Quote Originally Posted by joshuab View Post
... as expected, I ran into some hardware-related issues. I have passed pins 6 and 7 of the welder's connection into the box so that I can selectively indicate to the welder whether the box is attached or not. This way, I can power down the box if I want to, and return it to panel control, without having to disconnect the plug. These leads need to be Normally-Open, and then be Closed whenever the box is powered up. I tested an analog switch IC that I have, and with no power, the pins showed open on my multimeter. With power, they showed about 190 Ohms (which I believe to be the internal resistance of the IC, since that's present between all of its four switch-pin-pairs when they are closed). So I figured they would work perfectly, but they didn't. For some reason, even when the board is powered down, the welder detects that the device is plugged in. I am going to troubleshoot some more, just to make sure I haven't accidentally shorted a wire somewhere (I don't think so--multimeter says the pins are open), but in the mean time, I have ordered some Normally-Open optically-isolated solid-state relays. This will provide total isolation between the welder's electrical circuit and the Arduino's, and should be as close to just flipping a manually-operated switch as it comes.
Do you have a manual toggle for turning the box on? Could just make it a double pole switch and connect one pole to the power for the box, and the other to make the connection to let the welder know it is active.