Here is a great tutorial on building an automatic water gun using the Arduino microcontroller. Its a great project to integrate the use of a simple passive infrared (PIR) sensor into an arduino program along with some simple electronics. This one that is appropriate for middle and high school STEM students, and makes a nice corollary to robotics or other motion related projects that your students are working on.  Idea: A more advanced group of students might consider integrating a similar automatic water gun with a robot to create an autonomous fire fighting bot, or a roving small scale plant watering system.

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