L03 · Distance, Servo & Radar · Digital Challenge

01Before you start

What you’ll learn

By the end of this lab you can…

1Derive distance from echo time and the speed of sound.
2Trigger the HC-SR04 correctly (10 µs pulse) and use a pulseIn timeout.
3Send START / STOP from Processing and sweep a servo without freezing the loop.
4Combine angle + distance into a polar plot (radar).

Key vocabulary

HC-SR04: Ultrasonic ranger. TRIG sends a 40 kHz burst; ECHO stays HIGH for the round-trip time.
Speed of sound: About 343 m/s in dry air at 20 °C. It rises ~0.6 m/s per °C.
pulseIn(): Waits for a pin to go HIGH (or LOW) and returns the duration in microseconds. Always pass a timeout.
SG90: Hobby servo. Servo.write(0…180) asks for an angle. It needs 5 V and its own GND.
Blocking code: A for + delay that hogs loop() so serial commands (STOP) are ignored until the sweep ends.
Polar coordinates: A point as (distance, angle). Radar screens are polar plots.

02Kit list

Required equipment

Component	Qty	Purpose
Arduino Uno	1	Timing + servo PWM
HC-SR04	1	Distance
SG90 servo	1	Sweep / aim
Plastic holder	1	Mount the sensor on the horn
USB + both IDEs	1	Control + radar display

03Theory

The echo is a clock

The HC-SR04 clicks at ~40 kHz — too high to hear. The click hits a wall and comes back. Distance is half the trip, because the sound went there and back.

Formula

distance (cm) = duration (µs) × 0.0343 / 2

0.0343 cm/µs = 343 m/s. The 0.017 shortcut is the same number.

Note

Soft things (jumpers, wool, a classmate in a hoodie) absorb ultrasound. You will get timeouts, not magic. Aim at a hard, flat face.

InteractiveEcho labChange distance and air temperature. See how a 343 m/s constant lies a little when the room is cold.

Real distance60 cm

Air temperature20 °C

Speed of sound

343.4m/s

pulseIn()

3,494µs

Code (343 m/s)

59.9cm

Error

-0.1%

04Build 1

Exercise A — distance on a bar

HC-SR04	Arduino
VCC	5V
GND	GND
TRIG	D3
ECHO	D2

Sonar_Distance.inoArduino · C++

const int trigPin = 3;
const int echoPin = 2;

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  long duration = pulseIn(echoPin, HIGH, 30000); // µs, 30 ms timeout
  float distance = duration * 0.0343 / 2.0;
  Serial.println(distance);
  delay(80);
}

UpdatedpulseIn now has a 30 000 µs timeout so a missing echo cannot freeze the board for a full second.

DistanceBar.pdeProcessing · Java

import processing.serial.*;

Serial myPort;
float distance;

void setup() {
  size(640, 200);
  printArray(Serial.list());
  myPort = new Serial(this, Serial.list()[0], 9600);
  myPort.bufferUntil('\\n');
}

void draw() {
  background(16);
  fill(240);
  textSize(22);
  text("Distance: " + nf(distance, 0, 1) + " cm", 36, 48);
  fill(62, 232, 216);
  rect(36, 80, map(constrain(distance, 0, 200), 0, 200, 0, width - 80), 36, 4);
}

void serialEvent(Serial p) {
  String data = trim(p.readStringUntil('\\n'));
  distance = float(data);
}

05Build 2

Exercise B — START / STOP from Processing

Servo wire	Arduino
Brown / black (GND)	GND
Red (VCC)	5V
Orange / yellow (signal)	D9

Power

One SG90 is usually fine from the Uno’s 5 V. If the board resets when the servo kicks, power the servo from a separate 5 V pack and join the GNDs.

The original handbook swept with two blocking for loops. STOP sat in the serial buffer until the horn finished 180°. The sketch below moves one degree per loop and checks serial every time.

Servo_Sweep.inoArduino · C++

#include <Servo.h>

Servo servo;
bool sweeping = false;
int angle = 0;
int dir = 1;

void setup() {
  Serial.begin(9600);
  servo.attach(9);
  servo.write(0);
}

void loop() {
  if (Serial.available()) {
    String command = Serial.readStringUntil('\\n');
    command.trim();
    if (command == "START") sweeping = true;
    else if (command == "STOP") sweeping = false;
  }

  if (sweeping) {
    angle += dir;
    if (angle >= 180) { angle = 180; dir = -1; }
    if (angle <= 0) { angle = 0; dir = 1; }
    servo.write(angle);
    delay(12);
  }
}

UpdatedNon-blocking sweep. STOP is honoured on the next degree, not the next full cycle.

ServoPad.pdeProcessing · Java

import processing.serial.*;

Serial myPort;
boolean isSweeping = false;

void setup() {
  size(420, 220);
  printArray(Serial.list());
  myPort = new Serial(this, Serial.list()[0], 9600);
  textAlign(CENTER, CENTER);
}

void draw() {
  background(24);
  fill(230);
  textSize(18);
  text("Servo control", width / 2, 36);
  drawButton("START", 70, 100, isSweeping);
  drawButton("STOP", 240, 100, !isSweeping);
}

void drawButton(String label, int x, int y, boolean active) {
  fill(active ? color(62, 200, 140) : color(50));
  rect(x, y, 110, 48, 8);
  fill(active ? 10 : 230);
  text(label, x + 55, y + 24);
}

void mousePressed() {
  if (mouseX > 70 && mouseX < 180 && mouseY > 100 && mouseY < 148) {
    myPort.write("START\\n");
    isSweeping = true;
  } else if (mouseX > 240 && mouseX < 350 && mouseY > 100 && mouseY < 148) {
    myPort.write("STOP\\n");
    isSweeping = false;
  }
}

06Build 3

Exercise C — distance aims the servo

A common kit build: if something is closer than 50 cm, turn the horn to 90°; otherwise park at 0°. TRIG/ECHO move to D6/D7 in this example so you can keep the servo on D9.

Sonar_Aims_Servo.inoArduino · C++

#include <Servo.h>

const int TRIG_PIN = 6;
const int ECHO_PIN = 7;
const int SERVO_PIN = 9;
const int THRESHOLD_CM = 50;

Servo servo;

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  servo.attach(SERVO_PIN);
  servo.write(0);
}

void loop() {
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  float duration = pulseIn(ECHO_PIN, HIGH, 30000);
  float distance = 0.017 * duration;

  servo.write(distance > 0 && distance < THRESHOLD_CM ? 90 : 0);

  Serial.print("distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  delay(80);
}

Your challenge

The Arduino print is distance: 42.0 cm — not a bare number. Write a Processing sketch that strips the text, shows the number, draws a bar, and could later send START/STOP if you add buttons.

07Project

Exercise D — radar scanner

Mount the HC-SR04 on the servo horn. Arduino sweeps 0–180°, measures, and prints angle,distance. Processing plots each hit in polar coordinates. This is a favourite Digital Challenge project.

Radar_Scan.inoArduino · C++

#include <Servo.h>

Servo myServo;
const int trigPin = 9;
const int echoPin = 10;

void setup() {
  Serial.begin(9600);
  myServo.attach(6);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  for (int angle = 0; angle <= 180; angle += 2) {
    myServo.write(angle);
    delay(40);
    digitalWrite(trigPin, LOW);
    delayMicroseconds(2);
    digitalWrite(trigPin, HIGH);
    delayMicroseconds(10);
    digitalWrite(trigPin, LOW);
    long duration = pulseIn(echoPin, HIGH, 30000);
    float distance = duration * 0.0343 / 2.0;
    Serial.print(angle);
    Serial.print(",");
    Serial.println(distance);
  }
}

UpdatedTimeout on pulseIn. Prints angle,distance once per step. After 180° the loop restarts at 0 — add a down-sweep if you want a smooth return.

Radar.pdeProcessing · Java

import processing.serial.*;

Serial myPort;
int angle;
float distance;

void setup() {
  size(640, 400);
  printArray(Serial.list());
  myPort = new Serial(this, Serial.list()[0], 9600);
  myPort.bufferUntil('\\n');
  background(6);
}

void draw() {
  // fade so old blips die out
  noStroke();
  fill(6, 18);
  rect(0, 0, width, height);

  float r = map(constrain(distance, 0, 100), 0, 100, 0, 260);
  float x = width / 2 + r * cos(radians(angle));
  float y = height - 20 - r * sin(radians(angle));
  fill(62, 232, 216);
  ellipse(x, y, 6, 6);
}

void serialEvent(Serial p) {
  String data = trim(p.readStringUntil('\\n'));
  String[] parts = split(data, ",");
  if (parts.length == 2) {
    angle = int(parts[0]);
    distance = float(parts[1]);
  }
}

InteractiveRadar previewClick the dish to drop an obstacle. The sweep is the same `angle,distance` stream Arduino will send.

ANGLE 000°

DIST 000.0 cm

Serial: 0,0.0

Click the dish to drop an obstacle

08From the workshop

Student tasks

Basic

Show distance as text + bar.

A book sliding toward the sensor shortens the bar.

Intermediate

START / STOP buttons that actually stop mid-sweep.

The horn freezes within one degree.

Advanced

Distance aims the servo at 90° under 50 cm.

A wave of the hand flicks the horn.

Challenge

Full radar: sweep, serial CSV, polar plot.

You can “see” a chair leg on the screen.

09When it doesn’t work

Troubleshooting

Problem	Likely cause	Fix
Distance always 0.	Timeout (soft target) or TRIG/ECHO swapped.	Aim at a wall. Swap the two signal wires. Check 5 V.
Servo jitters / Uno resets.	The motor brown-out the USB 5 V rail.	External 5 V for the servo, common GND.
STOP does nothing for two seconds.	Blocking `for` loops from the original handbook.	Use the one-step-per-loop sketch.
Radar paints a spiral in the wrong place.	`cos`/`sin` origin or degrees vs radians.	Use `radians(angle)` and put the origin at the bottom centre.

Golden rule

Check the physical connections first, then the code. If nothing works, unplug, close both programs, reconnect and try again.

10Student edition

Exercises

Basic · 1Intermediate · 2Advanced · 2Challenge · 1

L03-E1BasicCalculate 6 min

How far?

pulseIn returns 2915 µs. Using 343 m/s, how far is the object in centimetres?

d = t × 0.0343 / 2.

2915 × 0.0343 / 2 ≈ 50.0 cm.

L03-E2IntermediateCalculate 10 min

Cold classroom

The code assumes 343 m/s. The room is 10 °C (v ≈ 337.4 m/s). A wall is truly 100 cm away. What distance does the sketch print, and what is the % error?

True echo time uses 337.4 m/s; the code multiplies that time by 0.0343.

t = 2 × 1.00 / 337.4 = 5.928 ms = 5928 µs. Code: 5928 × 0.0343 / 2 ≈ 101.7 cm. Error ≈ +1.7 %.

L03-E3IntermediateDebug 10 min

STOP is late

A student used the original blocking sweep. Explain in 4 sentences why STOP waits, then write the non-blocking idea in two lines of pseudocode.

for (angle = 0; angle <= 180; angle++) { servo.write(angle); delay(10); } does not call Serial.available() for 1.8 s. The STOP line sits in the UART buffer. Pseudocode: each loop: read serial → update sweeping flag → if sweeping, angle += dir, write once, delay 12 ms.

L03-E4AdvancedCode 12 min

Parse a messy line

Arduino prints distance: 42.0 cm. Write serialEvent that extracts 42.0. Do not crash on distance: 0.0 cm or a timeout line.

splitTokens(data, " :cm") or find the first number with a small scan.

serialEventProcessing · Java

void serialEvent(Serial p) {
  String data = trim(p.readStringUntil('\\n'));
  String[] bits = splitTokens(data, " :cm");
  for (int i = 0; i < bits.length; i++) {
    if (bits[i].length() > 0 && Character.isDigit(bits[i].charAt(0))) {
      distance = float(bits[i]);
      break;
    }
  }
}

L03-E5AdvancedCode 12 min

Polar plot one point

Window 600×600, origin at (300, 580). Angle 45°, distance 50 cm mapped onto r = 0…250 for 0…100 cm. Compute x and y of the blip.

r = 50/100 × 250 = 125. x = 300 + 125·cos(45°) ≈ 388. y = 580 − 125·sin(45°) ≈ 492. (y is flipped because screens grow downward; we subtract so 90° goes up.)

L03-E6ChallengeDesign 15 min

Parking sensor UX

Design a Processing UI for a reversing sensor: three zones (<30, 30–80, >80 cm), colour, optional beep, and a frozen “HOLD” button that stops the servo. Sketch the layout and list the serial messages in both directions.

Arduino → PC: angle,distance or just distance. PC → Arduino: START, STOP, maybe HOLD. UI: big number, coloured bar, zone chips, mute toggle. Beep interval shortens as distance falls (map 80→30 cm onto 800→80 ms). HOLD sends STOP and freezes the last bar.

11Check yourself

Self-check quiz

Progress

0/6 answered · 0 correct

Q01
We divide the echo time by 2 because…
Q02
A 10 µs HIGH on TRIG…
Q03
pulseIn(pin, HIGH) with no timeout can…
Q04
The original START/STOP sweep ignored STOP because…
Q05
Radar serial format in this lab is…
Q06
x = r cos θ, y = r sin θ is…

12Beyond the classroom

Real world & extensions

Where this is used

Parking sensorRobot bumperAutomatic doorMuseum “do not touch” beamDIY radar demo

Extension ideas

+Colour blips red under 30 cm, amber under 60 cm.
+Play a beep in Processing when anything is closer than 20 cm.
+Log millis,angle,distance for a heatmap of the room.

Cross-curricular connections

Subject	Connection
Physics	v = d/t, reflection, temperature dependence of sound.
Mathematics	Polar coordinates: x = r cos θ, y = r sin θ.
CS	Non-blocking state machines vs blocking loops.

13Think about it

Reflection & conclusion

Where did the radar “miss” objects, and what were those objects made of?

Why is a timeout kinder to a classroom demo than a 1 s freeze?

A car parking sensor beeps faster as you approach. Which Processing function would you use to map distance to beep interval?

Conclusion

You timed a sound, aimed a motor, and drew space. That is physics, control and graphics on one USB cable.