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Actually you can interface quite a few pieces of equipment to the vga port. Vga analog monitors are pretty standard now, but that was not always the case. To save money, people would make their own adapter cables for connecting competing brands of monitors. In fact, quite a few companies did quite well selling their own pre made cable adapters to save people all the soldering.

Now the interfacing is even more important with the introduction of the i2c interface in the vga port. which means you can interface all kinds of sensors from a compass, temperature sensors, to a host of other devices.

Note: Try this at your own risk. Get a professional to help if you are unsure. Not all cables have all 15 pins wired. 

Step 1: Vga Cable and Y-adapter.

You probably could just dissect the y-adapter and be done, but I wanted to preserve it for it's original use.

Needed:
1 - vga cable end from a dead monitor if possible.
1 - vga y-cable (so the monitor and the i2c bus can be used at the same time.
2 - resistors of same value from 4.7 k to 10 k ohm.
2 - 8 to 10 position screw down bar.

Ohm meter or continuity tester. (https://www.instructables.com/id/Yet-another-continuity-tester)
Wire strippers. (I like to use a pipe cutter for doing the outside sheath. )
Screwdriver.

Step 2: Make It.

Instead of buying a vga cable, I just cut one of a dead monitor.
Take the vga cable and strip off the outer shell at least a foot back or so.
Using a pipe cutter (also good for pvc) makes cutting the outer shell so much easier..
Some cables have the grounds encased in a second tube which needs to be stripped.
Strip the fifteen (if you have that many)  individual wires back a half an inch.
Take a continuity tester and find a wire and  where it connects on the db 15.
Connect that wire to the appropriate point on the screw down bars. 
Repeat till all the wires are done. 

Note: Our cable did not have all 15 pins wired so we had to use what we had. Also if the rgb lines are in-cased in shields, you will have to modify the wiring diagram.

Step 3: I2c Interface.

Attach the resistors.
Attach the ground.

Typical i2c application.

Step 4: Vga to Composite.

Never could get these to work with newer systems. You really need a special driver and video/monitor to use the cable. Did not try it with dvd player that had composite in. Have to make a new cable to try it.

Courtesy of Epanorama.net. 

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 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

 

Step 5: Fake Video Adapter. (aka Headless Server)

This is great for servers that need vga attached, but you access the system remotely and therefore you will not be using a monitor..
vga fake adapter:

That is…

red (pin 1) –> 75 ohm resistor –> red return (pin 6)
green (pin 2) –> 75 ohm resistor –> green return (pin 7)
blue (pin 3) –> 75 ohm resistor –> blue return (pin 8)

Note: Newer servers have the ability to overlook the monitor not being installed at boot time..

Step 6: Yet Another Vga to Composite Adapter.

Another format for a vga to composite adapter. I have not tried this yet so use at your own risk.

Step 7: Microcontrollers.

Very easy to connect to the arduino via i2c, You need to get the specific code from your video card maker for more details.

Interesting page: : http://www.paintyourdragon.com/?p=43

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