Testing the DS18B20 Temperature Sensor on Raspberry Pi

Testing the DS18B20 Temperature Sensor on Raspberry Pi

1. sudo modprobe w1-gpio
2. sudo modprobe w1-therm
3. cd /sys/bus/w1/devices
4. ls
5. cd 28-xxxx (replace with the serial number that appears)
6. cat w1_slave

The 1‑Wire interface can be a bit flaky, but it reliably reports whether a temperature value is available. Because the sensor exposes its data as a simple text file, all you need to do is read the file.

The first line ends with either YES or NO. If it’s YES, the second line contains the temperature in thousandths of a degree Celsius. In the example above, the sensor reports 20.687 °C and then 26.125 °C.

When multiple sensors are connected, each will appear as a separate 28-xxxx directory. Plug one in at a time, note the created directory, and label the sensor for future reference.

Working with Ultrasonic Sensors

The HC‑SR04 ultrasonic sensor is powered from the Pi’s +5 V rail (pin 2). The trigger pin (GPIO 23) initiates a measurement, while the echo pin returns a high pulse whose width corresponds to the round‑trip distance. Because GPIO pins can only tolerate 3.3 V, a resistor divider protects the echo line.

Example Code

#!/usr/bin/python
# Import required Python libraries
import time
import RPi.GPIO as GPIO

# Use BCM GPIO references instead of physical pin numbers
GPIO.setmode(GPIO.BCM)

# Define GPIO to use on Pi
GPIO_TRIGGER = 23
GPIO_ECHO = 24

print("Ultrasonic Measurement")

# Set pins as output and input
GPIO.setup(GPIO_TRIGGER,GPIO.OUT)  # Trigger
GPIO.setup(GPIO_ECHO,GPIO.IN)      # Echo

# Set trigger to False (Low)
GPIO.output(GPIO_TRIGGER, False)

# Allow module to settle
time.sleep(0.5)

# Send 10us pulse to trigger
GPIO.output(GPIO_TRIGGER, True)
time.sleep(0.00001)
GPIO.output(GPIO_TRIGGER, False)

start = time.time()
while GPIO.input(GPIO_ECHO)==0:
    start = time.time()

while GPIO.input(GPIO_ECHO)==1:
    stop = time.time()

# Calculate pulse length
elapsed = stop-start

# Distance pulse travelled in that time is time multiplied by the speed of sound (cm/s)
distance = elapsed * 34000

# That was the distance there and back so halve the value
distance = distance / 2

print("Distance : %.1f" % distance)

# Reset GPIO settings
GPIO.cleanup()

Ultrasonic‑Powered Rocket Launcher

The following script uses the ultrasonic sensor to trigger a rocket launch when an object is closer than a user‑defined distance. It also captures an image and lights an LED as a visual indicator.

#!/usr/bin/python
# Author : Julian and Kyle Milligan
# Date : 09/01/2013
# Import required Python libraries
import time
import RPi.GPIO as GPIO
from subprocess import call

# Use BCM GPIO references instead of physical pin numbers
GPIO.setmode(GPIO.BCM)

# Define GPIO to use on Pi
GPIO_TRIGGER = 23
GPIO_ECHO = 24
GPIO_FIRE = 4

print("Ultrasonic Measurement")

# Set pins as output and input
GPIO.setup(GPIO_TRIGGER,GPIO.OUT)  # Trigger
GPIO.setup(GPIO_ECHO,GPIO.IN)      # Echo
GPIO.setup(GPIO_FIRE,GPIO.OUT)     # Fire

# Set trigger to False (Low)
GPIO.output(GPIO_TRIGGER, False)

# Allow module to settle
time.sleep(0.5)

# Set distance to trigger
setdistance = input('Please enter a value to trigger the camera:')

while True:
    time.sleep(0.1)
    GPIO.output(GPIO_TRIGGER, True)
    time.sleep(0.00001)
    GPIO.output(GPIO_TRIGGER, False)

    start = time.time()
    s = start
    quit = 0

    while quit == 0:
        quit = GPIO.input(GPIO_ECHO)
        start = time.time()
        if start - s > 0.1:
            quit = 1
        if start - s < 0.1:
            while GPIO.input(GPIO_ECHO) == 1:
                stop = time.time()

    elapsed = stop-start
    distance = elapsed * 34300
    distance = distance / 2

    print("Distance : %.1f" % distance)

    if distance < setdistance:
        call(["raspistill -o image.jpg"], shell=True)  # take a picture with Pi Camera
        print("Fire")
        GPIO.output(GPIO_FIRE, True)  # Turn on GPIO pin 7 fire the rockets

# Reset GPIO settings
GPIO.cleanup()

Streaming a Webcam from the Raspberry Pi

To stream video from a USB webcam, ensure your Pi is connected to a network. Attach a recent webcam and follow the official guide for setting up MJPEG streaming. For a deeper dive, see the DS18B20 sensor test documentation.