SO YOU'RE 12 years old, and you have this pesky little sister who keeps sneaking into your room to read your top-secret journal.

Michael Carroll's got your back. With his "Sister Catcher 2.0" room alarm, the second your sibling steps into your sanctuary, the sudden sound of Rambo blasting away his enemies will send her scurrying and tip you to the intrusion.

Need to communicate with your friends without parents overhearing? Carroll created "Mecret Sessages" so you can write in code.

Such inventions are just two of dozens the Montgomery County fourth-grade teacher created to get students more excited about math and science - and get them to turn off their televisions, cellphones and video games.

"When I was a kid, we would make our fun, not buy it," said Carroll, 32, who has taught at Overlook Elementary School in Abington for five years. "Building [these inventions] helps kids turn off their video games and have fun experiences bonding with adults while using skills they'll need for future careers."

And unlike chefs who closely guard their most successful recipes, Carroll is eager to share his inventions.

He wrote a book aimed at kids in third through sixth grades that he expects to publish this spring. Dewey Mac and the Case of the Dog Gone Dog chronicles the adventures of a young detective/inventor who has to solve crimes, deal with his crazy family and outsmart a school bully named Zinc. Along the way, 12-year-old Dewey and his buddy Ched invent about 25 gadgets - with how-to's included - to further their sleuthing or just have fun.

Carroll hopes to eventually expand the book into a series.

He's made sure that all the ingredients for his inventions are affordable and easily available. His "Claw Gun," for example, which a kid spy can use to shoot a pencil at his enemies, is made up of a rubber band, dental floss, pill bottle, clothespin, a binder clip and some hot glue. Cardboard, yarn and tin foil are a few materials in the Sister Catcher 2.0 room alarm.

"I don't want these to be too hard or expensive to put together - you could go to the dollar store and find just about all you need," Carroll said.

Judging from reaction so far, the book has a promising future. Carroll's Kickstarter campaign raised more than three times as much money as he needed to self-publish. And in September, Carroll took about 30 of his gadgets to the World Maker Faire in New York, where he won the Editor's Choice Award.

At school, a full curriculum keeps Carroll from sharing most of his inventions during class. But students who stay in during recess or after school for enrichment clubs get to make some, and Carroll celebrates his students' birthdays by giving each a kit to make an "Interrogator 3000," a lie-detector test made of paper clips, wires and a few electrical bits from Radio Shack.

"He's a very imaginative teacher, extremely kid-friendly and full of enthusiasm," Overlook principal Carla Greene said. "He's always looking for ways to enhance the children's awareness of the sciences."

Tajmia Gray said her son Jordan, now in sixth grade, has never been crazy about science - except when he had Carroll in fourth grade and participated in Carroll's after-school enrichment club.

"That year was the only time my son was really into science - to the point that every time I turned on the light switch, he would tell me how it worked," Gray said. "Mr. Carroll really engaged my son."

Carroll traces his passion for making things back to his childhood, where his do-it-yourselfer father and grandfather inspired him. He remembers his grandfather, a mechanic, creating a car by attaching the front end of one wrecked Volkswagen with the back end of another.

Carroll's first inventions were decidedly smaller. As a skateboarding teen, he made a pinhole camera and rigged it to his board to get a ground-level view. He also "borrowed" his grandfather's shoeshine box to create a theremin, an unusual musical instrument one plays without touching it.

More recently, he built his own home arcade, putting video games from all sorts of systems inside an old boardwalk-style arcade cabinet.

"Everyone has a hobby, and this is mine. My wife is very tolerant," Carroll said, smiling. His wife, Kate, is a sixth-grade language-arts teacher in the Central Bucks School District.

The Chalfont couple are expecting their first child, a girl, in April. They plan to name her Lucy Drew - the Drew, of course, honoring famed fictional teenage detective Nancy Drew.