In order to build a trip wire, you will need the following: a knife, fishing line, a rope/string, sticks, and a large stone.
Whittle a tree branch until you have sharp points on either end. Cut notches in the stick. Secure a length of string around a rock. Toss one end of the string or rope over a large branch. Pull the rock into the air. This is your counter weight. Anchor the other end of the rope on the ground. Make a loop with the other end. Place the stick in the ground that you made notches in. Place another into the notch. Secure another stick at the other end. Place a stick in the ground in them middle of the rope. This is your trigger. Tie some fishing line to from the loops base to the stick.