Working of an Ultrasonic Sensor

Ultrasonic sensor’s are those sensors which is mainly used to detect object by measuring the distance of the targeted object by sending sound waves of high frequency which is not audible for humans and can be easily found in automobile safety systems i.e. anti-collision system

What is Ultrasonic Sensor

  • Ultrasonic sensor is an electronic device which generates ultrasonic sound waves to recognize the object or to estimate the distance between itself and the object
  • Ultrasonic sound waves are those waves whose frequencies are beyond audible range ( more than 20kilohertz). This ultrasonic sensors uses ultrasonic sound waves because its traveling speed in the air media is more than that of the speed of the audible sound waves
  • Ultrasonic sensors have two transducers one is transmitter and another one is for receiver attached in one head, that transmit back information about objects nearness
  • Transmitter converts electrical signal waves into ultrasonic sound waves whereas receiver converts the ultrasonic sound waves into the electrical signal waves

Working of Ultrasonic Sensor

Now let’s see figure 1  and understand the working step by step:


  • HC-SR04 (ultrasonic sensor module) has two transducers one is transmitter and other one is receiver
  • The transmitter firstly generates the ultrasonic sound waves of 40khz with the help of piezoelectric crystal and releases the original signal in the air medium
  • While traveling through the media,(here air)when the ultrasonic sound waves encounters an obstacle or object it bounces back
  • And, then the reflected signal is absorbed by the receiver site of the sensor


  • Use to power ON the ultrasonic sensor
  • Usually given with 5volt
  • It consumes very less current to operate thus can be easily powered by onboard power supply such as arduino or raspberry pi
  • This is the input trigger pin of sensor
  • Firstly it is been fed with 10microsecond pulses to start the ranging
  • Then HC-SR04 module emits 8 ultrasonic pulses at 40khz
  • When the original signal hits the object the output Echo pin is set in HIGH state
  • And, once the reflected signal gets absorbed at the receiver site after facing an obstacle this echo pin goes low thus it is used to calculate the distance


  • This is the ground pin
  • It has to be connected to the on board’s ground


Calculation of Distance

To calculate the actual distance between the sensor and the object we will use the formula DISTANCE=(SPEED*TIME)/2 here at 20 degree Celsius the speed of sound is 340m/sec  as the medium of transmission is air or  we can also say :-We are dividing it by 2 because as we are traveling the same distance twice one is the original signal traveling towards the object and other one is the reflected signal traveling from the object

Timing Diagram and it’s explanation

SR04 Timing Diagram


1. When there is no object encountered while ultrasonic wave traveling:-

  • First we will set trig on a HIGH state and send 10us pulse and transmit eight ultrasonic pulses
  • And, as no object therefore echo is 38ms (38ms because this is the time out period of echo pin in module HC-SR04 if no object detected)

2. When there is object encountered while ultrasonic wave traveling:-

  • Here again when we send 10us pulse and transmit eight ultrasonic pulses
  • And, as object detected therefore signal gets reflected back and comes back with the timing of 100us-18ms ,which is then used to determine the distance

Applications of Ultrasonic Sensor


  • Vehicle detection for car wash



Vehicle detection for car wash [Fig-3]

  • People detection such as counting
  • Robotic sensing in drones
  • Presence detection
  • Tank level

Tank level[fig-4]

Advantages of Ultrasonic Sensor


  • Low cost
  • Not affected by color or transparency of the object
  • Not affected by the dust
  • Greater accuracy
  • Better sensing capability for all material types


Limitations of Ultrasonic Sensor


  • Very sensitive in different temperature and humidity as traveling speed varies accordingly
  • Difficult to interpret readings from soft and curved objects
  • Needs an media to send ultrasonic waves
  • Have a limited range



Program code on Arduino for HC-SR04


const int trigPin = 9;    //defining the trigger pin
const int echoPin = 10;   //defining the echo pin
float duration;           //‘duration’ will hold the length of sound wave
float distance;           //‘distance’ will store data of distance of object                         
void setup()               
 pinMode(trigPin, OUTPUT); //declaring trig pin as output pin 
 pinMode(echoPin, INPUT);  // declaring echo pin as the input pin 
 Serial.begin(9600);       //serial communication takes place 
void loop() 
 digitalWrite(trigPin, LOW); //setting the trig pin in LOW state  
 delayMicroseconds(2);       //for 2microseconds
 digitalWrite(trigPin, HIGH);//setting the trig pin in HIGH state 
 delayMicroseconds(10);      //for 10us which emits 8pulses from the transmitter and bounces back once hit by an object in the receiver site
 digitalWrite(trigPin, LOW);
 duration = pulseIn(echoPin, HIGH);//duration is a variable that starts timing once the echo pin is high by using arduino function pulseIn()
 distance = (duration*.0343)/2;   //calculating distance by putting the value of duration
 Serial.print("Distance: ");      //printing the results of the distance

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