Instructables

Converting VGA Monitor Splitter into computer-controlled Monitor Switcher

Picture of Converting VGA Monitor Splitter into computer-controlled Monitor Switcher
This instructable explains how a cheap (20 EURO) VGA monitor splitter which is used to connect one PC to two monitor can be converted into a computer controlled-monitor switcher. The final device is controlled via the parallel port and allows to turn on both monitors independently.

I use the final device in my HTPC set-up: one PC, one 15" TFT and a SVGA projector. For watching the latest news, the 15" is fine but for movies the projector is used. But then, the 15" TFT has to be turned off. I didn't want to physically turn it off and there was no easy-to-use way to control it by the PC other than switching its power on/off. This comes especially handy, if your PC (e.g. a silent barebone) only has a single VGA out. With the monitor splitter both the projector and the TFT can be used to show the PC screen. To disable a VGA output its sync signals (HSYNC+VSYNC) are disabled.

I've got the monitor switcher from pearl.de (german site)
 
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Step 1: Analysis of the Monitor Splitter

Picture of Analysis of the Monitor Splitter
schematic-1-640.png
I've traced the HSYNC (pin 13) and the VSYNC (pin 14) of the PC VGA connector to the 74LS244 logic chip, a tristate bus driver. Both output enable signals were connected to ground to enable all lines. This chip interestingly allows to tristate its 8 lines in groups of 4 and by this this bus driver already provides the flexibility to turn the sync signal for both outputs independently on/off. See schematics and datasheet.

Step 2: Desolder Bus Driver

Picture of Desolder Bus Driver
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As I had to rewire the signals to the IC, I desoldered the chip from the PCB (accidentaly destroyed all pads , too). Then I used double sided tape to glue it on a VGA connector for easier soldering.

Step 3: Wiring the Monitor Switcher

Picture of Wiring the Monitor Switcher
schematic-2-640.png
I had to re-wire it in such a way that the sync signals for the first VGA output were enabled by the upper half of the logic chip and the second by the lower half. To enable its use without a computer, I've used 2 pull-down resistors with 10 k to enable both outputs per default. The resistores are shown in the next step. See schematics.
LasVegas7 years ago
Assuming all other connections are passed through directly, I suggest that you also toggle pin 9 high (+5v) to the active monitor. This would turn on that monitor's DDC circuitry so the computer sees the monitor switch and adjusts the resolution accordingly.
mringwal (author)  LasVegas7 years ago
Thanks. Good point. What I've build is actually not a "switcher" but a splitter with additional on/off feature. The hack allows to independently turn on/off both monitors, so if both would be on, there is no 'active montior'. With this splitter, the DDC circuitry is only connected to one of the monitor outs. (see picture of the back in step 4). For a real "monitor switcher" this would have to be changed and then toggling pin 9 of both monitors as you suggestied is required to support the DDC and automatic switching. In my use case, I will rely on the projector to downscale 1024x768 to 800 x600, or if the quality is worse than expected to just tell the XServer to use the lower resolution when switching to the projector.
Hi mringwal, I have just a question about the vga splitter you used:
Are the other signals (RED, GREEN, BLUE) connected directly from the VGA PC connector to the two monitor VGA connector? Or are they also buffered (74LS244)?
Please, answer me!!

Thank you!!
mringwal (author)  stornara2 years ago
AFAIK they are also buffered. The 74LS244 provides 8 lines, which are enough for all signals.
thanks for the datasheets.
i am working on a projekt to make a pixel monitor wall without a computer.
i hoped that i can just give a small signal to each of the RGB oprts of the VGA-plug from the monitor to control a solid color on a monitor.
i am getting crazy by "circuitbend" each of the pins on the vga plug.

by your datasheet i hope i can make bigger stepsin getting to my goal.

can anybody help me what would be the key-move to get a first result in bringing up the monitor jsut show a red(or green or blue) display?

i would be so thankful if anybody had tips on building that.
i wanted to use multiple monitors (up to 4 x 4 = 16) to make such a pixel wall and just wanted to connectit to small microcontroller.

please help me

i am sure i posted on the wrong site but i don´t know a other way out.

thnaks
biberce7 years ago
Excellenet job! I was an eyewitness of the demolishing part of your project :) and to be truly honest, I was convinced that you have wasted your 20 euro vga splitter. But thanks to your patience and creativity, you've ended with a real thing that suit your needs! Congratulations!!!
:D :D :D i'm so confused about this ideas...did you sell it in ebay?
mringwal (author)  bijikenyot4 years ago
I used it for a while at home to turn an LCD on/off
do you want know happy story ? in my country, vga splitter with mocrocontroller jsut like you build, with on/off signal can make bucks some $100 per unit
mringwal (author)  bijikenyot4 years ago
Sure, that's a niche use case ("professional"), which increases the cost automatically, although the functionality is already available in the cheap version. That's what's instructables is for :)
but in my version, i only add manual switch to cut off/on horizontal & yertical syn :D :D :D
mringwal (author)  biberce7 years ago
Thanks. I wasn't sure about it either after removing the chip.. :) (but I still could have got another IC, it would just have taken some time)
GreenD4 years ago
I'm still learning electrical engineering, but I'm confused what you did with the chip - and why was the chip's tristate enables always set to enable?? Seems pointless to me??
mringwal (author)  GreenD4 years ago
hi. the original device amplifies the video signal and provides it at two ports. i rewired the chip, so that each port is driven by one half of the chip, and be controlled via the enable lines. the enable lines have a pull-down (high resistance) resistor to define its state when no controlling computer is connected. if a compurt is connected it can over-ride the enable state without damage,as the pull-downs are high ohm. so... it works as before without control, but can shut off  the two output ports individually.
vmathew5 years ago
OMG PLEASE ANSWER ME!!!! DOES THIS LET YOU USE BOTH MONITORS SIMULTANIOUSLY?!?
mringwal (author)  vmathew5 years ago
Yes. A VGA Monitor Splitter allows to use two monitors simultaneously. This Instructable allows to selectively turn them on/off independently.