Grayscale composite video adapters

First prototype

This is a very simple black and white composite video adapter for experimenters. The circuit is very simple, so the signal is not exactly according the video standard, but you should at least get some kind of picture.

The circuit needs VGA to TV drivers which output negative polarity HSYNC and VSYNC signals. I haven't tested which of the drivers on my web page output negative polarity sync signals (active at logic zero). I have only made tests with Matrox Millinum card which allows easily changing the sync polarities with the configuration utility.

                        ____
 RED   1 >-+-----------|____|----+
           |  ____     82 ohm    |
           +-|____|-+            |
            180 ohm |            |
 BGND  6 -----------+            |
                        ____     |
GREEN  2 >-------------|____|----+-------------> VIDEO OUT
                       22 ohm    |
                        ____     |
 BLUE  3 >-+-----------|____|----+
           |  ____     240 ohm   |
           +-|____|-+            |
            100 ohm |            |
 BGND  8 -----------+            |
                        ____     |
HSYNC 13 >-------------|____|----+
                       470 ohm   |
                        ____     |
VSYNC 14 >-------------|____|----+
                       470 ohm    

  GND  5 --------------------------------------- VIDEO GND

The circuit itself is so small that it can be easily built inside a VGA connector shell (that's how I built my prototype). The circuit is suitable for borh PAL and NTSC systems.

Another circuit idea

I have another idea how the sync signal might be producted better. Here I have used a transistor to combine HSYNC and VSYNC signals instead of resistors like in my first circuit. In theory this circuit should work better, but I have not yet tested this version. This circuit also needs that HSYNC and VSYNC signal are negative polarity.

                        ____
 RED   1 >-+-----------|____|----+
           |  ____     82 ohm    |
           +-|____|-+            |
            180 ohm |            |
 BGND  6 -----------+            |
                        ____     |
GREEN  2 >-------------|____|----+-------------> VIDEO OUT
                       22 ohm    |
                        ____     |
 BLUE  3 >-+-----------|____|----+
           |  ____     240 ohm   |
           +-|____|-+            |
            100 ohm |            |
 BGND  8 -----------+            |
                                 |
                  BC548     ___  |    
HSYNC 13 >------      __---|___|-+
                 \     /|  680 ohm     
                  \   /
                 ---------
             ____   |
VSYNC 14 >--|____|--+
             1 kohm

  GND  5 --------------------------------------- VIDEO GND

SCART to gray scale composite video

Here is an untested idea how to generate composite vidoe signal from the SCART RGB output from by VGA to TV circuit.

                        ____
 RED  15 >-+-----------|____|----+
           |  ____     82 ohm    |
           +-|____|-+            |
            180 ohm |            |
 BGND 13 >----------+            |
                        ____     |
GREEN 11 >-------------|____|----+-------------> VIDEO OUT
                       22 ohm    |
                        ____     |
 BLUE  7 >-+-----------|____|----+
           |  ____     240 ohm   |
           +-|____|-+            |
            100 ohm |            |
 BGND  5 -----------+            |
                        ____     |
CSYNC 20 >-------------|____|----+
                       220 ohm    

  GND 17 --------------------------------------- VIDEO GND

Sources

• Circuit for converting spearate sync signals to SYNC-in-GREEN by Peter Stieglecker
• `True' VGA Grey-Scale adapter by Kees Stravers

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Tomi Engdahl <[email protected]>