Introduction: Touch Free Walk by Alarm (No Electricity)

About: I am an author and a maker. My current project is Santa's Shop. I'm working on a science fiction type book--more later. @EngineerRigsby

This sounds like one of those AP Physics challenges: Build an audible alarm to indicate that a person has walked past, but the subject can't touch the device and the device uses no electricity.


The delicately balanced piece of cardboard becomes unstable due to air movement of a person walking past. When the cardboard falls, it has enough weight to pull the "trigger" of a rubber band powered arm. The arm swats a metallic can (filled with coins) to the floor, creating quite a racket.





Metal can



Rubber Band


Step 1:

Cut two pieces of cardboard and glue together to make the base for the detector.

Step 2:

Cut the stabilizer strips and glue one to each side of the upright piece.

Step 3:

Cut three pieces of cardboard, 10 inches by 1 1/2 inches. Make two holes (about 1/4" diameter); one hole is 1/2 inch from the end, the other is 2 1/4 inches from the end. Glue the pieces together.

Step 4:

Insert a pencil in one hole and thread a rubber band into the other.

Step 5:

Cut 13 pieces of cardboard to the size shown in the photo. Glue them together as shown.

Special Note: If you are not using an electric drill to create the 9/16" hole for the pencil after this glue step, then you need to poke a hole for the pencil (1 1/2" and 2 1/2" from the lower right edge as shown) before gluing these pieces together.

Step 6:

Insert the pencil and arm into the thick base. Mark the end of the arm position on the base.

Step 7:

Cut cardboard (three pieces thick) in a shape to create a "trigger slot) on the base. Glue these into position.

Step 8:

Place coins (or gravels, marbles, whatever) into a can and tape paper over the open end of the can.

Step 9:

Cut the "sensor" piece of cardboard.

Step 10:

Cut a trigger.

Step 11:

Take 12 inches of thread and tape one end to the trigger.

Step 12:

Tape the other end to the sensor.

Step 13:

With the sensor, trigger and arm base on a table, place the trigger in the trigger slot.

Step 14:

Pull the arm back and adjust the trigger so that it barely prevents the arm from moving. Set the can on the edge of the base. Be sure that the assembly is at a table edge so that the can will fly off onto the floor for more noise.

Step 15:

Balance the sensor cardboard (top of the T assembly).

When you walk by, the air disturbance will upset the balance, triggering the noise maker--good luck!

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