The situation is such that I'm running POE+ to a remote location with very limited options for leveraging another power supply. So I have an Ethernet cable with POE connected to a POE splitter that separates the power from the data (it is 802.3af/at compliant).
The splitter I'm using is this one: https://www.amazon.com/gp/product/B08HS4NT13/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&th=1. The reason I selected this model is because they claim it is electrically isolated to 1500V.
The DC is split out onto a barrel connector. I then have that splitter connected to a simple Y-cable so that there are 2 DC barrel connectors on the ends.
One end I connect to the barrel connector on the Arduino Mega 2560 with POE, and the Ethernet cable connects to the Arduino, which now starts up properly.
Then I have a wire from 5V pin on Arduino to DC+ on the opto-isolated low/high trigger relay module. Then I have a wire from GND on the Arduino to DC- on that same relay module. This is a link to the opto-isolated low/high trigger relay module: https://www.amazon.com/gp/product/B00LW15A4W/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
The second barrel connector is terminated to a breakout so that I can run wires from that breakout's 12VDC+ to Common on the relay contact. Then I have a 12VDC horn that's connected to NC. The horn draws a max of 600mA.
I have very few options for bringing another power supply. THere's simply nowhere to plug into. And this location is literally in the middle of nowhere. Is there anything I can do to keep the output circuit from damaging the control circuit?
I read that when standard compliant POE device does DC-DC conversion, that's considered isolation. And I also read that POE compliant switch ports are required to be electrically isolated (I'm using a Cisco 9200 24 port POE+ switch). But I'm not sure if that takes into account extenders and splitters.
Then I have one of the Arduino pins (12) programmed to go high when some condition is met. This occurs, and 5VDC activates the coil and switches the relay NC/NO contact.
However, the more I read (and obsess), I'm pretty confident that this isn't a good practice, and I'm worried that some issue on the output side will make its way back to the micro-controller and kill it. I'm looking for input and guidance.
It seems to me that it would be safer to run a second POE cable to the remote location. Of course, I'd still then have to rely on the manufacturers to be following the standard for port isolation. Running that cable would come at an expense, but if that's the recommended action, I can push it up the ladder.
Also, as a side note, the switch is POE+ (30W max output per port).