Build a clap- or loud-sound-responsive lighting system using an LM393-based sound sensor.
Measure distance with a Sharp infrared sensor based on optical triangulation, as a practical alternative to ultrasonic sensing.
Build an educational Arduino PIR motion alarm, including wiring, warm-up time, sensitivity adjustment, false-trigger reduction, and safe limitations.
Use the TCS3200 sensor to recognize colors by measuring reflected light frequency, showing the result with LEDs.