How to Make an Alarm Circuit

Alarm circuits are based on one of two models, differentiated by how they activate their siren or buzzer. A closed circuit alarm normally has electricity running through it, which prevents the siren from going off. When the circuit is broken, the electricity turns off and the siren sounds. An open circuit alarm is the exact opposite: There is no electricity moving through it until the trigger is activated. That completes the circuit and sends electricity into the siren, which then goes off. This article describes how to make a simple open circuit system.

4 pieces of insulated wire 1 buzzer or alarm 1 9-volt battery 1 piece of cardboard 2 pieces of aluminum foil Electrician's tape Lightweight rug

Instructions

- 1
  Fold a piece of cardboard in half so that the two ends touch. Wrap each of the ends in a piece of aluminum foil.
- 2
  Strip the ends off four pieces of insulated wire so that the metal is showing.
- 3
  Use electrician's tape to attach the end of one wire to one of the pieces of aluminum foil and the end of a second wire to the other piece of aluminum foil.
- 4
  Connect the positive terminal of the battery to the positive terminal of the buzzer using the two remaining pieces of wire.
- 5
  Connect one of the wires taped to the foil to the negative terminal in the battery and the wire taped to the other piece of foil to the negative end of the buzzer.
- 6
  Lay the cardboard under a lightweight rug directly in front of the door you wish to protect. Place the battery and buzzer in an unobtrusive location nearby (make sure the wires are long enough). The rug needs to be light enough to keep the ends of the cardboard from touching. When someone steps on the rug, it should complete the circuit and set off the buzzer.

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