I am pretty sure at some point, you were out camping, sitting by the campfire, worried that there might be something or someone walking towards you. I know it has happened to me before.
These two videos will show you two different ways to make a trip wire alarm. The idea is simple, when someone trips on the line they would cause the wire to connect and trigger the alarm or the fire in the two different cases.
Watch the videos, I hope you like it.