A Tinker Birthday Party: Making a Simple Alarm System

Image: (c) Bernd Grobauer

My son likes making and tinkering, and so do several of his friends. So we decided to have a “Tinker Birthday Party.” Searching for inspiration of what to make together with a dozen children, I found the “Talking Booby Trap” in Bob Knetzger’s  Make: Fun, in which he uses a clothes pin as switch for activating a sound playback. Replacing the sound playback unit with a simple buzzer gave me something that was simple enough and cheap enough to use as project for the birthday party: a small alarm system.

For each child, I prepared a bag with the parts required for the alarm system – though I decided to hand out the battery after each child had completed the circuit so as to prevent the children from accidentally shorting the battery.

Parts of the alarm system. From left to right: a battery holder for a 1.5V AA cell; the battery itself; a clothes pin (with a hole drilled into the handle); a buzzer; a luster terminal; two pieces of aluminum foil; a piece of string; a cable tie. Because the long pin of the buzzer can just be held by the luster terminal, I only had to solder a cable to the shorter pin. Image: (c) Bernd Grobauer

I started out the project by demonstrating the alarm system to the children. I had built two examples and used them to secure a book on a shelf and a door, then asked two children to “steal” the book and open the door, respectively … buzzzzzz!

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This door is secure! When somebody tries to open the door from the other side, the little piece of cardboard separating the two tips of the clothes pin is pulled out, the circuit is closed and the alarm goes off. Image: (c) Bernd Grobauer

Then I cut the cable binder, took apart the clothes pin and demonstrated the circuit to the children as follows:

The alarm system circuit. Image: (c) Bernd Grobauer

The tricky part of making the alarm system is building the two contact areas at the tips of the clothes pin: The cables have to be wrapped tightly into aluminum foil and then secured with adhesive tape. By providing the children with several choices of adhesive tape, they were able to decorate their alarm systems in different colors (though, this being an alarm system, red was the color of choice for many).

Making contact … Image: (c) Bernd Grobauer

Once each circuit was ready, we put in the battery, reassembled the clothes pin, and tied everything together with the cable binder.

Things went well, though I wish my the battery in my voltmeter had not died just then and there. Not all circuits worked right away and troubleshooting without the voltmeter was cumbersome. But finally, all alarm systems worked and we used them to build a small obstacle course, which the children had to traverse as fast as possible without triggering an alarm. After all that tinkering, some running about was a welcome change! Looking at how much fun we had, I have the feeling that this was not our last “Tinker Birthday Party.”

A selection of the alarm systems made at the “Tinker Birthday Party.” Image: (c) Bernd Grobauer
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This post was last modified on January 15, 2018 2:51 pm

Bernd Grobauer

Bernd Grobauer is a computer scientist and father of two geeks in the making. He hails from Munich, Germany.

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