devices for measuring tension or detecting "slackness" in fishing wire
This really needs a drawing, but alas, I don't have time to draw one up.  Here's a super simple method of detecting a slack line:

Start with two plates/pulleys/rods/whatever.  Let's assume they are plates.  There is a hole in each plate.  Run the line through the hole in each plate.  When the line is tight, the line forms a nice straight line between the two holes.  When the line is slack, it is not a straight line.

Halfway between the plates, put a microswitch.  When the line is tight, the switch is pressed.  When the line is not tight, the switch is not pressed.  You might need a lever or something for the line to push against-- which in turn pushes the switch.  I would probably make this lever have a small hole in it that the line goes through.  The length of the lever, and the overall mechanics of things, will determine how much force is required to press the switch (and thus how much weight you need on the line).

This would be super simple to 3-D print.  Just saying.