Anyone have ideas for easy-setup early warning systems and tripwires.
I don't have a dog and possible alternatives to fishing line running through soda/beer cans.
Thanks.

get a dog...preferably one that barks at strangers...and if it is barking and it stops suddendly then that is telling you something, too!

A one foot long piece of 2x4 with a nail in it that the point sticks through a quarter of an inch. Bury them about six inches apart and four and a half inches deep. Before burying them, set them in your holes, and take a two and three quarter inch piece of one inch pvc and center it on the nails. Fill in the dirt around the pvc and VERY carefully set a two and three quarter inch 12ga shell in each one. Cover with leaves and don't forget where you buried them

run heavy weight fishing line around the perimeter of your yard. Run the lines into your house close to the ceiling. Tie a pop can full of sand to the line. Someone trips the line and the can falls. Put something for the can to land on like a pie tin to make just enough noise to wake you up. And make the trip lines strong enough so rabbits and cats wont trip them.
And with a strong enough line the invaders will recognize them and might think twice about entering the premises.
I forgot to add. Add at least 4 cans and designate a can per area so you know which way they are coming from.

If you add a secondary tripline set right up against your house you will know exactly where they are entering from.