Using an Ultrasonic Sensor with Smart Pi One

This page describes how to use an ultrasonic sensor (e.g., HC-SR04) with the Smart Pi One, providing detailed steps, wiring instructions, and code examples in both Python and C.

Required Materials

• Smart Pi One
• Ultrasonic sensor (e.g., HC-SR04)
• Connecting wires
• Breadboard (optional for easier connections)
• 1kΩ and 2kΩ resistors (for voltage divider)

Wiring Diagram

Below is a sample wiring diagram for connecting an ultrasonic sensor to the Smart Pi One:

HC-SR04 Pin	Smart Pi One Pin	Description
VCC	5V	Powers the sensor (5V supply)
GND	GND	Ground connection
TRIG	GPIO7	Trigger pin connected to GPIO
ECHO	GPIO8	Resistors used to step down voltage from 5V to 3.3V to protect the GPIO pin - 1kΩ + 2kΩ (Voltage Divider)

Important Note:

• ECHO pin (GPIO8) requires a voltage divider to step down the 5V output signal from the sensor to 3.3V, which is safe for the GPIO pin on the Smart Pi One.
• Connect a 1kΩ resistor between the ECHO pin and the GPIO pin.
• Connect a 2kΩ resistor between the GPIO pin and GND to complete the voltage divider.

Using Python

Prerequisites: Configuration of smartpi-gpio

1. Open a terminal on your Smart Pi One.
2. Create a new Python file using nano:

nano ultrasonic_sensor.py

1. Copy and paste the following Python code into the file:

import time
from smartpi_gpio.gpio import GPIO

# Initialize GPIO
gpio = GPIO()
TRIG = 7
ECHO = 8

# Set GPIO pins
gpio.setup(TRIG, gpio.OUT)
gpio.setup(ECHO, gpio.IN)

try:
    while True:
        # Send a pulse
        gpio.output(TRIG, gpio.HIGH)
        time.sleep(0.00001)  # 10 microseconds
        gpio.output(TRIG, gpio.LOW)

        # Wait for the echo to start
        while gpio.input(ECHO) == gpio.LOW:
            start_time = time.time()

        # Wait for the echo to stop
        while gpio.input(ECHO) == gpio.HIGH:
            end_time = time.time()

        # Calculate distance
        duration = end_time - start_time
        distance = (duration * 34300) / 2  # Speed of sound is 34300 cm/s

        print(f"Distance: {distance:.2f} cm")
        time.sleep(1)  # Wait 1 second before next measurement
except KeyboardInterrupt:
    pass
finally:
    gpio.cleanup()  # Clean up GPIO

1. Save the file by pressing CTRL + X, then Y, and finally Enter.

Running the Python Script

To run the Python script, use the following command:

python3 ultrasonic_sensor.py

Using a C Program

Creating the C Program

1. Open a terminal on your Smart Pi One.
2. Create a new C file using nano:

nano ultrasonic_sensor.c

1. Copy and paste the following C code into the file:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include "smartpi_gpio.h"

int main() {
    // Initialize GPIO
    smartpi_gpio_init();
    int TRIG = 7;
    int ECHO = 8;

    // Set GPIO pins
    smartpi_gpio_set_mode(TRIG, OUTPUT);
    smartpi_gpio_set_mode(ECHO, INPUT);

    while (1) {
        // Send a pulse
        smartpi_gpio_write(TRIG, HIGH);
        usleep(10);  // 10 microseconds
        smartpi_gpio_write(TRIG, LOW);

        // Wait for the echo to start
        while (smartpi_gpio_read(ECHO) == LOW);
        struct timespec start, end;
        clock_gettime(CLOCK_MONOTONIC, &start);

        // Wait for the echo to stop
        while (smartpi_gpio_read(ECHO) == HIGH);
        clock_gettime(CLOCK_MONOTONIC, &end);

        // Calculate distance
        double duration = (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) / 1e9;
        double distance = (duration * 34300) / 2;  // Speed of sound is 34300 cm/s

        printf("Distance: %.2f cm\n", distance);
        sleep(1);  // Wait 1 second before next measurement
    }

    // Release GPIO resources (this will never be reached)
    smartpi_gpio_cleanup();
    return 0;
}

1. Save the file by pressing CTRL + X, then Y, and finally Enter.

Compiling and Running the C Program

To compile and run the C program, use the following commands:

gcc -o ultrasonic_sensor ultrasonic_sensor.c -I/path/to/smartpi_gpio/include -L/path/to/smartpi_gpio/lib -lsmartpi_gpio
./ultrasonic_sensor