How To Make A Tripwire Alarm

A tripwire is a trigger mechanism. It is considered to be a passive device that will only set the alarm when it detects or reacts to a movement.

This experiment will help you create a basic tripwire alarm system. We will focus on creating an alarm that is powered by a battery. This mechanism requires complete circuit so the current from the battery can power the buzzer that will be used as the alarm.

What you will need:

• 1 Clothespin
• 1 Buzzer
• 1 Battery
• Piece of foil
• 6” Electrical wire
• 18” or more fishing line
• 1 Popsicle stick
• 2 Dowels
• 2 Blocks of wood
• Scissors
• Electrical tape
• Glue gun
• Wire strippers
• Drill

What you need to do:

• Strip the two ends of the electric wire and put one end on top of the clothespin. The stripped end of the wire should be at the clamping edge of the clothespin. Wrap the tip of the clothespin with the stripped wire inside. Secure it with an electrical tape to make sure the foil and wire will stay in place. See the photo for reference.

• Take the negative wire from the buzzer and attach it to the other side of the clothespin. Do the same procedure of wire stripping, foil wrapping and electric taping.
• Take the positive wire of the buzzer and connect it to the positive terminal of the battery. Secure the connection with the electric tape.
• Take the first electric wire and connect it to the negative terminal of the battery to complete the circuit. Secure with the electric tape. The buzzer should work if the circuit is completed.

• Get the popsicle stick and put a hole in it. Put the popsicle stick between the clamp of the clothespin to break the circuit and stop the buzzer from making a noise.
• Use the glue gun to glue the battery to the buzzer and then the clothespin to the battery. Then take one dowel and glue it to the battery as well. See photo for reference.

• Take the two wooden blocks and drill a hole in each of them. The hole should be big enough to fit the dowel tightly so it will stand without wobbling. Make the dowel with the circuit stand in one of the wooden blocks.
• Take the fishing line and thread one end through the hole in the popsicle stick. The other end should be tied to the second dowel.
• Place the second dowel into the other wooden block. Put both stand far from each other so the fishing line is stretched. Weigh the wooden blocks so it will not topple over.
• Put the tripwire alarm in a place where people walk through (e.g. hallway or doorway).
• When someone passes through and trips over the fishing line, the popsicle stick will be removed from the clothespin and trigger the buzzing sound.

What just happened:

A tripwire is a simple illustration of how a circuit works. While the popsicle stick is clamped between the clothespin, the circuit is broken, thus keeping the buzzer from making a sound. That is an illustration of an open circuit.

When someone trips over the fishing line and causes the popsicle stick to be removed from the clothespin, the two foil ends will meet and complete the circuit. That is a closed circuit. This will cause the buzzer to be activated.

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