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)
Step 1: Analysis of the Monitor Splitter
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
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
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.
Step 4: Adding the Control
A normal 3.5 " headphone jack was added into the rear terminal to access the control lines.
Looks like it was designed for this in the first place.