Hello, I have this same SainSmart 4-channel 5v relay that you have. Here's my issue, I would REALLY appreciate your input (or anyone's in the know, I suppose). So there are many sources claiming that with this exact relay board, to control it using the Rpi, you need to solder 1kohm resistors over the existing ones in order for the 3.3v of the Rpi to control the 5v relays. However, there are clearly others who don't seem to have done this and are controlling it just fine. Did you need to do this? Do I need to do this? I'm trying to do exactly what you've done, I just want to be able to remotely power on and off my printer, and a light. Though my Octopi setup is powered through my 3D printer's PSU via a 12v to 5v step down converter, so I will need the main power supply to remain on at all times.
I'm not new to electronics but I am new to this whole Rpi thing, and connecting it to other stuff as well, lol, so it's a lot to take in and it doesn't help when there's sort of conflicting information out there. I just want to make sure I hook it up correctly, I literally have the 1kohm resistors and the relay board sitting in front of me and I can solder them on right now if I have to, but if I don't have to I'd obviously rather not. I hope you see this, thanks for your time. (sorry for the length, brevity is NOT my strong suit)