HC-SR04 Ultrasonic Tape Measure

Time the echo of a 40 kHz ping and convert microseconds into millimetres — your first sensor that actually measures something physical with sound.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$7.50. 3-part bill of materials with live supplier pricing. A digital tape measure built from an HC-SR04 ultrasonic range finder and an Arduino.