How do you create a simple and cheap tension alarm?
A piece of non conductive material and a clothes pin. Cover both sides of the clothes pin in foil and connect to your circuit. Tie the non conductive material (plastic, paper, cardboard) to the string and put it in the clothespin as to break the circuit. Place it so that the slightest bump names the wire fall out. Instant trip wire. 

https://youtu.be/wB6BEHAUZow