HC-SR04 Ultrasonic Distance Sensor Module

The HC-SR04 is the classic distance sensor that every maker encounters early in their journey. Simple, cheap, and remarkably effective for basic obstacle detection and distance measurement.

Operation is straightforward: send a 10-microsecond pulse on the trigger pin, measure the echo pulse duration, and calculate distance using the speed of sound. The NewPing library handles all this in a single function call.

Range is 2cm to 400cm with about 3mm accuracy in ideal conditions. The 15-degree beam angle provides decent spatial resolution for obstacle avoidance in small robots. For tighter beams, look at time-of-flight sensors like the VL53L0X.

The 5V requirement means you'll need a level shifter when pairing with 3.3V boards like the ESP32. This is the biggest usability drawback — newer ultrasonic modules like the JSN-SR04T offer 3.3V compatibility.

Reliability is good indoors but outdoor performance degrades with wind, temperature extremes, and soft absorptive surfaces. Fabric, foam, and angled surfaces can cause missed readings.