These have a multitude of uses. From training- to humans and predators.. They are trip wire triggered..You can get them at Lowes in the smoke-alarms section for about 8$ each. They are semi water resistant.
They are called “Body Alarms”
I made covers for them from pieces of inner tubing..
I simply fed the chain through the one end, sewed it up and added permatex silicone to seal out the water..so it was actually attached to the chain not the main body of the alarm. It sort of floating around the alarm and it didn’t diminish any of his hell hound raping a banchi like screeching noise effect..