The HC-SR04 is the most widely-used ultrasonic distance sensor in maker projects. It uses sonar — just like bats and dolphins — to measure distance to an object with high accuracy and stability. Send a 10 µs pulse to the TRIG pin, and the sensor emits an 8-cycle 40 kHz burst. When the ECHO pin reads the reflection, measure the pulse width: distance in cm = pulse width (µs) ÷ 58. Works perfectly for robot obstacle avoidance, parking sensors, water-level monitors, and people counters.