VGA Background Lighting Circuit Diagram

More and more people are using a PC (conventional or notebook) to view films. The VGA output can be used to provide a matching ‘Ambilight’ effect for this. If you restrict your-self to a single RGB LED, you can also draw the power for this circuit from the VGA connector, along with the RGB signals. 

The following pins of the 15-way VGA connector (three rows of five pins) are used for  this circuit: 

Pin 1:  Red video signal
Pin 2:  Green video signal
Pin 3:  Blue video signal
Pin 5:  GND
Pin 9:  +5 V 

The video signals for the red, green and  blue channels are available at the RGB out-puts. These signals have an amplitude of 1 to 1.35 V, and they output the screen imagery at the rate of dozens of frames per second. This produces the visible image on the screen. The circuit described here drives an RGB LED according to the average values of each of these three signals. Of course, this is not a full-fledged ‘Ambilight’ system, but the RGB LED will produce a nice green light during a  football match or an orange hue if a sunset is shown on the screen.

A sawtooth generator is built around IC1 and T1. It supplies a nice sawtooth signal to opamp IC2a via R6. The frequency of the sawtooth signal is approximately 850 Hz, and its amplitude ranges from 1.6 to 3.4 V. IC2A subtracts approximately 1.6 V from this due to voltage divider R4/R5. After this, voltage  divider R10/R11 reduces the peak value of the sawtooth to around 1.35 V. The resulting sawtooth signal is buffered by IC2b and  used to drive the three comparators in IC3. The level of the red video signal is averaged  by the R12/C2 network. IC3a constantly com-pares the previously generated sawtooth signal with the average value of the red video  signal. If the image has a high red content, the output of IC3a will be logic Low a good deal of the time, while with a low red content  it will be Low less often. This comparator circuit thus implements a PWM driver for the red LED. The same arrangement is used for the green and blue channels.

Note that with a notebook computer you always have to enable the VGA first, usually by pressing Fn-F5. If you use a desktop or tower PC, you can tap off the video signals from an adapter connected between the video cable and the monitor. 

You can also use several LEDs or a LED strip (available from Ikea and other sources) in place of a single RGB LED. In this case you will need an external power supply for the LEDs, but the control circuit can still be powered from the PC. If you use multiple LEDs or a LED strip, connect the cathodes (negative leads) of the LEDs to the comparator outputs of IC3 as shown on the schematic diagram, and connect all the anodes (positive leads) to the external power supply. Resistors R15–R17 are often already integrated in the LED strip. There’s no harm in using an external supply with a higher working voltage, such as 12 V. Remember to connect the ground terminal of the external supply to the ground of the control circuit. 

IC3 can handle a current of 15 mA on each  output. If this is not enough, swap the connections to the inverting and non-inverting inputs of the three comparators in IC3 and  connect their outputs to the bases of three  BC547 transistors. Connect a 10-kΩ resistor between each base and the positive supply  line (+5 V). Connect the emitter of each transistor to ground, and connect the collector  to the LED strip. A BC547 can switch up to  100 mA with this arrangement, and a BC517  can handle up to 500 mA.

Author : Heino Peters

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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. 
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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.
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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.
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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.
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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

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