Parents, Educators & Engineers

Hidden Alarm

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Here's a chance for a little mischief—just a little. Design an alarm that you can turn on and off and is small enough to hide. Make your friends and family ask, "What's buzzing?"


  • 1 AA battery
  • 1-2 feet of electrical wire (Ask an adult to help you strip the plastic coating off the ends to expose the wires.)
  • 1 buzzer (wires attached preferred)
  • Tape (duct or masking)
  • Thin cardboard (Also called chipboard; you can use cereal boxes, too.)
  • Tin—foil
  • Scissors
  • Wire strippers


The thing that makes a hidden alarm cool is that it can be hidden and it can sound an alarm. That's why we call it a hidden alarm! Before you start, think about:

  • where you want to hide your alarm
  • how small you think it needs to be to fit in your hiding place
  • how you'll turn your alarm on and off


  1. Sound your alarm
    • Check the buzzer. To make your buzzer buzz, you need to make a complete pathway for electricity to get from the battery to the buzzer. To do this, connect the buzzer, battery, and wires. This makes a closed circuit.
    • Did it buzz? Ours didn't the first time we tried. That's because the buzzer's red wire needs to be attached to the positive (+) side of the battery and the black wire to the negative (—) side. Check the wires and reverse them, if necessary.
  2. Add a switch
    As you build, you also want to think about ways to turn your alarm on and off. A switch starts and stops the flow of electricity. When the switch is closed (called a closed circuit), electricity flows to the buzzer and it buzzes and buzzes and buzzes. Would somebody please open that switch!
  3. Put it all together
    Mount everything (your circuit, battery, etc.) onto a cardboard frame. We turned our frame into a switch—the folded cardboard acted like a spring that could open and close our circuit.


Did your alarm buzz on command? Did it fit in its hiding place? Did you trick anyone? When we were building our buzzer, the wires sometimes got loose and our alarm stopped working. If that happens to you, check your connections.


Try to make your hidden alarm more reliable or make it even smaller. Is there another hiding place you want to try?


Like your hidden alarm, computers basically work by switching circuits on and off. The first computer, called the Electronic Numerical Integrator and Computer (ENIAC), was built in the 1940s (not so long ago, really). ENIAC was so big it filled a small Building and weighed 30 tons! Since then, engineers have been making computers smaller and smaller and smaller. Today, the average laptop computer weighs just 6 pounds. That means it would take 10,000 laptops to weigh as much as ENIAC. We don't even want to think about carrying all those around. On top of that, today's laptops are even more powerful than ENIAC. How'd they do it? By making the parts smaller and making them better conductors of electricity. Just think, ENIAC, laptops, and the alarm you made work the same basic way—by switching circuits on and off.

Picture of some kind of alarm contraption