Documentation

const int SENSOR_PIN = A0;
float CAL = 250.0;   // scale factor - tune against a multimeter

void setup() {
  Serial.begin(9600);
}

void loop() {
  const unsigned long windowMs = 200;  // 10 cycles @ 50 Hz, 12 @ 60 Hz
  unsigned long start = millis();
  unsigned long n = 0;
  double sum = 0, sumSq = 0;

  while (millis() - start < windowMs) {
    int raw = analogRead(SENSOR_PIN);
    sum += raw;
    sumSq += (double)raw * raw;
    n++;
  }

  double mean = sum / n;                       // DC bias (~512)
  double variance = sumSq / n - mean * mean;
  double rmsCounts = sqrt(variance > 0 ? variance : 0);
  double volts = rmsCounts * (5.0 / 1023.0) * CAL;

  Serial.print("AC voltage: ");
  Serial.print(volts, 1);
  Serial.println(" V");
  delay(500);
}

const int SENSOR_PIN = 34;   // ADC1 input-only pin
float CAL = 250.0;           // tune against a multimeter

void setup() {
  Serial.begin(115200);
  analogSetPinAttenuation(SENSOR_PIN, ADC_11db);  // full 0-3.3 V range
}

void loop() {
  const unsigned long windowMs = 200;
  unsigned long start = millis();
  unsigned long n = 0;
  double sum = 0, sumSq = 0;

  while (millis() - start < windowMs) {
    int raw = analogRead(SENSOR_PIN);   // 0-4095
    sum += raw;
    sumSq += (double)raw * raw;
    n++;
  }

  double mean = sum / n;
  double variance = sumSq / n - mean * mean;
  double rmsCounts = sqrt(variance > 0 ? variance : 0);
  double volts = rmsCounts * (3.3 / 4095.0) * CAL;

  Serial.printf("AC voltage: %.1f V\n", volts);
  delay(500);
}

import time
import math
import board
import busio
import adafruit_ads1x15.ads1115 as ADS
from adafruit_ads1x15.analog_in import AnalogIn

# pip3 install adafruit-circuitpython-ads1x15

i2c = busio.I2C(board.SCL, board.SDA)
ads = ADS.ADS1115(i2c)
ads.data_rate = 860          # fastest rate - needed for 50/60 Hz
chan = AnalogIn(ads, ADS.P0)

CAL = 250.0                  # tune against a multimeter

def read_rms(window_s=0.2):
    samples = []
    end = time.monotonic() + window_s
    while time.monotonic() < end:
        samples.append(chan.voltage)
    mean = sum(samples) / len(samples)
    var = sum((v - mean) ** 2 for v in samples) / len(samples)
    return math.sqrt(var) * CAL

while True:
    print("AC voltage: {:.1f} V".format(read_rms()))
    time.sleep(1)

from machine import ADC
import math
import time

adc = ADC(26)                # GP26 / ADC0
CAL = 250.0                  # tune against a multimeter

def read_rms(window_ms=200):
    n = 0
    total = 0
    total_sq = 0
    t_end = time.ticks_add(time.ticks_ms(), window_ms)
    while time.ticks_diff(t_end, time.ticks_ms()) > 0:
        raw = adc.read_u16()
        total += raw
        total_sq += raw * raw
        n += 1
    mean = total / n
    var = total_sq / n - mean * mean
    rms = math.sqrt(var if var > 0 else 0)
    return rms * (3.3 / 65535) * CAL

while True:
    print("AC voltage: {:.1f} V".format(read_rms()))
    time.sleep(1)