Reddit mentions of 8 Channel USB GPIO Module With Analog Inputs (Pack of two)

Of course! Here is an example with a small switcher: AW-HS50 In the specifications on that page you will see a listing at the bottom in the section called TALLY/GPI. As you see, it has 5 inputs and 7 outputs. The inputs can be assigned to do specific things on the switcher but the outputs/tally are usually assigned for a purpose (turn on the tally light on a camera or a multi-viewer, here is a video that goes over the setup on a larger, slightly out of date swticher), to turn on/activate when the source assigned to it has been selected on the program bus (larger switchers that have multiple program feeds with have the option to assign a source and program feed to use so you know which program feed went live with that specific source). You will have to dig into the manual for the specific switcher you want in order to set that up as most manufacturers do it a different way. When a tally is activated, it switches from being either negative to positively charged, or positively to negatively charged. This wire is then connected to another system that can take in this boolean value (on or off) and then assigned to a specific function on that system. Here is an example of what I meant by a GPIO USB device. With all of this being said, feel free to jump to my TLDR if you don't care.

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TL;DR

As others have said, just get a PCI capture card and set the inputs into OBS. Should be a lot easier and if you really want a physical switcher, just get some device that you can assign the buttons to keyboard shortcuts and then assign those shortcuts in OBS to change sources.

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Hope this helps and feel free to reach out with more questions. I don't mind.