Dumb and simple.
Metal strip that slips into the gap between door and top jamb. Hang a tin can or two on it. Door opens, cans make a racket when they hit the floor.
No mods to door or floor, no wires, no batteries, no uplinks to the net. Will not prevent entry, but will give you a warning. Will probably scare away an intruder anyhow since he she or it will know you've been alerted.
Dumb and simple.
Oh, and cheapicity approaches zero.
Terry, 230RN
Poor pix:
Can hanging from top of door. Actually the bottom half of an ash tray which makes a rewardingly sonorous racket when it hits the floor. First pic is the metal strip stuck in the top of the door on which the can hangs.