Lesson 2: Traffic-Light Distance Indicator

Add three LEDs that act like a traffic light for distance: green when it is far, yellow when it gets close and red when it is near.

One of 9 projects in the Arduino Ultrasonic Distance & Parking Learning Kit — the parts below are from that kit.

Before you start

What you build and learn

• Which LED leg is + and why every LED needs its own resistor
• Turning a measurement into a decision with if / else if / else
• Choosing zones with named numbers (CLEAR_CM, NEAR_CM)
• Why "no echo" must never look like "all clear"

New words

LED

Light-emitting diode. It only lights one way round: long leg +.

Resistor

A part that limits current. The lessons use 220 Ω: red-red-brown bands.

Zone

A range of distances that gets the same light, such as 20-49 cm = yellow.

if / else

Code that does one thing if something is true, and something else if not.

What this project uses

Every item is in the kit — nothing extra to buy.

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3

In the kit

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043

In the kit

◇

red/yellow/green LEDs

In the kit

resistor assortment RESISTOR ASSORTED 220-1K-10K DIY-213

In the kit

400 tie-point breadboard 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238

In the kit

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212

In the kit

Build this with

Arduino Ultrasonic Distance & Parking Learning Kit

All 9 parts in one box, with this project and 8 more to build from them.

AED 85.00 See the kit Add kit to cart

Need only these parts? Order them loose · from AED 87.65 ▾

Used in this project tick what you still need

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3 · ×1

AED 26.39

In stock

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043 · ×1

AED 15.31

In stock

resistor assortment RESISTOR ASSORTED 220-1K-10K DIY-213 · ×1

AED 9.35

In stock

400 tie-point breadboard 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238 · ×1

AED 11.58

In stock

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212 · ×1

AED 25.02

In stock

Selected parts AED 87.65

Add 5 parts to cart

One of each part. Packs (resistor assortment, jumper wires) already hold more than a single build needs. red/yellow/green LEDs: included in the kit, ask us for it loose. Delivery is AED 20, free on orders AED 100+.

Connections

Unplug USB before you change any wire. Pin names are the labels printed on the board.

Step by step

One thing at a time. Tick nothing off until it matches the diagram.

1. Unplug the USB cable.Watch out: Always unplug the USB cable before you add, move or remove a wire.
2. Leave the sensor and the rail wires from Lesson 1 where they are.
3. Push the red LED into the breadboard with its two legs in two different rows. Note which row has the long leg.Watch out: The long leg is +. If the legs are cut, the flat edge on the LED's rim marks the − side.
4. Put a 220 Ω resistor (red-red-brown) from the long-leg row to an empty row, and wire that row to UNO pin 5.Watch out: Every LED needs its own resistor.
5. Do the same for the yellow LED (pin 6) and the green LED (pin 7).
6. Wire each LED's short-leg row to the − rail.
7. Check the three LEDs against the diagram, plug in USB, then copy the Lesson 2 code into the IDE and upload it.Watch out: If anything gets hot or smells, unplug straight away.
8. Watch the light test (red, yellow, green), then move a book slowly towards the sensor from about 70 cm away.

Sketch for this project

Written for the board in the kit. Copy it into the Arduino IDE; change the pin numbers if you wire it differently.

/*
  DIY KIT - Arduino Ultrasonic Distance & Parking Learning Kit
  Lesson 2 of 9: Traffic-Light Distance Indicator

  WIRING (unplug the USB cable before you wire)
    HC-SR04 as in Lesson 1: VCC 5V, TRIG pin 9, ECHO pin 10, GND.
    Red LED    long leg -> 220 ohm resistor -> UNO pin 5
    Yellow LED long leg -> 220 ohm resistor -> UNO pin 6
    Green LED  long leg -> 220 ohm resistor -> UNO pin 7
    All three short legs -> GND (- rail)

  WHAT SHOULD HAPPEN
    Green  = 50 cm or more   (CLEAR)
    Yellow = 20 to 49 cm     (CAUTION)
    Red    = under 20 cm     (NEAR)
    Yellow blinking = no echo: the sensor does not know. Unknown is never shown as green.
    The Serial Monitor (9600 baud) prints the zone and the distance.
*/

const int RED_LED = 5;
const int YELLOW_LED = 6;
const int GREEN_LED = 7;
const int TRIG_PIN = 9;
const int ECHO_PIN = 10;

const int CLEAR_CM = 50;   // this far or more: green
const int NEAR_CM = 20;    // closer than this: red

const unsigned long ECHO_TIMEOUT_US = 30000;
const int MIN_CM = 2;
const int MAX_CM = 400;

long readOnceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long us = pulseIn(ECHO_PIN, HIGH, ECHO_TIMEOUT_US);
  if (us == 0) return 0;          // no echo
  long cm = us / 58;
  if (cm < MIN_CM || cm > MAX_CM) return 0;
  return cm;
}

// Turn each LED on (true) or off (false).
void showLights(bool red, bool yellow, bool green) {
  digitalWrite(RED_LED, red ? HIGH : LOW);
  digitalWrite(YELLOW_LED, yellow ? HIGH : LOW);
  digitalWrite(GREEN_LED, green ? HIGH : LOW);
}

void setup() {
  pinMode(RED_LED, OUTPUT);
  pinMode(YELLOW_LED, OUTPUT);
  pinMode(GREEN_LED, OUTPUT);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  Serial.begin(9600);
  Serial.println("Lesson 2: Traffic-Light Distance Indicator");

  // Light test: red, yellow, green, one after another.
  showLights(true, false, false);  delay(300);
  showLights(false, true, false);  delay(300);
  showLights(false, false, true);  delay(300);
  showLights(false, false, false);
}

void loop() {
  long cm = readOnceCm();

  if (cm == 0) {
    // No echo: blink yellow - on for half a second, off for half a second.
    unsigned long halfSeconds = millis() / 500;   // half-seconds since the UNO started
    bool blinkOn = (halfSeconds % 2 == 0);        // even: on, odd: off
    showLights(false, blinkOn, false);
    Serial.println("NO_ECHO");
  } else if (cm < NEAR_CM) {
    showLights(true, false, false);
    Serial.print("NEAR ");
    Serial.println(cm);
  } else if (cm < CLEAR_CM) {
    showLights(false, true, false);
    Serial.print("CAUTION ");
    Serial.println(cm);
  } else {
    showLights(false, false, true);
    Serial.print("CLEAR ");
    Serial.println(cm);
  }

  delay(100);
}

Upload it

1. Click Copy (or Download .ino and open the file).
2. In the Arduino IDE choose File → New Sketch, select everything in it and paste.
3. Check the bottom of the window says Arduino Uno and your port (Start here, step 5).
4. Click Upload (the → arrow) and wait for Done uploading.

How the code works

Change it and see

What you should see or hear

• At start: red, yellow, green each light for a moment.
• Green at 50 cm and more, yellow from 20 to 49 cm, red under 20 cm.
• Point the sensor at an empty space: yellow blinks and the Serial Monitor shows NO_ECHO.

If it doesn't work

Small experiment

Find the exact distance where yellow changes to red. Move the book 1 cm at a time. Is it always at the same place, or does it flicker at the edge?

Check what you learned

Next

• Lesson 3 adds the buzzer, so it beeps when something gets close. Keep the LEDs where they are.

Hi DIY KIT, I'm stuck on the Arduino Ultrasonic Distance & Parking Learning Kit.
Lesson: __
Step number: __
What should happen: __
What happens instead: __
Error message (copied exactly): __
I'll attach a photo of my wiring.

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