HC-SR04 Ultrasonic Sonar Distance Sensor + 2 x 10K resistors

If you're like me, you've dreamed of being a dolphin - smoothly gliding through the water. Using your echo-location abilities to detect tasty fish treats. Until genetic modifications catches up with our desires, we'll just have to make do with these handy HC-SR04 Ultrasonic Sonar Distance Sensors and a pair of flippers. These ubiquitous sensors are really common in robotics projects, but they can also be used for automation, interactive art and motion sensing. They work at about 2cm to 400cm away, but we think 10cm-250cm will get you the best results.