How to Use Ultrasonic Sensor with Raspberry Pi: Simple Guide

To use an ultrasonic sensor with a Raspberry Pi, connect the sensor's trigger and echo pins to the Pi's GPIO pins, then write a Python script to send a pulse and measure the echo time. This time is converted to distance by calculating how long the sound wave took to bounce back.

Syntax

Here is the basic Python syntax to use an ultrasonic sensor with Raspberry Pi's GPIO pins:

• GPIO.setup(pin, GPIO.OUT): Set the trigger pin as output.
• GPIO.setup(pin, GPIO.IN): Set the echo pin as input.
• GPIO.output(trigger_pin, True/False): Send a pulse to start measurement.
• GPIO.input(echo_pin): Read the echo pin to detect pulse return.
• Calculate distance using the time difference between sending and receiving the pulse.

python

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

TRIG = 23
ECHO = 24

GPIO.setup(TRIG, GPIO.OUT)
GPIO.setup(ECHO, GPIO.IN)

GPIO.output(TRIG, False)
print("Waiting for sensor to settle")
time.sleep(2)

GPIO.output(TRIG, True)
time.sleep(0.00001)
GPIO.output(TRIG, False)

while GPIO.input(ECHO) == 0:
    pulse_start = time.time()

while GPIO.input(ECHO) == 1:
    pulse_end = time.time()

pulse_duration = pulse_end - pulse_start

distance = pulse_duration * 17150

distance = round(distance, 2)

print(f"Distance: {distance} cm")

GPIO.cleanup()

Example

This example shows how to measure distance using an HC-SR04 ultrasonic sensor connected to Raspberry Pi GPIO pins 23 (trigger) and 24 (echo). It sends a short pulse, waits for the echo, calculates the distance, and prints it.

python

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

TRIG = 23
ECHO = 24

GPIO.setup(TRIG, GPIO.OUT)
GPIO.setup(ECHO, GPIO.IN)

try:
    while True:
        GPIO.output(TRIG, False)
        time.sleep(0.5)

        GPIO.output(TRIG, True)
        time.sleep(0.00001)
        GPIO.output(TRIG, False)

        while GPIO.input(ECHO) == 0:
            pulse_start = time.time()

        while GPIO.input(ECHO) == 1:
            pulse_end = time.time()

        pulse_duration = pulse_end - pulse_start

        distance = pulse_duration * 17150
        distance = round(distance, 2)

        print(f"Distance: {distance} cm")

except KeyboardInterrupt:
    print("Measurement stopped by user")
    GPIO.cleanup()

Output

Distance: 15.23 cm
Distance: 15.10 cm
Distance: 15.05 cm
... (repeats every 0.5 seconds)

Common Pitfalls

• Incorrect GPIO pin numbering: Use BCM mode consistently to avoid confusion.
• Not waiting for sensor to settle: Always wait 2 seconds after setup before measuring.
• Timing errors: Make sure to measure pulse start and end correctly to avoid wrong distances.
• Not cleaning up GPIO: Always call GPIO.cleanup() to reset pins.
• Wiring mistakes: Connect trigger to output pin and echo to input pin properly; wrong wiring causes no readings.

python

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

TRIG = 23
ECHO = 24

# Wrong: Setting echo as output (should be input)
GPIO.setup(TRIG, GPIO.OUT)
GPIO.setup(ECHO, GPIO.IN)  # Mistake fixed here

# Correct way:
# GPIO.setup(ECHO, GPIO.IN)

GPIO.cleanup()

Quick Reference

Ultrasonic Sensor Pins:

• VCC: 5V power
• GND: Ground
• Trig: Trigger pin (output from Pi)
• Echo: Echo pin (input to Pi)

Distance Calculation: Distance (cm) = (Time (seconds) × 34300) / 2 = Time × 17150

GPIO Setup: Use GPIO.setmode(GPIO.BCM) and set trigger as output, echo as input.

Key Takeaways

Connect the ultrasonic sensor's trigger to a GPIO output pin and echo to a GPIO input pin on Raspberry Pi.

Send a short 10 microsecond pulse on the trigger pin to start measurement.

Measure the time the echo pin stays high to calculate distance using speed of sound.

Always wait 2 seconds after setup before measuring to let the sensor settle.

Clean up GPIO pins after use to avoid conflicts in future runs.