How distance is calculated

Assuming the speed of sound is 340 m/s, the time needed to travel 1 cm in air is:

340 × 100 × 10⁻⁶ = 29 μs

Because the sound wave travels to the object and back, the total travel distance is double. Therefore, the formula is:

distance = (time / 29) / 2 = time / 58

Important: Voltage considerations

The HC-SR04 operates at 5V. When the ECHO pin outputs a 5V HIGH signal, it can potentially damage the GPIO pins of the BW21-CBV-Kit.

To avoid this, you must step down the voltage using resistors or a level shifter.

Connection diagram (BW21-CBV-Kit + resistors)

Use a 1:2 resistor divider (any suitable values are fine; avoid extremely high resistance). If you don’t have resistors, you can use a logic level converter instead.

Example Code

Open the example from: File → Examples → AmebaGPIO → HCSR04_Ultrasonic

Compile and upload it to the BW21-CBV-Kit, then press the reset button. Open the Serial Monitor —

the measurement result is printed every 2 seconds.

Notes

Since the HC-SR04 uses reflected sound waves, the measured distance may vary depending on the material of the object's surface:

• Rough surfaces may scatter sound
• Soft surfaces may absorb sound

Code Reference

Trigger the measurement by pulsing TRIG HIGH for 10 µs:

digitalWrite(trigger_pin, HIGH);delayMicroseconds(10);digitalWrite(trigger_pin, LOW);

Measure the duration of the HIGH pulse on the ECHO pin:

duration = pulseIn(echo_pin, HIGH);

Calculate the distance:

distance = duration / 58;