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Custom Video Switch

Hey, so I've been tossing this idea around for a while, and I'm getting closer and closer.

so my big issue is that for KVM's, component signal, etc, the actual switches are expensive and hard to come by unless you want to pay out the arse for them.

so I've been spending the last couple months researching a custom solution instead, based around a bus switch circuit IC.

effectively what this is, is a method to make a video (and possibly data switch with some tweaking), for less than $25.

to start with, there's articles online, however they're somewhat dated, and not ideal.

so building on the work of an old endgadget article going over a custom switch designed by the very well known ben heck, I've been busy studying what are called bus switch IC's.

effectively they can take a number of inputs and depending on if the pin is high or low can switch that many inputs on/off simultaneously.

the upside is that this makes things fairly easy, the down side though is there needs to be a circuit to prevent interference, effectively, if you turn one circuit to allow traffic through, the rest have to block it.

this might help explain it better.

[Image: pByAPma.png]

Effectively, when pin 20 is set to high, no signal get's through, however when pin 20 is set to low, A connects to B.

that's 8 signal lines, either in state 1 or state 0, on or off.

from there you simply route the signals to the output bus like so.

[Image: bus_engadget_howto.jpg]

fairly simple right?

well this is a powered switch, it needs to be for it to work, so there's a bit of power circuitry going on.


I know unpowered switches exist, I have a few sitting around, so I decided to take one apart to see what's different.

[Image: SCIq9Tf.jpg]

[Image: 5mlxqAV.jpg]

What's special about the video switch isn't the circuit, it's essentially the same, just traces going from the inputs to the output.

what's unique is the switching mechanism.

basically it breaks/Connects the traces mechanically, and there's a spring loaded setup across the middle that pushes the other switches to an off state when one is set to an on state (you can still hold down two switches simultaneously however but you'll get crappy signal if you do that because the two will compete.)

it's a mechanical version of the circuit I'm looking to create with the bus switches, and unfortunately this one appears to be a special component, one I can't get off the shelf...

it also actually degrades the signal slightly because of it's nature.

back to square 3...


so here's the circuit in a nut shell, you tie your grounds together, rig a switch from the power input circuit to send 5v to the bus switches when set to off, and nothing when set to on (remember high = voltage, low = no voltage, it's kind of an inversion of common sense)

and you take your inputs, be they data, video, or audio, and you route them to the same output bus, with a bus switch in between the inputs and the output bus to switch them on/off.

it's simple and it works, I'll take pictures when I have a working prototype, the chips take more than a month to get here from china., but so far this is what I've worked out.

just one bus chip should support 2 USB ports, so I'd need to tie two chips together per switch to make a KVM switch with three separate output bus lines and three inputs per input, USB 1, USB2, Video/Audio.

it is possible to use a 16 bit bus switch, but they cost a lot more than 2 8 bit bus switches, still it's something I've been thinking about regarding the KVM variant.
"I reject your reality and subsitute my own." - Adam Savage, Mythbusters
[Image: 5.jpg]

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