How are mines triggered in Captain Sonar?
You must STOP in order to trigger a mine, and the Engine Room can't have a breakdown for the MINE + TORPEDO system.

This thread over on BGG relays information from the designers. In order to detonate a mine, the Engineer needs weapon systems online:


  And to detonate a mine you also need to have ALL the red "lights" clear in the Engine room


The last line also clarifies that a STOP needs to occur during detonation:


  With these answers, I think a good way to describe the rules is that each team is only allowed 1 STOP (either system activation or mine detonation) between movements.