Connect the devices as per the guidelines given here and enjoy the coding in the next section. You will need these things on a neat wooden table or a table with an insulation sheet:

Now, we are ready to build the circuit. The circuit will look like the following diagram if you build it correctly. This diagram is also called as schematics, which represents every terminal of the components and their symbols.

For RasPi, we used the connector in the center with all its 26 pins (for RasPi 1 model B). We already know that RasPi 1 Model B, Model B+, and RasPi 2 Model B have the same functionality on the first 26 pins on header. Let's connect it step by step. It's more enjoyable if you understand the connections and connect it by yourself on the breadboard.

However, I have shown the connections on the breadboard in the following figure, which makes the connections easy to understand. The connections are also described here in a step by step. Black dots on the breadboard in the figure show the physical connections of the wire onto the breadboard. We will perform the following steps:

So far, the connections we have done are good to go. Cross-check the connection twice before you power up the RasPi. We have done the procedure on the hardware. We need to write the software (code) to tell the RasPi that we have connected this sensor to these pins and to calculate the distance based on the formulas we derived.

Let's start writing the code in Python. This code can be separated into the modules. These modules can then be integrated as a single code to get the distance. Let's summarize the steps here:

Let's see each of them in detail.

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
print "Measuring Distance"

GPIO.setup(23,GPIO.OUT)
GPIO.setup(24,GPIO.IN)
GPIO.output(23, False)
print "Setting Trigger pin to zero by default"
time.sleep(1)

Sending and receiving the pulses

If we analyze the timing diagram of the ultrasonic sensor to measure the distance, we can easily understand how the sequence should be. The following timing diagram shows the sequence. In the previous step, we made the trigger pin zero by default. Then, we provided a 10µs pulse to the trigger pin of the sensor from the RasPi. Once the sensor receives the trigger signal, it sends the 40 KHz pulse / ultrasonic burst from the built-in transmitter (contains eight pulses).

We should expect the echo signal after the ultrasonic burst signal. The length of the echo signal provides us with the timings for the distance calculations. How do we write in a code? The simple way to create a trigger pulse is by turning the pin high for 10µs, as shown in the following code:

GPIO.output(23, True)
time.sleep(0.00001)
GPIO.output(23, False)

Using this piece of code, we sent a pulse, and the ultrasonic sound burst is generated internally and transmitted into the air. It will reflect and come back to generate the echo pulse. We have to check it continuously until we get the echo pulse. As we saw in the previous figure, the echo pin remains high for a certain period of time. So, we need to calculate the amount of time while the echo signal received stays on. We can use the while() loop to continuously monitor the echo pin. We can use the time function to capture the time duration of echo while it remains high:

while GPIO.input(24) == 0:
     start_time = time.time()

This piece of code creates a kind of forever running loop until the echo pin changes its state from 0 (low) to 1 (high). While the echo pin is 0 (or low), we keep updating the timing value (timestamp) in the start_time variable. Once the echo pin goes high, the while() loop breaks, and we get the value of time when the echo pulse went from low to high low state. Next, we need to use the same logic to get the timestamp for the time it remains high. At this instance, we used another variable to store the timestamp so that later on, we can use both the variables to determine the time duration:

while GPIO.input(24) == 1:
     end_time = time.time()

The while() loop here keeps updating the variable with the latest time while the condition is true.

We have the start time and the end time of the echo pulse. So, we have to subtract both the values to get the pulse duration. Once we get the pulse duration, we can then use it for the distance calculation:

time = end_time - start_time

Initially, in the chapter, we derived a formula for distance calculation, which will be useful here. Let's recall it:

We considered the two-way journey of sound and then divided the time by the factor of 2.

Converting this formula into the Python script, we get the following code:

distance = 17150 * time
print "Measured Distance is :", distance, "cms"

We have understood the operation in parts. Let's join them together to get a completely working sensor with the RasPi. Write sudo nano distance.py in the PuTTY command line and start typing the following code:

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
print "Measuring Distance"
print "Press ctrl+c to stop me"
GPIO.setup(23,GPIO.OUT)
GPIO.setup(24,GPIO.IN)
time.sleep(0.02)
GPIO.output(23, False)
print "Setting Trigger pin to zero by default"
time.sleep(1)
while True:
     GPIO.output(23, True)
     time.sleep(0.00001)
     GPIO.output(23, False)
     while GPIO.input(24) == 0:
          start_time = time.time()
     while GPIO.input(24) == 1:
          end_time = time.time()
     time = end_time – start_time
     distance = 17150 * time
     print "Measured Distance is:", distance, "cms."

Press Ctrl + X followed by Y and then press Enter to exit and save the code.

Compared to the steps to set up software, there are some minor changes in the compiled code. You will find that we used GPIO.setwarnings(False). This command turns off the warnings generated while compiling the code. When the program is started and the GPIOs are configured, give the RasPi module some time to get ready to take the readings. There is a need to use the while() loop. Using this forever-running loop, we can see the live distance variation by pointing the sensor towards different targets. Before we start to compile the project, check the connections again. Just ensure that the ultrasonic sensor is positioned well and pointed towards the target. Alternatively, just put the sensor perpendicular to the table so that the cylinders remain parallel to the table surface.

And here we go! Type sudo python distance.py and press Enter. Wonderful, isn't it? As we programmed and executed it, it now shows the output as Measured Distance is: 9.1321cms. We have created the ultrasonic distance meter. You can point it at various objects to measure the distance. I have tried to measure the distance from my table to the kitchen to know how much distance I walk every time I go to drink some water. You can stop the code by pressing Ctrl + C. This kills any script currently running in the Linux shell. You can use this command if the program is stuck in an infinite loop. This may mean that there is a software or a hardware bug.