Raspberry Pi Computing

Tuesday, 24 April 2018

Ultrasonic Distance Measurement with a Raspberry Pi

This post is part of the book Raspberry Pi Computing: Ultrasonic Distance Measurement which can be downloaded from Leanpub for free (or donate if you wish).

Ultrasonic Distance Measurement

This project will measure distance using an ultrasonic sensor module. We will use the JSN-SR04T sensor since it utilises a single waterproof send/receive audio module that will be useful for my particular application, but an alternative sensor called the HC-SR04 is a drop in replacement that combines the electronic and audio modules. It also breaks out the send and receive components which allows for measuring closer distances down to 2cm.

The JSN-SR04T Sensor

The JSN-SR04T is an ultrasonic ranging sensor that comprises an electronics module and a waterproof audio module connected by a long cable. It is designed to measure distance by sending out a high frequency pattern of pulses when triggered and to return a signal that corresponds to the length of time that it has taken for the pulses to travel out and be reflected back to the audio module.

Electronics and Audio modules for JSN-SR04T
Electronics and Audio modules for JSN-SR04T

JSN-SR04T Specifications

Specifications:
  • Operating voltage: DC 5V
  • Quiescent current: 5mA
  • Total current work: 30mA
  • Acoustic emission frequency: 40khz
  • Farthest distance: 4.5m
  • Shortest distance: 25cm
  • Module size: 41mm x 28.5mm
  • Resolution: about 0.5cm
  • Angle: less than 50 degrees
  • Working temperature: -10 to 70°C
Wiring:
  • +5V (Power supply)
  • Trig (Receives the trigger to operate)
  • Echo (Signals time taken for audio send/receive trip)
  • GND (Ground)


The device is touted as a drop-in replacement for the HC-SR04 which combines the electronics and audio modules and which separates the transmit and receive sensors.

HC-SR04. Image Courtesy sparkfun.com
HC-SR04. Image Courtesy sparkfun.com

Measure

Hardware required

  • 1 x JSN-SR04T sensor
  • 1k Ohm resister
  • 2k Ohm resister
  • Jumper cables with Dupont connectors on the end
  • Solder
  • Heat-shrink

Connect

For the sake of keeping the connecting pins together on the Pi’s GPIO block we will be connecting pins 4,6,8 and 10 to the JSN-SR04T electronics module. The following diagram provides a high level overview of the complete chain.

Electronics and Audio modules for JSN-SR04T
Electronics and Audio modules for JSN-SR04T

  • Pin 4 connects directly to the +5V connector
  • Pin 6 Connects directly to the GND connector, but is incorporated into the voltage divider for the Echo pin.
  • Pin 8 connects to the centre of the voltage divider
  • Pin 10 connects directly to the Trig connector.
As we noted earlier, the Raspberry Pi provides general purpose digital input/output pins (called GPIO pins) that you can use for reading digital logic signals. In our case, we will want to apply the output signal from the ‘Echo’ pin to them. Unfortunately the output voltage from the Echo pin is 5V and the GPIO pins on the Raspberry Pi are only able to operate safely at voltage levels of 3.3V.
To safely apply the correct voltage to the Pi we can employ a voltage divider which divides the signal in a ratio corresponding to the resistance of two connected resistors. In the following diagram we can see that when a 5V signal is applied from the JSN-SR04T, 1.7V is dropped across the 1k Ohm resistor and 3.3V is across the 2k Ohm resistor.

How the voltage divider works
How the voltage divider works

Basic Operation

A pulse of at least 10us is applied to the trigger (Trig) pin of the electronics module.

A trigger signal starts the process
A trigger signal starts the process

This will automatically generate 8 audio pulses at 40kHz from the audio transmitter / receiver module.

This generates 8, 40kHz pulses
This generates 8, 40kHz pulses

The 8 pulses will reflect off a surface and be received back at the audio module.

The 8 pulses are reflected back to the audio module
The 8 pulses are reflected back to the audio module

The electronics module will generate a signal from the Echo pin that corresponds to the time between the audio pulses being transmitted and the reflections being received. If no reflected audio is received, a signal of time, t = 38ms will be generated.

The signal from the Echo pin corresponds to the time
The signal from the Echo pin corresponds to the time

The distance from the audio module to the reflecting surface can be found by multiplying the time that the echo pin was high (represented as 12ms in the diagram)by the speed of sound (340m/s) and dividing by 2 (since the sound travelled out and back). Therefore;

calculating the distance
calculating the distance

Our object is 2.04 metres away!

Test

The following python program can be saved onto our Pi as distance.py and run from the command line using;

#!/usr/bin/python
#encoding:utf-8

import RPi.GPIO as GPIO                    #Import GPIO library
import time                                #Import time library
GPIO.setmode(GPIO.BCM)                     #Set GPIO pin numbering 

TRIG = 15                                  #Associate pin 15 to TRIG
ECHO = 14                                  #Associate pin 14 to Echo

print "Distance measurement in progress"

GPIO.setup(TRIG,GPIO.OUT)                  #Set pin as GPIO out
GPIO.setup(ECHO,GPIO.IN)                   #Set pin as GPIO in

while True:

  GPIO.output(TRIG, False)                 #Set TRIG as LOW
  print "Waiting For Sensor To Settle"
  time.sleep(2)                            #Delay of 2 seconds

  GPIO.output(TRIG, True)                  #Set TRIG as HIGH
  time.sleep(0.00001)                      #Delay of 0.00001 seconds
  GPIO.output(TRIG, False)                 #Set TRIG as LOW

  while GPIO.input(ECHO)==0:               #Check if Echo is LOW
    pulse_start = time.time()              #Time of the last  LOW pulse

  while GPIO.input(ECHO)==1:               #Check whether Echo is HIGH
    pulse_end = time.time()                #Time of the last HIGH pulse 

  pulse_duration = pulse_end - pulse_start #pulse duration to a variable

  distance = pulse_duration * 17150        #Calculate distance
  distance = round(distance, 2)            #Round to two decimal points

  if distance > 20 and distance < 400:     #Is distance within range
    print "Distance:",distance - 0.5,"cm"  #Distance with calibration
  else:
    print "Out Of Range"                   #display out of range

This program is available as distance.py with the code samples that can be downloaded with the book from Leanpub.
When executed the sensor will settle and then start displaying the distance every two seconds.

Waitng For Sensor To Settle
Distance: 53.44 cm
Waitng For Sensor To Settle
Distance: 52.95 cm

Move the audio module about and confirm that it is operating as expected.
For more info on using the Raspberry Pi or for a range of other free to download books (there are no catches) check out the list here.

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