This circuit is connected to a trial spawner and detects when it is inactive or when the fight has ended. This way you can connect it to a door and make a dungeon room that closes its doors at the start of the fight and opens them when it ends. If there is space in a trial chamber, it can be implemented. You can move the circuit away and carry the observers signal through cables, and vice versa with the circuit if there is no space. Below there is a link to a video I made trying to explain it and showing an example of application. Although it is in not very good English.
I have put compatible versions from the original version where I built it up to the most recent one, but it will probably work in earlier and later versions.