3. Cloud-Connected Parking Sensor

Measure an object’s distance, classify it as safe, slow, or stop, and activate a buzzer in the nearest zone.

Level: Intermediate · Time: 40 minutes

Electrical warning and wiring

HC-SR04 ECHO may output 5 V. Do not connect it directly to ESP32. A 1 kΩ resistor from ECHO to GPIO and 2 kΩ from GPIO to GND forms an approximate 3.3 V divider.

Part	ESP32
HC-SR04 VCC / GND	5V / GND
TRIG	GPIO 5
ECHO	GPIO 18 through divider
Active buzzer SIG	GPIO 23

Complete sketch


#include <PxServ.h>
 
const char* WIFI_SSID = "your_wifi_ssid";
const char* WIFI_PASSWORD = "your_wifi_password";
const char* PXSERV_API_KEY = "your_pxserv_api_key";
const int TRIG_PIN = 5;
const int ECHO_PIN = 18;
const int BUZZER_PIN = 23;
 
PxServ client(PXSERV_API_KEY);
 
float readDistanceCm() {
  digitalWrite(TRIG_PIN, LOW); delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long duration = pulseIn(ECHO_PIN, HIGH, 30000);
  return duration == 0 ? -1 : duration * 0.0343f / 2.0f;
}
 
void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  digitalWrite(BUZZER_PIN, LOW);
  PxServ::connectWifi(WIFI_SSID, WIFI_PASSWORD);
}
 
void loop() {
  float distance = readDistanceCm();
  String zone;
  if (distance < 0) zone = "no-reading";
  else if (distance < 20) zone = "stop";
  else if (distance < 50) zone = "slow";
  else zone = "safe";
 
  digitalWrite(BUZZER_PIN, zone == "stop" ? HIGH : LOW);
  client.setData("parking/zone", zone);
  if (distance >= 0) client.setData("parking/distance-cm", String(distance, 1));
  delay(1000);
}

Test

Move a flat board through the 50 cm and 20 cm boundaries and compare with a ruler. Soft, angled, or small surfaces may reflect ultrasound poorly.

Reference: Arduino HC-SR04 library entry