of IC1 is used. Pin 6 & 5 are inputs and 7 is the output.

Circuit Diagram

VGA to BNC Adapter (Converter)

There are monitors which only have three BNC inputs and which use composite synchronization (‘sync on green’). This circuit has been designed with these types of monitor in mind. As can be seen, the circuit has been kept very simple, but it still gives a reasonable performance. The principle of operation is very straightforward. The RGB signals from the VGA connector are fed to three BNC connectors via AC-coupling capacitors. These have been added to stop any direct current from entering the VGA card. A pull-up resistor on the green output provides a DC offset, while a transistor (a BS170 MOSFET) can switch this output to ground. It is possible to get synchronisation problems when the display is extremely bright, with a maximum green component.

In this case the value of R2 should be reduced a little, but this has the side effect that the brightness noticeably decreases and the load on the graphics card increases. To keep the colour balance the same, the resistors for the other two colors (R1 en R3) have to be changed to the same value as R2. An EXOR gate from IC1 (74HC86) combines the separate V-sync and H-sync signals into a composite sync signal. Since the sync in DOS-modes is often inverted compared to the modes commonly used by Windows, the output of IC1a is inverted by IC1b. JP1 can then by used to select the correct operating mode. This jumper can be replaced by a small two-way switch, if required.


This switch should be mounted directly onto the PCB, as any connecting wires will cause a lot of interference. The PCB has been kept as compact as possible, so the circuit can be mounted in a small metal (earthed!) enclosure. With a monitor connected the current consumption will be in the region of 30 mA. A 78L05 voltage regulator provides a stable 5 V, making it possible to use any type of mains adapter, as long as it supplies at least 9 V. Diode D2 provides protection against a reverse polarity. LED D1 indicates when the supply is present. The circuit should be powered up before connecting it to an active VGA output, as otherwise the sync signals will feed the circuit via the internal protection diodes of IC1, which can be noticed by a dimly lit LED. This is something best avoided.

Resistors:
R1,R2,R3 = 470Ω
R4 = 100Ω
R5 = 3kΩ3
Capacitors:
C1,C3,C5 = 47µF 25V radial
C2,C4,C6,C7,C10 = 100nF ceramic
C8 = 4µF7 63V radial
C9 = 100µF 25V radial
Semiconductors:
D1 = LED, high-efficiency
D2 = 1N4002
T1 = BS170
IC1 = 74HC86
IC2 = 78L05
Miscellaneous:
JP1 = 3-way pinheader with jumper
K1 = 15-way VGA socket (female), PCB mount (angled pins)
K2,K3,K4 = BNC socket (female), PCB mount, 75Ω

Video-Out Coupling

If you want to connect a video signal to several destinations, you need a distribution amplifier to match the 75-ohm video cable. A distribution amplifier terminates the incoming cable in 75 ohms and provides several outputs, each with 75-ohm output impedance. Since this is usually achieved by putting a 75-ohm series resistor in the output lead of each video opamp (current-feedback amplifier), the opamps must be set up for a gain of 2 in order to achieve an insertion gain of 1 (0 dB). The disadvantage of this arrangement is that if the amplifier or its power supply fails, no signal is available at any of the outputs. This can be remedied by using a high input impedance amplifier, which can be tapped into a video line without having to have its own 75-ohm termination resistor.


In order to eliminate hum interference and voltage differences between the cable screen and the circuit earth, the circuit exploits the common-mode rejection of the opamp. This can be optimized with resistor RG1. With the indicated LT1396 video opamp, more than 40 dB of common-mode rejection can be achieved. The signal bandwidth of the circuit can be optimized using the trimpots. It reaches to more than 10 MHz, which is quite acceptable for video signals. Thanks to the high-impedance connection to the video line, the video signal is not affected when the power for the coupled amplifier is switched off. You can learn more about the LT1396 from its data sheet at http://www.linear-tech.com.

Video Isolator Circuit Diagram

These days many more audio-visual devices in the home are connected together. This is especially the case with the TV, which may be connected to a DVD player, a hard disk recorder, a surround-sound receiver and often a PC as well. This often creates a problem when earth loops are created in the shielding of the video cables, which may cause hum and other interference. The surround-sound receiver contains a tuner that takes its signal from a central aerial distribution system.

The TV is also connected to this and it’s highly likely that the PC has a TV-card, which again is connected to the same system. On top of this, there are many analogue connections between these devices, such as audio cables. The usual result of this is that there will be a hum in the audio installation, but in some cases you may also see interference on the TV screen.
The ground loop problem can be overcome by galvanically isolating the video connections, for example at the aerial inputs of the surround-sound receiver and the TV.

Special adaptors or filters are sold for this purpose, known as video ground loop isolators. Good news: such a filter can also be easily made at home by yourself. There are two ways in which you can create galvanic isolation in a TV cable. The first is to use an isolating transformer with two separate windings. The other is to use two coupling capacitors in series with the cable. The latter method is easily the simplest to implement and generally works well enough in practice. The simplest way to produce such a ‘filter’ is as an in-line adapter, so you can just plug it onto either end of a TV aerial cable.

Diagram and snapshoot:

Video Isolator Circuit

The only requirements are a male and female coax plug and two capacitors. The latter have to be suitable for high-frequency applications, such as ceramic or MKT types. It is furthermore advisable to choose types rated for high voltages (400 V), since the voltages across these capacitors can be higher than you might expect (A PC that isn’t connected to the mains Earth can have a voltage as high as 115 V (but at a very low, safe current), caused by the filter capacitors in its power supply.

These capacitors don’t need to be high value ones, since they only have to pass through frequencies above about 50 MHz. Values of 1 nF or 2.2 nF are therefore sufficient. To make the isolator you should connect one capacitor between the two earth connections of the coax plugs and the other between the two signal connections. The mechanical construction has to be sturdy enough such that the connections to the capacitors won’t break whenever the inline adapter is removed forcibly.

A good way to do this is to make a cover from a piece of PVC piping for the central part. Wrap aluminium foil round the outside and connect it to one of the plugs, so that the internal parts are properly shielded from external interference. Make sure that the aluminium foil doesn’t make contact with the other plug, otherwise you lose the isolation. The majority of earth loops will disappear when you connect these filters to all used outputs of the central aerial distribution system where the signal enters the house.

Harry Baggen
Elektor Electronics 2008