Mains voltage monitor with low/high alerts (ESP32 + ZMPT101B)

// Mains voltage monitor — ESP32 DevKit V1 + ZMPT101B + SSD1306 OLED
// ZMPT101B OUT on D34 (ADC1, input-only). Module powered from 3V3 so its output can never
// exceed the ESP32's 3.3 V ADC limit. The 230 V side is wired by an electrician only.
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

constexpr uint8_t SENSE_PIN = 34;
// Calibrate: compare with a true-RMS multimeter and adjust until the readings match.
float voltsPerCount = 0.33f;
constexpr float LOW_V = 190.0f;   // below this, many appliances misbehave
constexpr float HIGH_V = 250.0f;  // above this, protect your appliances

Adafruit_SSD1306 oled(128, 64, &Wire, -1);

// RMS over 10 mains cycles (200 ms at 50 Hz).
float readVrms() {
  const unsigned long window = 200;
  unsigned long t0 = millis();
  double sum = 0, sumSq = 0;
  unsigned long n = 0;
  while (millis() - t0 < window) {
    int v = analogRead(SENSE_PIN);
    sum += v;
    sumSq += (double)v * v;
    n++;
  }
  double mean = sum / n;
  double variance = sumSq / n - mean * mean;  // removes the module's DC offset
  return variance > 0 ? sqrt(variance) * voltsPerCount : 0;
}

void setup() {
  Serial.begin(115200);
  analogSetPinAttenuation(SENSE_PIN, ADC_11db);  // full 0–3.3 V range
  Wire.begin(21, 22);
  if (!oled.begin(SSD1306_SWITCHCAPVCC, 0x3C)) Serial.println("OLED not found at 0x3C");
}

void loop() {
  float v = readVrms();
  const char* state = v < 20 ? "NO SUPPLY" : v < LOW_V ? "LOW" : v > HIGH_V ? "HIGH" : "NORMAL";
  Serial.printf("V=%.1f %s\n", v, state);

  oled.clearDisplay();
  oled.setTextColor(SSD1306_WHITE);
  oled.setTextSize(3);
  oled.setCursor(0, 8);
  oled.printf("%3.0fV", v);
  oled.setTextSize(2);
  oled.setCursor(0, 44);
  oled.print(state);
  oled.display();
  delay(800);
}

// Mains voltage monitor — ESP32 DevKit V1 + ZMPT101B + SSD1306 OLED
// ZMPT101B OUT on D34 (ADC1, input-only). Module powered from 3V3 so its output can never
// exceed the ESP32's 3.3 V ADC limit. The 230 V side is wired by an electrician only.
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

constexpr uint8_t SENSE_PIN = 34;
// Calibrate: compare with a true-RMS multimeter and adjust until the readings match.
float voltsPerCount = 0.33f;
constexpr float LOW_V = 190.0f;   // below this, many appliances misbehave
constexpr float HIGH_V = 250.0f;  // above this, protect your appliances

Adafruit_SSD1306 oled(128, 64, &Wire, -1);

// RMS over 10 mains cycles (200 ms at 50 Hz).
float readVrms() {
  const unsigned long window = 200;
  unsigned long t0 = millis();
  double sum = 0, sumSq = 0;
  unsigned long n = 0;
  while (millis() - t0 < window) {
    int v = analogRead(SENSE_PIN);
    sum += v;
    sumSq += (double)v * v;
    n++;
  }
  double mean = sum / n;
  double variance = sumSq / n - mean * mean;  // removes the module's DC offset
  return variance > 0 ? sqrt(variance) * voltsPerCount : 0;
}

void setup() {
  Serial.begin(115200);
  analogSetPinAttenuation(SENSE_PIN, ADC_11db);  // full 0–3.3 V range
  Wire.begin(21, 22);
  if (!oled.begin(SSD1306_SWITCHCAPVCC, 0x3C)) Serial.println("OLED not found at 0x3C");
}

void loop() {
  float v = readVrms();
  const char* state = v < 20 ? "NO SUPPLY" : v < LOW_V ? "LOW" : v > HIGH_V ? "HIGH" : "NORMAL";
  Serial.printf("V=%.1f %s\n", v, state);

  oled.clearDisplay();
  oled.setTextColor(SSD1306_WHITE);
  oled.setTextSize(3);
  oled.setCursor(0, 8);
  oled.printf("%3.0fV", v);
  oled.setTextSize(2);
  oled.setCursor(0, 44);
  oled.print(state);
  oled.display();
  delay(800);
}