The VGA video fader came about by personal frustration with our church's video system. Our church uses a computer and projector to display the liturgy for the congregation to follow. Often the projectionist would need to select a slide out of the order of the sequence saved on the computer. The result was a distracting flip through the pages until the correct slide was found, or worse, exiting the slide program to show the desktop of the computer.

I had a NTSC composite video fader that I wanted to use, but the church had chosen to use a VGA signal to feed the projector. I looked on the net to see if I could find a VGA fader or switch that could blank out the screen, but I was surprised that none were being made inexpensively with just the simple fade to black or blank black screen. I started to do some research on a solution to this problem and I discovered that most large projection units have the black out as a feature of the projector. However, our projector did not have such a feature, and if it did we would have to have some kind of remote control for the button to use it since the projector is 50 feet from the computer.

I found a simple passive composite video fader circuit on line

I used this basic idea and converted it to work with each of the three video signals that VGA uses. I ended up using MOSFETs to do the work of fading the image to black by shorting the VGA signal to ground. I then used a digital potentiometer IC to control the MOSFETs. I used the hysteresis of a 4093 quad NAND gate to be a variable speed oscillator that would generate the steps needed by the digital potentiometer to step through the voltage levels. I used the extra NAND gates to debounce the switch inputs in the form of a simple R/S flipflop configuration. The basic idea was to gang three MOSFETs together, one for each video signal (Red, Green, Blue), and then control them by an on switch and an off switch with video fading in and out as an option.

With this design, I went to build the circuit into one box that could be adjusted for different video sources, and with it's own on board power supply. As a cool side effect of blanking out just the RGB signals, the sync signals stay active and the the projector never knows that the signal is being blanked out. If you simply switch off the VGA signal, by unplugging it from the projector it caused the projector to go into video input search mode and the screen becomes very distracting to the viewers as the screen turns blue and prints the input number. The video fader eliminates all that distraction, and just fades the projector to black and the projectionist can flip through the slides on his computer screen at will without anything being seen from the projector.

Another cool side effect of the design is that when the circuit is off the VGA signals are just passed through as if no circuit were in between the computer and projector. It is a truly passive connection.