DIY Intruder Alarms

Push a thin 3-inch by 3-inch piece of steel or other strong metal between the top of your closed door and the jamb. Rest a small container filled with marbles on top of the flat piece of metal. When the door is opened from the outside the metal will dislodge, allowing the marbles to fall to the floor and make a loud noise. This does not work well with a carpeted floor.

Screw a small metal eyelet to each side of the door about 4 feet off the ground. Balance a container filled with marbles on the edge of a shelf or table near the door. Tie a string to the container and run it through the eyelet closest to it. When the door is closed, continue the string across the door and through the other eyelet on the other side. When the door is opened from the outside, the push on the string will pull the container off the shelf, alerting everyone with the noise that the door has opened.

Attach a store-bought buzzer to a 9-volt battery by connecting a thin, insulated wire from each terminal on the battery to the contacts on the buzzer. Cut and strip 1/4 inch of insulation along both sides of one of the wires and wrap the bare wires around the ends of an alligator clip. While the alligator clip is closed, the buzzer will sound. Insert a small piece of cardboard between the clip jaws, breaking the circuit and shutting off the buzzer. Attach the cardboard to the string attached to the door. When the door is opened, the string pulls the cardboard from the clip and the buzzer sounds.