Pro Mini at 3.3 V

// Dial: a potentiometer on A7 fades an LED on pin 9, and reports volts.
//
// The potentiometer's ends go to VCC and GND, and on this board VCC is 3.3 V.
// The ADC measures against AVCC, which is the same rail, so a full turn still
// reads 1023: the count is a fraction of the supply, not a number of volts.
// To turn it back into volts the sketch has to know the supply, and that is
// the line a sketch copied from a 5 V board gets wrong.
//
// A7 is an analogue-only input, as on the Nano: analogRead reaches it and
// nothing else does. Pin 9 is OC1A, so analogWrite is hardware PWM, at
// 245 Hz here rather than 490 Hz, because the timer runs from an 8 MHz clock.

const int DIAL_PIN = A7;
const int LAMP_PIN = 9;
const long SUPPLY_MV = 3300;

unsigned long lastReport = 0;

void setup() {
  Serial.begin(57600);
  pinMode(LAMP_PIN, OUTPUT);
  Serial.println("turn the dial on A7");
}

void loop() {
  int raw = analogRead(DIAL_PIN);
  analogWrite(LAMP_PIN, raw >> 2);

  unsigned long now = millis();
  if (now - lastReport >= 250) {
    lastReport = now;
    long mv = (long)raw * SUPPLY_MV / 1023;
    Serial.print("A7 ");
    Serial.print(raw);
    Serial.print("  ");
    Serial.print(mv);
    Serial.println(" mV");
  }
}