Tag Archives: Echolocation

In this video lesson we explore creating a portable distance measurement widget using the Raspberry Pi Pico W, the HC-SR04 Ultrasonic Sensor, and the SSD1306 OLED display.

The schematic we use for this project is as follows:

Schematic for Portable Distance Sensor Project

In this work, we use the State Machines on the Pi Pico to ensure precise timing. Below, we include the code developed in the lesson above:

import rp2
from machine import Pin,I2C  #***************
import time
from ssd1306 import SSD1306_I2C  #**************

i2c2=I2C(1,sda=Pin(2), scl=Pin(3), freq=400000)
dsp=SSD1306_I2C(128,64,i2c2)

@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)
def pulse_program():
    wrap_target()
    pull(block)
    mov(x,osr)
    set(pins,1)[31-1]
    set(pins,0)
    wait(1,pin,0)
    label("land")
    in_(pins,1) # 1 Cycle
    mov(y,isr)  # 1 Cycle
    jmp(not_y,"moveOn") # 1 Cycle
    jmp(x_dec,"land")  # 1 Cycle
    label("moveOn")
    mov(isr,invert(x))
    push()
    wrap()
triggerPin = Pin(18,Pin.OUT)
echoPin = Pin(19, Pin.IN)
sm=rp2.StateMachine(0,pulse_program, freq=1000000, set_base=triggerPin,in_base=echoPin)
sm.active(1)
while True:
    sm.put(0xFFFFFFFF)
    print("Pulse Launched")
    clockCycles=(sm.get())*4/2
    distance=clockCycles*.0342
    print("Distance to Target: ",distance)
    dsp.text("Distance: "+str(distance),0,16)
    dsp.show()
    dsp.fill(0)
    time.sleep(.25)

import rp2

from machine import Pin,I2C #***************

import time

from ssd1306 import SSD1306_I2C #**************

i2c2=I2C(1,sda=Pin(2), scl=Pin(3), freq=400000)

dsp=SSD1306_I2C(128,64,i2c2)

@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)

def pulse_program():

wrap_target()

pull(block)

mov(x,osr)

set(pins,1)[31-1]

set(pins,0)

wait(1,pin,0)

label("land")

in_(pins,1) # 1 Cycle

mov(y,isr) # 1 Cycle

jmp(not_y,"moveOn") # 1 Cycle

jmp(x_dec,"land") # 1 Cycle

label("moveOn")

mov(isr,invert(x))

push()

wrap()

triggerPin = Pin(18,Pin.OUT)

echoPin = Pin(19, Pin.IN)

sm=rp2.StateMachine(0,pulse_program, freq=1000000, set_base=triggerPin,in_base=echoPin)

sm.active(1)

while True:

sm.put(0xFFFFFFFF)

print("Pulse Launched")

clockCycles=(sm.get())*4/2

distance=clockCycles*.0342

print("Distance to Target: ",distance)

dsp.text("Distance: "+str(distance),0,16)

dsp.show()

dsp.fill(0)

time.sleep(.25)

Raspberry Pi Pico

Precision Ultrasonic Distance Measurement With PIO State Machines on Raspberry Pi Pico W

February 4, 2025 admin

In this video lesson we will show you how to get precise distance measurements with the HC-SR04 Ultrasonic sensor. We show how measurement stability and accuracy can be improved by using the Raspberry Pi Pico PIO State Machines to trigger and monitor the sensor. For your convenience, the code is presented below.

import rp2
from machine import Pin
import time

@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)
def pulse_program():
    wrap_target()
    pull(block)
    mov(x,osr)
    set(pins,1)[10-1]
    set(pins,0)
    wait(1,pin,0)
    label("land")
    mov(y,pins) # 1 clock cycle
    jmp(not_y,"moveOn") #1 clock cycle
    jmp(x_dec,"land") # 1 clock cycle
    label("moveOn")
    mov(isr,invert(x))
    push()
    wrap()
triggerPin = Pin(16,Pin.OUT)
echoPin = Pin(17, Pin.IN)
sm=rp2.StateMachine(0,pulse_program, freq=1000000, set_base=triggerPin,in_base=echoPin)
sm.active(1)
while True:
    sm.put(0xFFFFFFFF)
    print("Pulse Launched")
    clockCycles=sm.get()*3/2
    distance=clockCycles*.0342
    print("Distance to Target: ",distance)
    time.sleep(.25)

import rp2

from machine import Pin

import time

@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)

def pulse_program():

wrap_target()

pull(block)

mov(x,osr)

set(pins,1)[10-1]

set(pins,0)

wait(1,pin,0)

label("land")

mov(y,pins) # 1 clock cycle

jmp(not_y,"moveOn") #1 clock cycle

jmp(x_dec,"land") # 1 clock cycle

label("moveOn")

mov(isr,invert(x))

push()

wrap()

triggerPin = Pin(16,Pin.OUT)

echoPin = Pin(17, Pin.IN)

sm=rp2.StateMachine(0,pulse_program, freq=1000000, set_base=triggerPin,in_base=echoPin)

sm.active(1)

while True:

sm.put(0xFFFFFFFF)

print("Pulse Launched")

clockCycles=sm.get()*3/2

distance=clockCycles*.0342

print("Distance to Target: ",distance)

time.sleep(.25)

Raspberry Pi Pico

Measure Distance Using the HC-SR04 Ultrasonic Sensor on the Raspberry Pi Pico

January 28, 2025 admin

In this Video Lesson I will teach you how to use echolocation to measure distances. We will use the HC-SR04 Ultrasonic Sensor, and will use the Raspberry Pi Pico W. We will explain how echolocation works, and show you how the sensor works. We will do the math, and then create the code for making measurements.

import machine
import time

TRIG_PIN = 16
ECHO_PIN = 17

TRIG = machine.Pin(TRIG_PIN,machine.Pin.OUT)
ECHO = machine.Pin(ECHO_PIN,machine.Pin.IN)

def distance():
    TRIG.low()
    time.sleep_us(2)
    TRIG.high()
    time.sleep_us(10)
    TRIG.low()
    while (ECHO.value()==0):
        pass
    time1=time.ticks_us()
    while (ECHO.value()==1):
        pass
    time2=time.ticks_us()
    pingTime=time.ticks_diff(time2,time1)/2
    d=.0343*pingTime
    return d
while True:
    dist=distance()
    print("Distance to Target: ",dist," cm")
    time.sleep_ms(250)

import machine

import time

TRIG_PIN = 16

ECHO_PIN = 17

TRIG = machine.Pin(TRIG_PIN,machine.Pin.OUT)

ECHO = machine.Pin(ECHO_PIN,machine.Pin.IN)

def distance():

TRIG.low()

time.sleep_us(2)

TRIG.high()

time.sleep_us(10)

TRIG.low()

while (ECHO.value()==0):

pass

time1=time.ticks_us()

while (ECHO.value()==1):

pass

time2=time.ticks_us()

pingTime=time.ticks_diff(time2,time1)/2

d=.0343*pingTime

return d

while True:

dist=distance()

print("Distance to Target: ",dist," cm")

time.sleep_ms(250)

Elegoo Smart Car Tutorial

Robotics Training LESSON 18: Obstacle Detection and Collision Avoidance

December 9, 2020 admin

In this lesson we explore using the HC-SR04 to detect obstacles, and then avoid collisions by stopping the car. This requires a more sophisticated control algorithm for the desired distance for the car to travel. Making it even more tricky, after the obstacle is removed, the car needs to resume its programmed mission. The video shows my solution to this problem

int Trig=A5;
int Echo=A4;

int ENA=5;
int ENB=6;
int IN1=7;
int IN2=8;
int IN3=9;
int IN4=11;
float d=1;
int degRot=90;
int left;
int right;
float v=1.2;
int rv;
char cmd;
void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
pinMode(Trig,OUTPUT);
pinMode(Echo,INPUT);


pinMode(ENA,OUTPUT);
pinMode(ENB,OUTPUT);
pinMode(IN1,OUTPUT);
pinMode(IN2,OUTPUT);
pinMode(IN3,OUTPUT);
pinMode(IN4,OUTPUT);
digitalWrite(ENA,HIGH);
digitalWrite(ENB,HIGH);
}

void loop() {
int wv;
//v=1.2;
//d=1;
wv=(v-.35)/.0075;
left=wv;
right=wv;
setSpeed(left,right);
while (Serial.available()==0){
}
cmd=Serial.read();
if (cmd=='G'){
  Serial.println("Forward");
  forward(d,v);  
}
if (cmd=='B'){
  Serial.println("Backward");
  backward(d,v);  
}
if (cmd=='R'){
  Serial.println("Right");
  turnRight(degRot,wv);  
}
if (cmd=='L'){
  Serial.println("Left");
  turnLeft(degRot,wv);  
}
//If Command Was Pound, Set Distance
if (cmd=='d'){
  Serial.println("Set distance");
  delay(500);

  while (Serial.available()==0){
  }
cmd=Serial.read();

    if (cmd=='1'){
    Serial.println("Distance=1");
    d=1;
  }
    if (cmd=='2'){
    Serial.println("Distance=2");
    d=2;
  }
      if (cmd=='3'){
    Serial.println("Distance=3");
    d=3;
  }
      if (cmd=='4'){
    Serial.println("Distance=4");
    d=4;
  }
      if (cmd=='5'){
    Serial.println("Distance=5");
    d=5;
  }
      if (cmd=='6'){
    Serial.println("Distance=6");
    d=6;
  }
      if (cmd=='7'){
    Serial.println("Distance=7");
    d=7;
  }
      if (cmd=='8'){
    Serial.println("Distance=8");
    d=8;
  }
      if (cmd=='9'){
    Serial.println("Distance=9");
    d=9;
  }
}

//If Command Was star, Set Speed
if (cmd=='v'){
  Serial.println("Set speed");
  delay(500);
   while (Serial.available()==0){
    
  }
  delay(100);
  cmd=Serial.read();
    if (cmd=='1'){
    Serial.println("rv=1");
    rv=1;
  }
    if (cmd=='2'){
    Serial.println("rv=2");
    rv=2;
  }
      if (cmd=='3'){
    Serial.println("rv=3");
    rv=3;
  }
      if (cmd=='4'){
    Serial.println("rv=4");
    rv=4;
  }
      if (cmd=='5'){
    Serial.println("rv=5");
    rv=5;
  }
      if (cmd=='6'){
    Serial.println("rv=6");
    rv=6;
  }
      if (cmd=='7'){
    Serial.println("rv=7");
    rv=7;
  }
      if (cmd=='8'){
    Serial.println("rv=8");
    rv=8;
  }
      if (cmd=='9'){
    Serial.println("rv=9");
    rv=9;
  }
  v=(1.16/9.)*rv+1.1;




}
}
void setSpeed(int leftVal,int rightVal){
  analogWrite(ENA,leftVal);
  analogWrite(ENB,rightVal);
}
void forward(float d, float v){
float tTarget;
float dObs;
int tOn;
int tStart;
bool carBlocked=false;
int tStopped=0;
int t1;
int t2;
tStart=millis();
digitalWrite(IN1,HIGH);
digitalWrite(IN2,LOW);
digitalWrite(IN3,LOW);
digitalWrite(IN4,HIGH);
tTarget=d/v*1000;
tOn=millis()-tStart-tStopped;
while (tOn<=tTarget){
tOn=millis()-tStart-tStopped;
dObs=measDist();
t1=millis();
while(dObs<=12){
  stopCar();
  dObs=measDist();
  carBlocked=true;
}
t2=millis();
if (carBlocked==true){
  tStopped=tStopped+(t2-t1);
  carBlocked=false;
  digitalWrite(IN1,HIGH);
digitalWrite(IN2,LOW);
digitalWrite(IN3,LOW);
digitalWrite(IN4,HIGH);
}
}
stopCar();
}
void backward(float d, float v){
float t;
digitalWrite(IN1,LOW);
digitalWrite(IN2,HIGH);
digitalWrite(IN3,HIGH);
digitalWrite(IN4,LOW);
t=d/v*1000;
delay(t);
stopCar();
}
void turnRight(int deg, int wv){
  float t;
stopCar();
delay(100);
analogWrite(ENA,125);
analogWrite(ENB,125);
digitalWrite(IN1,HIGH);
digitalWrite(IN2,LOW);
digitalWrite(IN3,HIGH);
digitalWrite(IN4,LOW);
t=(deg+6)/136.29*1000.;
delay(t);
stopCar();
analogWrite(ENA,wv);
analogWrite(ENB,wv);
}
void turnLeft(int deg, int wv){
float t;
analogWrite(ENA,125);
analogWrite(ENB,125);
digitalWrite(IN1,LOW);
digitalWrite(IN2,HIGH);
digitalWrite(IN3,LOW);
digitalWrite(IN4,HIGH);
t=(deg+6)/136.29*1000.;
delay(t);
stopCar();
analogWrite(ENA,wv);
analogWrite(ENB,wv);
}
void stopCar(){
digitalWrite(IN1,LOW);
digitalWrite(IN2,LOW);
digitalWrite(IN3,LOW);
digitalWrite(IN4,LOW);
}

void calF(){
digitalWrite(IN1,HIGH);
digitalWrite(IN2,LOW);
digitalWrite(IN3,LOW);
digitalWrite(IN4,HIGH);
delay(5000);
stopCar();
}
void calB(){
digitalWrite(IN1,LOW);
digitalWrite(IN2,HIGH);
digitalWrite(IN3,HIGH);
digitalWrite(IN4,LOW);
delay(5000);
stopCar();
}
void calR(int wv){
stopCar();
analogWrite(ENA,125);
analogWrite(ENB,125);
digitalWrite(IN1,HIGH);
digitalWrite(IN2,LOW);
digitalWrite(IN3,HIGH);
digitalWrite(IN4,LOW);
delay(3000);
analogWrite(ENA,wv);
analogWrite(ENB,wv);
stopCar();
}
void calL(int wv){
analogWrite(ENA,125);
analogWrite(ENB,125);
digitalWrite(IN1,LOW);
digitalWrite(IN2,HIGH);
digitalWrite(IN3,LOW);
digitalWrite(IN4,HIGH);
delay(5000);
analogWrite(ENA,wv);
analogWrite(ENB,wv);
stopCar();
}

unsigned int pingTime(){
  unsigned int pingTravelTime;
  digitalWrite(Trig, LOW);   
  delayMicroseconds(2);
  digitalWrite(Trig, HIGH);  
  delayMicroseconds(20);
  digitalWrite(Trig, LOW);   
  pingTravelTime = pulseIn(Echo, HIGH);        
  return pingTravelTime; 
}
float measDist(){
 unsigned int pingTravelTime;
  float Dist;
  pingTravelTime=pingTime();
  Dist=(pingTravelTime*761.*5280.*12.)/(1000000.*3600.);
  Dist=Dist/2;
  return Dist;
}

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int Trig=A5;

int Echo=A4;

int ENA=5;

int ENB=6;

int IN1=7;

int IN2=8;

int IN3=9;

int IN4=11;

float d=1;

int degRot=90;

int left;

int right;

float v=1.2;

int rv;

char cmd;

void setup() {

// put your setup code here, to run once:

Serial.begin(9600);

pinMode(Trig,OUTPUT);

pinMode(Echo,INPUT);

pinMode(ENA,OUTPUT);

pinMode(ENB,OUTPUT);

pinMode(IN1,OUTPUT);

pinMode(IN2,OUTPUT);

pinMode(IN3,OUTPUT);

pinMode(IN4,OUTPUT);

digitalWrite(ENA,HIGH);

digitalWrite(ENB,HIGH);

}

void loop() {

int wv;

//v=1.2;

//d=1;

wv=(v-.35)/.0075;

left=wv;

right=wv;

setSpeed(left,right);

while (Serial.available()==0){

}

cmd=Serial.read();

if (cmd=='G'){

Serial.println("Forward");

forward(d,v);

}

if (cmd=='B'){

Serial.println("Backward");

backward(d,v);

}

if (cmd=='R'){

Serial.println("Right");

turnRight(degRot,wv);

}

if (cmd=='L'){

Serial.println("Left");

turnLeft(degRot,wv);

}

//If Command Was Pound, Set Distance

if (cmd=='d'){

Serial.println("Set distance");

delay(500);

while (Serial.available()==0){

}

cmd=Serial.read();

if (cmd=='1'){

Serial.println("Distance=1");

d=1;

}

if (cmd=='2'){

Serial.println("Distance=2");

d=2;

}

if (cmd=='3'){

Serial.println("Distance=3");

d=3;

}

if (cmd=='4'){

Serial.println("Distance=4");

d=4;

}

if (cmd=='5'){

Serial.println("Distance=5");

d=5;

}

if (cmd=='6'){

Serial.println("Distance=6");

d=6;

}

if (cmd=='7'){

Serial.println("Distance=7");

d=7;

}

if (cmd=='8'){

Serial.println("Distance=8");

d=8;

}

if (cmd=='9'){

Serial.println("Distance=9");

d=9;

}

//If Command Was star, Set Speed

if (cmd=='v'){

Serial.println("Set speed");

delay(500);

while (Serial.available()==0){

}

delay(100);

cmd=Serial.read();

if (cmd=='1'){

Serial.println("rv=1");

rv=1;

}

if (cmd=='2'){

Serial.println("rv=2");

rv=2;

}

if (cmd=='3'){

Serial.println("rv=3");

rv=3;

}

if (cmd=='4'){

Serial.println("rv=4");

rv=4;

}

if (cmd=='5'){

Serial.println("rv=5");

rv=5;

}

if (cmd=='6'){

Serial.println("rv=6");

rv=6;

}

if (cmd=='7'){

Serial.println("rv=7");

rv=7;

}

if (cmd=='8'){

Serial.println("rv=8");

rv=8;

}

if (cmd=='9'){

Serial.println("rv=9");

rv=9;

}

v=(1.16/9.)*rv+1.1;

}

void setSpeed(int leftVal,int rightVal){

analogWrite(ENA,leftVal);

analogWrite(ENB,rightVal);

}

void forward(float d, float v){

float tTarget;

float dObs;

int tOn;

int tStart;

bool carBlocked=false;

int tStopped=0;

int t1;

int t2;

tStart=millis();

digitalWrite(IN1,HIGH);

digitalWrite(IN2,LOW);

digitalWrite(IN3,LOW);

digitalWrite(IN4,HIGH);

tTarget=d/v*1000;

tOn=millis()-tStart-tStopped;

while (tOn<=tTarget){

tOn=millis()-tStart-tStopped;

dObs=measDist();

t1=millis();

while(dObs<=12){

stopCar();

dObs=measDist();

carBlocked=true;

}

t2=millis();

if (carBlocked==true){

tStopped=tStopped+(t2-t1);

carBlocked=false;

digitalWrite(IN1,HIGH);

digitalWrite(IN2,LOW);

digitalWrite(IN3,LOW);

digitalWrite(IN4,HIGH);

}

stopCar();

}

void backward(float d, float v){

float t;

digitalWrite(IN1,LOW);

digitalWrite(IN2,HIGH);

digitalWrite(IN3,HIGH);

digitalWrite(IN4,LOW);

t=d/v*1000;

delay(t);

stopCar();

}

void turnRight(int deg, int wv){

float t;

stopCar();

delay(100);

analogWrite(ENA,125);

analogWrite(ENB,125);

digitalWrite(IN1,HIGH);

digitalWrite(IN2,LOW);

digitalWrite(IN3,HIGH);

digitalWrite(IN4,LOW);

t=(deg+6)/136.29*1000.;

delay(t);

stopCar();

analogWrite(ENA,wv);

analogWrite(ENB,wv);

}

void turnLeft(int deg, int wv){

float t;

analogWrite(ENA,125);

analogWrite(ENB,125);

digitalWrite(IN1,LOW);

digitalWrite(IN2,HIGH);

digitalWrite(IN3,LOW);

digitalWrite(IN4,HIGH);

t=(deg+6)/136.29*1000.;

delay(t);

stopCar();

analogWrite(ENA,wv);

analogWrite(ENB,wv);

}

void stopCar(){

digitalWrite(IN1,LOW);

digitalWrite(IN2,LOW);

digitalWrite(IN3,LOW);

digitalWrite(IN4,LOW);

}

void calF(){

digitalWrite(IN1,HIGH);

digitalWrite(IN2,LOW);

digitalWrite(IN3,LOW);

digitalWrite(IN4,HIGH);

delay(5000);

stopCar();

}

void calB(){

digitalWrite(IN1,LOW);

digitalWrite(IN2,HIGH);

digitalWrite(IN3,HIGH);

digitalWrite(IN4,LOW);

delay(5000);

stopCar();

}

void calR(int wv){

stopCar();

analogWrite(ENA,125);

analogWrite(ENB,125);

digitalWrite(IN1,HIGH);

digitalWrite(IN2,LOW);

digitalWrite(IN3,HIGH);

digitalWrite(IN4,LOW);

delay(3000);

analogWrite(ENA,wv);

analogWrite(ENB,wv);

stopCar();

}

void calL(int wv){

analogWrite(ENA,125);

analogWrite(ENB,125);

digitalWrite(IN1,LOW);

digitalWrite(IN2,HIGH);

digitalWrite(IN3,LOW);

digitalWrite(IN4,HIGH);

delay(5000);

analogWrite(ENA,wv);

analogWrite(ENB,wv);

stopCar();

}

unsigned int pingTime(){

unsigned int pingTravelTime;

digitalWrite(Trig, LOW);

delayMicroseconds(2);

digitalWrite(Trig, HIGH);

delayMicroseconds(20);

digitalWrite(Trig, LOW);

pingTravelTime = pulseIn(Echo, HIGH);

return pingTravelTime;

}

float measDist(){

unsigned int pingTravelTime;

float Dist;

pingTravelTime=pingTime();

Dist=(pingTravelTime*761.*5280.*12.)/(1000000.*3600.);

Dist=Dist/2;

return Dist;

}

Elegoo Smart Car Tutorial

Robotics Training LESSON 17: Measuring Distance to Obstacle using the HC-SR04

December 2, 2020 admin

In this video lesson we show how to use the HC-SR04 ultrasonic sensor to detect the distance to obstacles in a robotic application. We use the sensor to measure the time it takes for a ping to travel from the sensor, to the target, and then back. We use this measured time, and the known speed of sound to calculate the distance to the target. This will be used in future lessons to keep the robot from running into an obstacle.

int Trig=A5;
int Echo=A4;
int pingT;
float targetDistance;
void setup() {
  // put your setup code here, to run once:
pinMode(Trig,OUTPUT);
pinMode(Echo,INPUT);
Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
targetDistance=measDist();
Serial.println(targetDistance);
delay(500);

}

int pingTime(){
  int pingTravelTime;
  digitalWrite(Trig, LOW);   
  delayMicroseconds(2);
  digitalWrite(Trig, HIGH);  
  delayMicroseconds(20);
  digitalWrite(Trig, LOW);   
  pingTravelTime = pulseIn(Echo, HIGH);        
  return pingTravelTime; 
}
float measDist(){
  int pingTravelTime;
  float Dist;
  pingTravelTime=pingTime();
  Dist=(pingTravelTime*761.*5280.*12.)/(1000000.*3600.);
  Dist=Dist/2;
  return Dist;
}

int Trig=A5;

int Echo=A4;

int pingT;

float targetDistance;

void setup() {

// put your setup code here, to run once:

pinMode(Trig,OUTPUT);

pinMode(Echo,INPUT);

Serial.begin(9600);

}

void loop() {

// put your main code here, to run repeatedly:

targetDistance=measDist();

Serial.println(targetDistance);

delay(500);

}

int pingTime(){

int pingTravelTime;

digitalWrite(Trig, LOW);

delayMicroseconds(2);

digitalWrite(Trig, HIGH);

delayMicroseconds(20);

digitalWrite(Trig, LOW);

pingTravelTime = pulseIn(Echo, HIGH);

return pingTravelTime;

}

float measDist(){

int pingTravelTime;

float Dist;

pingTravelTime=pingTime();

Dist=(pingTravelTime*761.*5280.*12.)/(1000000.*3600.);

Dist=Dist/2;

return Dist;

}

Technology Tutorials

Tag Archives: Echolocation

Portable Distance Measurement Project With HC-SR04 Ultrasonic Sensor

Precision Ultrasonic Distance Measurement With PIO State Machines on Raspberry Pi Pico W

Measure Distance Using the HC-SR04 Ultrasonic Sensor on the Raspberry Pi Pico

Robotics Training LESSON 18: Obstacle Detection and Collision Avoidance

Robotics Training LESSON 17: Measuring Distance to Obstacle using the HC-SR04

Making The World a Better Place One High Tech Project at a Time. Enjoy!