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Burglar Alarm

Have you ever wondered how a burglar alarm works?

Burglar alarm's science revolve around complete and incomplete electric circuits.

A real burglar alarm is triggered when the intruder breaks a light or laser beam, which is invisible to the human eye, but needed to complete a circuit. When the beam is broken, the electronic system detects the change in the circuit, and sounds the alarm.

In this activity students will make a burglar alarm that they can use to protect something precious to them.

In this burglar alarm, a piece of cardboard prevents completion of the circuit by preventing the foil-lined tips of the clothespin from touching. Thus, when the alarm is set, the light is off.

If someone steals the protected item, the thread and the attached cardboard are pulled away. This permits the foil-lined tips of the clothespin to touch, completing the circuit and allowing the electrons to flow. The light turns on, to warn you that the item was stolen.


  • Describe the components required to complete an electric circuit.


  • Per Class:
    wire cutters
    wire strippers

  • Per student:
    baggie or small paper bag
    D-cell battery
    paper plate
    2 pieces of aluminum foil
    1 Christmas tree mini-light bulb with wires attached, ends stripped of insulation
    20 cm length of insulated wire, ends stripped of insulation
    small piece of cardboard
    30 cm length of fishing line

Key Questions

  • What will happen to the light if someone steals your item?
  • What is preventing the light bulb from switching on?
  • What other materials could be used in place of the piece of cardboard?
  • Are these materials conductors or insulators?

What To Do


  1. Put together a kit of supplies in a baggie for each student.


  1. Tape the battery to the plate.
  2. Wrap aluminum foil around each leg of the clothespin.
  3. Tape the one end of the light bulb’s wire to one leg’s foil and one end of the length of wire to the other.
  4. Tape the free end of the length of wire to one battery terminal, and the free end of the light bulb’s wire to the other terminal.
  5. Make sure the light comes on.
  6. Tape one end of the fishing line to the piece of cardboard and put the cardboard between the legs of the clothespin. The light should go out.
  7. Tape the other end of the fishing line to something precious and watch what happens if someone tries to steal it.


  • To make your burglar alarm closer to the real thing, you could replace the light bulb with a buzzer.
  • Smoke alarms work in a very similar way to burglar alarms. Take one apart and see if you can identify all the parts of the circuit. Compare the inside to your burglar alarm.