Lamp power meter on an Arduino Uno

// Lamp power meter: a MAX471 current sensor board in a lamp's supply lead,
// and the voltage, current and power on a 16x2 I2C LCD.
//
//   A0      the MAX471 board's AT (1 V an amp)
//   A1      the board's VT (a fifth of the supply voltage)
//   GND     the board's GND, which is also the supply's minus
//   VIN     the bench supply's plus;  VOUT  to the lamp
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The MAX471 puts 500 uA out for every amp through its 35 milliohm resistor,
// and the board turns that into a voltage across 2 kohm: 1 V an amp, so on a
// 5 V Uno one converter step is 4.9 mA. VT is the supply through a 30 k and
// 7.5 k divider, a fifth of it, so a step is 24 mV.
//
// Press the button to light the lamp. A cold tungsten filament has about a
// tenth of its hot resistance, so the current jumps and then settles as it
// warms up. The peak line keeps the highest reading, which with a reading
// every tenth of a second or so is the tail of that surge, not its top.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int AT_PIN = A0;
const int VT_PIN = A1;
LiquidCrystal_I2C lcd(0x27, 16, 2);
long peakMa = 0;

// The average of 16 readings, in millivolts.
long readMv(int pin) {
  long sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(pin);
  return sum * 5000L / 1023L / 16;
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: lamp power meter");
  lcd.init();
  lcd.backlight();
}

void loop() {
  long mA = readMv(AT_PIN);        // 1 mV on AT is 1 mA
  long mV = readMv(VT_PIN) * 5;    // VT is a fifth of the supply
  long mW = mV * mA / 1000;
  if (mA > peakMa) peakMa = mA;

  lcd.setCursor(0, 0);
  lcd.print(mV / 1000);
  lcd.print('.');
  lcd.print((mV % 1000) / 100);
  lcd.print(" V  ");
  lcd.print(mA);
  lcd.print(" mA     ");
  lcd.setCursor(0, 1);
  lcd.print(mW);
  lcd.print(" mW pk ");
  lcd.print(peakMa);
  lcd.print("   ");

  Serial.print("V: ");
  Serial.print(mV);
  Serial.print(" mV  I: ");
  Serial.print(mA);
  Serial.print(" mA  P: ");
  Serial.print(mW);
  Serial.print(" mW  peak: ");
  Serial.println(peakMa);
  delay(100);
}