Still, the relay you used was low enough power if the coil resistance was 100 ohms. That would be well below the 300mA that the output is supposedly rated for. So I think your original choice was fine. That article does show a protection diode placed around the relay coil, and that's good practice. And I do think that would have prevented the problem you were having. I really wish we could get some good info from Dahua about the design of their alarm outputs. If you keep doing what you just did, though, with that DC SSR, everything should be just dandy. So looking into this further is just a time-killer that you might do if you have some spare time to waste. I fully understand that!