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Summary

Obstacle Avoidance with Ultrasonic Sensors

Use an ultrasonic sensor to detect obstacles and prevent your robot from crashing into things.

By Kevin McAleer,    2 Minutes

Page last updated May 10, 2025

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Your robot needs to be aware of its surroundings. With an ultrasonic sensor like the HC-SR04, you can measure the distance to objects and make your robot avoid collisions.

📏 How the Ultrasonic Sensor Works

The HC-SR04 sends out a burst of ultrasound and listens for the echo. By measuring the time it takes for the echo to return, it can calculate distance.

  • Trigger: Start a measurement
  • Echo: Pulse length corresponds to distance

Distance is calculated as:
distance (cm) = duration (µs) / 58

🔌 Wiring the HC-SR04 to the Pico

HC-SR04

  • VCC → 3.3V (The Pico needs the 3.3v versino of the HC-SR04)
  • GND → GND
  • TRIG → GP8
  • ECHO → GP9 (use a voltage divider or level shifter to avoid damaging the Pico!)

Voltage divider for ECHO:

ECHO → 1kΩ → Pico GP9
           ↓
         2kΩ
           ↓
         GND

🧪 Reading Distance with MicroPython

from machine import Pin, time_pulse_us
from time import sleep

trigger = Pin(8, Pin.OUT)
echo = Pin(9, Pin.IN)

def get_distance():
    trigger.low()
    sleep(0.002)
    trigger.high()
    sleep(0.01)
    trigger.low()

    duration = time_pulse_us(echo, 1, 30000)  # timeout after 30ms
    distance_cm = duration / 58
    return distance_cm

while True:
    dist = get_distance()
    print("Distance:", dist, "cm")
    sleep(0.5)

🧠 Avoiding Obstacles

You can use the measured distance to decide what your robot should do:

  • Distance < 10 cm → Stop or turn
  • Distance ≥ 10 cm → Keep moving
distance = get_distance()

if distance < 10:
    stop()
    turn_right()
else:
    forward()

🧩 Try It Yourself

Adjust the threshold to make your robot more or less cautious.

  • Add random turns when an obstacle is detected.
  • Mount the sensor on a servo and scan side-to-side.

With obstacle detection working, your robot now reacts to the real world! Next up: Building a Two-Wheel Drive Robot

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