It's like a really small hook with a shiny, reflective material on it. It is usually rigged with 4 other similiar hooks with a weight at the top and people catch a bunch of baitfish with it. This is a saltwater rig by the way.
Most backup alarms are controlled by a piezio buzzer. These can only make one sound. You could probably replace the buzzer with a circuit like this: xpower that goes to backup alarm ysoundboard with sonar sound on it zspeaker x y z Other than that, I couldn't think of any other way to do it. Ask someone at your local electronics store (Radioshack, etc) where to get the parts or repost the question in the electronics section.
Yes they are. You don't see them much on private homes but I know several people who wished they had them.
