Arduino parking sensor

/*
  Parking sensor — Robot Maker Toolkit
  Arduino UNO + HC-SR04 + 3 LEDs + buzzer

  Distance drives a traffic light and a beep that speeds up as you close in,
  exactly like a car's reversing sensor.

  Pins follow the HC-SR04 page in the app (TRIG D9, ECHO D10).

  Zones:
    > 100 cm      green,  silent
    40 - 100 cm   yellow, slow beep
    20 - 40 cm    red,    fast beep
    < 20 cm       red,    solid tone  (STOP)
*/

const int TRIG_PIN   = 9;
const int ECHO_PIN   = 10;
const int LED_GREEN  = 2;
const int LED_YELLOW = 3;
const int LED_RED    = 4;
const int BUZZER_PIN = 5;

// 25 ms ~= 4.3 m. Caps how long pulseIn() can stall the loop when nothing
// is in front of the sensor; without a timeout it blocks for a full second.
const unsigned long ECHO_TIMEOUT_US = 25000UL;

const float ZONE_STOP_CM = 20.0;
const float ZONE_CLOSE_CM = 40.0;
const float ZONE_NEAR_CM = 100.0;

unsigned long lastBeepToggle = 0;
bool beepOn = false;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_GREEN, OUTPUT);
  pinMode(LED_YELLOW, OUTPUT);
  pinMode(LED_RED, OUTPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(9600);
}

// One reading, in cm. Returns -1 when nothing echoes back in time.
float readDistanceCm() {
  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 -1.0;

  // 343 m/s = 0.0343 cm/us, halved because the pulse travels there and back.
  return (us * 0.0343) / 2.0;
}

// Median of three. A single HC-SR04 reading occasionally returns a wild
// value off a soft or angled surface; the median throws those away without
// the lag that averaging adds.
float readDistanceStable() {
  float a = readDistanceCm();
  delay(15);
  float b = readDistanceCm();
  delay(15);
  float c = readDistanceCm();

  if (a < 0 || b < 0 || c < 0) return -1.0;
  float hi = max(a, max(b, c));
  float lo = min(a, min(b, c));
  return a + b + c - hi - lo;
}

void showZone(int green, int yellow, int red) {
  digitalWrite(LED_GREEN, green);
  digitalWrite(LED_YELLOW, yellow);
  digitalWrite(LED_RED, red);
}

// Non-blocking beep: toggles on its own schedule so distance keeps updating.
// interval 0 = silent, -1 = solid tone.
void updateBuzzer(long interval) {
  if (interval == 0) {
    noTone(BUZZER_PIN);
    beepOn = false;
    return;
  }
  if (interval < 0) {
    tone(BUZZER_PIN, 2000);
    beepOn = true;
    return;
  }
  if (millis() - lastBeepToggle >= (unsigned long)interval) {
    lastBeepToggle = millis();
    beepOn = !beepOn;
    if (beepOn) tone(BUZZER_PIN, 2000); else noTone(BUZZER_PIN);
  }
}

void loop() {
  float cm = readDistanceStable();

  if (cm < 0) {
    // Out of range or no echo — treat as clear.
    showZone(HIGH, LOW, LOW);
    updateBuzzer(0);
    Serial.println("no echo");
  } else if (cm > ZONE_NEAR_CM) {
    showZone(HIGH, LOW, LOW);
    updateBuzzer(0);
  } else if (cm > ZONE_CLOSE_CM) {
    showZone(LOW, HIGH, LOW);
    updateBuzzer(400);
  } else if (cm > ZONE_STOP_CM) {
    showZone(LOW, LOW, HIGH);
    updateBuzzer(120);
  } else {
    showZone(LOW, LOW, HIGH);
    updateBuzzer(-1);
  }

  if (cm >= 0) {
    Serial.print(cm, 1);
    Serial.println(" cm");
  }
  delay(50);
}

{
  "version": 1,
  "author": "Infinitx Labs",
  "editor": "wokwi",
  "parts": [
    { "type": "wokwi-arduino-uno", "id": "uno", "top": 130, "left": 0, "attrs": {} },
    { "type": "wokwi-hc-sr04", "id": "dist", "top": -60, "left": 100, "attrs": { "distance": "80" } },
    { "type": "wokwi-led", "id": "ledGreen", "top": 20, "left": 320, "attrs": { "color": "green" } },
    { "type": "wokwi-led", "id": "ledYellow", "top": 20, "left": 370, "attrs": { "color": "yellow" } },
    { "type": "wokwi-led", "id": "ledRed", "top": 20, "left": 420, "attrs": { "color": "red" } },
    { "type": "wokwi-resistor", "id": "rGreen", "top": 120, "left": 320, "attrs": { "value": "220" } },
    { "type": "wokwi-resistor", "id": "rYellow", "top": 160, "left": 370, "attrs": { "value": "220" } },
    { "type": "wokwi-resistor", "id": "rRed", "top": 200, "left": 420, "attrs": { "value": "220" } },
    { "type": "wokwi-buzzer", "id": "buzzer", "top": 20, "left": 200, "attrs": { "volume": "0.1" } }
  ],
  "connections": [
    [ "dist:VCC", "uno:5V", "red", [] ],
    [ "dist:GND", "uno:GND.1", "black", [] ],
    [ "dist:TRIG", "uno:9", "blue", [] ],
    [ "dist:ECHO", "uno:10", "purple", [] ],

    [ "uno:2", "rGreen:1", "green", [] ],
    [ "rGreen:2", "ledGreen:A", "green", [] ],
    [ "ledGreen:C", "uno:GND.2", "black", [] ],

    [ "uno:3", "rYellow:1", "gold", [] ],
    [ "rYellow:2", "ledYellow:A", "gold", [] ],
    [ "ledYellow:C", "uno:GND.2", "black", [] ],

    [ "uno:4", "rRed:1", "red", [] ],
    [ "rRed:2", "ledRed:A", "red", [] ],
    [ "ledRed:C", "uno:GND.2", "black", [] ],

    [ "uno:5", "buzzer:1", "orange", [] ],
    [ "buzzer:2", "uno:GND.3", "black", [] ]
  ]
}