This happens to me all the time. I need to put a big thing in the refrigerator, so I clear out a spot. Our fridge tends to be full, so this can be quite a puzzle. Then before I can put the thing away, another family member puts something else right in the middle of the space I just cleared!

What’s a cook to do?

Sweetpea knows! A motion-detecting All-Seeing Fridge Ninja™!

The Fridge Ninja™ has 4 main parts, plus a few assorted LEGO pieces:

A LEGO EV3 Infrared Sensor
watches the problem area for signs of intruders
A LEGO EV3 Intelligent Brick
reads sensor inputs, plays sounds, and controls the position of the motor to move the ninja
A LEGO EV3 Servo Motor
turns a wheel to raise and lower a ninja on a string
A ninja
serves as the avatar for the system. Equal parts cute and threatening, I have no idea where ours came from. It just showed up inside the freezer one day. That’s not even a joke. One day we just had a ninja in our freezer.

You install the Fridge Ninja on a shelf above the space you’re trying to protect. It remains vigilant, watching the space below for incursions. If it detects motion, it will lower the ninja and play a nice ”hee-YAH!”. The ninja will stay there until the intruder retreats. When the space is clear again, it will say “OK” and raise the ninja.

Sweetpea (11 years old) wrote the code herself. Most of the time was spent testing and adjusting the sensor’s threshold values to make the sensor sensitive, but not too sensitive. It turns out you need different values for “too close” and “far enough” to make it work.

It constantly monitors the sensor. When something gets too close it:

plays a “hee-YAH!”
Runs the motor to lower the ninja
waits until the invader is far enough away

plays an “OK.”
Runs the motor backward to raise the ninja

It took a lot of fiddling to get the right values for too close and far enough, but she figured it out.

It’s an AWESOME engineering project. She’s very modest about her success, but I’m super proud of her.