Reference: DIY USB Examples

🧰 What you need:

• ESP32 board (like a DevKitC or NodeMCU-ESP32)
• USB-A cable with exposed 5V (red) and GND (black) wires
• Two resistors for voltage divider (e.g., 10kΩ + 20kΩ → drops 5V to ~3.3V)
• Breadboard and jumper wires

🔌 Wiring

USB Wire | Connect to

Red (5V) | Voltage divider → ESP32 GPIO (e.g., GPIO 18)

Black (GND) | ESP32 GND

Voltage divider setup:

USB 5V → [20kΩ] → GPIO 18
           ↓
        [10kΩ] → GND

This drops 5V down to ~3.3V safely.

✅ Example Code (ESP32, digital read)

const int usbPowerPin = 18;  // GPIO18 gets the divided signal
bool lastState = LOW;

void setup() {
  pinMode(usbPowerPin, INPUT);
  Serial.begin(115200);
}

void loop() {
  bool currentState = digitalRead(usbPowerPin);
  if (currentState != lastState) {
    if (currentState == HIGH) {
      Serial.println("USB Power ON");
      // YOUR ALERT START CODE GOES HERE
    } else {
      Serial.println("USB Power OFF");
      // YOUR ALERT STOP CODE GOES HERE
    }
    lastState = currentState;
  }
  delay(100); // debounce / polling delay
}

⚠️ Important:

• Do NOT connect 5V directly to ESP32 pins — always drop it to ≤3.3V.
• ESP32 pins are usually labeled GPIO on your board — you can pick most, but avoid strapping pins (like GPIO0, GPIO2, GPIO15).
• Use Serial.begin(115200) — ESP32 typically uses higher serial baud rates than Arduino Uno.