I have made a few posts before regarding the building of a relay control module... Rather than start from scratch I decided I could use a 16 port KVM switch I had in the basement! Perfect!!! Push a button, enable a channel, push another button, diconnect and enable another channel... Exactly what I need...

I tied 5v to pin 4 of each of keyboard ports inside thus enabling each channel without actually having to have any channel connected to a computer so now each channel runs stand-alone...

Now all I need to do is pull some juice or a ground from each channel to use to trigger my relays and I'm done...

I wanted to pick the brains of those in the forum before start permanently altering the box beyond repair...

(To give a little background - I'm using a TrendNet 1601D rackmount 16 channel KVM as break-before-make relay control module... Keyboard and Mouse Ports are emulated via an onboard processor in the KVM and the video lines are from what I can tell, simply rerouted.)

Since I'll be pulling from what would be the input of each of the 16 channels of the KVM I can't use the "pin-9" 5v DDC line since I'm pulling from the wrong end. I have used a multimeter and can get about 4.4v from the vsync and hsync pins at 13 and 14 but that's a little low as the lowest coil hi-current relays I can find are a 15A contact at 6v coil so 4.4v won't quite be enough to trigger... 5v will do it... Also I don't know how stable a vsync and hsync line will be under load... Any ideas? Does the monitor put out a 5v signal (or more) as a return that I could then tap on it's way out the KVM to trigger my relay? Or maybe I'm even looking in the wrong spot???

Thanks guys/gals!

 

Just to clarify, I won't be connecting a monitor to the output in order to send a return (if there is even a useable return)... I just figured I could use the pin that would be the return to the VGA card and simply tie a 5v (or even 6v or even better 12v) to the monitor side of the kvm so I can catch it on the way out the other end of the kvm on each channel to trigger the relays that way...

 

Looking on wikipedia real fast (you should try it) it appears the VGA standard allows for 75ohm interconnects at a max of 9.3ma's at 6.5mw's 0-0.7 peak to peak voltages. The voltage is so low you'd have to use a transformer as a primary stage and then rectify it. You won't be able to drive a normal relay with it, but you might be able to pulse the input coil of a latching relay using a capacitor bank, but it would be VERY low duty cycle like switches per minute.

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If I understand the problem correctly, you could use the low voltage signal you have to drive a transistor, which would then drive the relay.

 

But he wants power from the VGA signal, which you can't get. Amplifying the signal with a transistor will work, IF you have an external power source, you're not going to be able to get more than a minuscule at best amount of power from a VGA 'signal' lines. It would be workable with a transformer, latching relays and a large capacitor buffer, for very low duty cycle infrequent pulses. But that's a LOT of circuit to build to avoid having to run a separate power line or put a battery in.

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