• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

vga/rgb cut-out switch

Status
Not open for further replies.

dtowers

New Member
I am trying to put together a circuit that will make the screen of a VGA monitor go black (controlled by a switch). The idea is to "ground" just the RGB lines so that the monitor does not go into a power down mode. I've tried a simple circuit that just routes the RGB lines through a 4066 quad switch without luck - in the on state, the signals were obviously attenuated. I was thinking about trying a relay, but I don't have very much experience dealing with this kind of circuit. My question is can the RGB lines just be switched to ground, or should there be resistors in there? My guess would be that the off state should actually route the signal to ground through 75Ω resistors, but that really is just a guess (my experience consists entirely of digital circuits and some simple analog ones where impedance wasn't such an issue). If that's the way to go, should the resistors be on both ends (i.e. put a resistor between ground and the RGB lines on both the computer and monitor ends of the line). I would really prefer to keep the whole thing solid state, so if anyone has suggestions along those lines I'd appreciate that too.
 

dtowers

New Member
Sorry, should have said what it is for. I work in a psychology lab, and we use a video splitter to run the signal to both the monitor in the control room and the monitor the subject sees. We run several experiments, and enter a number of parameters between them - those are the times when we want the subject monitor to be black, but the experimenter monitor to be normal.
 

blueroomelectronics

Well-Known Member
One thing you could try is to disable the H or V sync signal, they're digital and should be easier than tampering with the analog VGA signal (almost guaranteed to make the normal signal worse as even a poor quality VGA cable can make for a poor video signal)
 

dtowers

New Member
Sorry, another detail ... we use a KVM switch since our experiments switch between two computers (one running DOS, the other windows), and we have a parallel port switch so that each of the computers can control our equipment. I'd like to control the video cut-out switch off of an extra parallel port line rather than add a third switch (e.g. another KVM) that out experimenters have to keep track (I have the parallel port interface working, and was using that to control the 4066).
 

dtowers

New Member
One thing you could try is to disable the H or V sync signal, they're digital and should be easier than tampering with the analog VGA signal (almost guaranteed to make the normal signal worse as even a poor quality VGA cable can make for a poor video signal)
Yes, that was the first stab at it. Worked fine, but would display "powering down in 5 seconds" when first switched off (similar messages on other monitors). This might end up being the only way to go, but we would like to avoid that if possible. I know how picky it sounds, but we tested a few people, and they reported the message as being distracting - annoying, but in this context that sort of thing trumps design simplicity.
 

cnra

New Member
Hi dtowers,

I have the exact same issue (psyc lab, splitting video signal between a control room and a testing chamber). Have you had any success in solving this problem. I'm sure this a widespread issue in social science research, and solution you've come up with would be extremely helpful to many researchers.

Thanks
 

Mr RB

Well-Known Member
The analog switches should be fine, they use these in many televisions to switch the AV inputs, since TVs often have 2 or 3 sets of AV inputs these days. Anyway they will switch the RGB analog signals with no problems.

You need to use 2 analog switches, similar to a divider so if the top switch is on the signal is on, and if the top is off and bottom is on it will be properly attenuated.
 
Last edited:
A few years back we had to do something similar. We had a multiple monitor setup in a public place, which needed to be started up/shutdown whilst the customers could see the screens. We used a small PIC, and generated a very simple graphic on the screens until our computers had full started, then the monitor were switched to display the computer screens. We used this to hide the bios/starting windows screens. The code was modified from a PIC games code that we found using google.
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top