Raspberry Pi Ultrasonic Sensor (HC-SR04)

One may want to connect an ultrasonic sensor with a Raspberry Pi. In fact, there many posts talks about this already but I have it here for my own record and in fact most of the content of this post inspired by this site.

I have got HC-SR04 ultrasonic sensor which is a fairly cheap sensor in hand. An ultrasonic sensor can be used to measure the distance between the object itself and an object in front of it by sending out an ultrasonic pulse and listening for its echo (a reply or response). In practice one could connect this sensor with other devices other than Raspberry Pi such as Arduino.

Here is the connectivity (you can also see here):

Vcc --> 5V pin
Trig --> GPIO pin (GPIO 17)
Echo --> Series with 1K Ohm and goes to GPIO pin (GPIO 27)
GND --> Ground pin

Note for Raspberry Pi model B: GPIO 17 is pin 11, GPIO 27 is pin 13, 5V is pin 2 and Ground is pin 6.

One may use any other GPIO pins. In this case GPIO 17 will be an output from Raspberry Pi to the sensor’s Trig pin. This pin is used to trigger the sensor to send out an ultrasonic signal. According to the datasheet a signal of 10 microseconds is a required to trigger the sensor.One may use voltage divider to lower down 5V to 3.3V instead.

In addition, a 1K Ohm resister is required to drop down the output of the Echo pin to 3.3V since Raspberry Pi GPIO pins support 3.3V. Hence, the GPIO 27 used in this case acts as an input pin from the sensor to the Pi.

Here is the code (slightly modified from here)

#!/usr/bin/python

# remember to change the GPIO values below to match your sensors
# GPIO output = the pin that's connected to "Trig" on the sensor
# GPIO input = the pin that's connected to "Echo" on the sensor

def reading(sensor):
    import time
    import RPi.GPIO as GPIO
    
    # Disable any warning message such as GPIO pins in use
    GPIO.setwarnings(False)
    
    # use the values of the GPIO pins, and not the actual pin number
    # so if you connect to GPIO 25 which is on pin number 22, the 
    # reference in this code is 25, which is the number of the GPIO 
    # port and not the number of the physical pin
    GPIO.setmode(GPIO.BCM)
    
    if sensor == 0:
        
        # point the software to the GPIO pins the sensor is using
        # change these values to the pins you are using
        # GPIO output = the pin that's connected to "Trig" on the sensor
        # GPIO input = the pin that's connected to "Echo" on the sensor
        GPIO.setup(17,GPIO.OUT)
        GPIO.setup(27,GPIO.IN)
        GPIO.output(17, GPIO.LOW)
        
        # found that the sensor can crash if there isn't a delay here
        # no idea why. If you have odd crashing issues, increase delay
        time.sleep(0.3)
        
        # sensor manual says a pulse ength of 10Us will trigger the 
        # sensor to transmit 8 cycles of ultrasonic burst at 40kHz and 
        # wait for the reflected ultrasonic burst to be received
        
        # to get a pulse length of 10Us we need to start the pulse, then
        # wait for 10 microseconds, then stop the pulse. This will 
        # result in the pulse length being 10Us.
        
        # start the pulse on the GPIO pin 
        # change this value to the pin you are using
        # GPIO output = the pin that's connected to "Trig" on the sensor
        GPIO.output(17, True)
        
        # wait 10 micro seconds (this is 0.00001 seconds) so the pulse
        # length is 10Us as the sensor expects
        time.sleep(0.00001)
        
        # stop the pulse after the time above has passed
        # change this value to the pin you are using
        # GPIO output = the pin that's connected to "Trig" on the sensor
        GPIO.output(17, False)

        # listen to the input pin. 0 means nothing is happening. Once a
        # signal is received the value will be 1 so the while loop
        # stops and has the last recorded time the signal was 0
        # change this value to the pin you are using
        # GPIO input = the pin that's connected to "Echo" on the sensor
        while GPIO.input(27) == 0:
          signaloff = time.time()
        
        # listen to the input pin. Once a signal is received, record the
        # time the signal came through
        # change this value to the pin you are using
        # GPIO input = the pin that's connected to "Echo" on the sensor
        while GPIO.input(27) == 1:
          signalon = time.time()
        
        # work out the difference in the two recorded times above to 
        # calculate the distance of an object in front of the sensor
        timepassed = signalon - signaloff
        
        # we now have our distance but it's not in a useful unit of
        # measurement. So now we convert this distance into centimetres
        distance = timepassed * 17000
        
        # return the distance of an object in front of the sensor in cm
        return distance
        
        # we're no longer using the GPIO, so tell software we're done
        GPIO.cleanup()

    else:
        print "Incorrect usonic() function varible."

        
print reading(0)

The above code is saved with file name ‘usonic.py’. Ensure that this code is executable.

chmod +x usonic.py

The code must be run with sudo (super user) since it requires accessing hardware.

sudo ./usonic.py