An analog input on the shield

The blue potentiometer is on analog pin A0. Turning it changes the resistance, and analogRead(A0) returns a value from 0 to 1023.

The first sketch just prints the reading to the Serial Monitor at 9600 baud so you can watch it change as you turn the knob with a screwdriver. The second adds a threshold: below 400 all four LEDs come on, above it they go off (remember the LEDs are active-low).

A threshold like this is how you turn one knob into a mode selector: one range for a clock, another for a stopwatch, another for a timer.

1. Print the potentiometer value

#define POT_PIN A0

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

void loop() {
  int reading = analogRead(POT_PIN);
  Serial.print("Potentiometer reading: ");
  Serial.println(reading);
  delay(500);
}

2. Threshold switches the LEDs

#define POT_PIN A0

const byte LEDS[] = {13, 12, 11, 10};

void setup() {
  Serial.begin(9600);
  for (byte i = 0; i < 4; i++) {
    pinMode(LEDS[i], OUTPUT);
    digitalWrite(LEDS[i], HIGH);   // active-low: HIGH = off
  }
}

void loop() {
  int reading = analogRead(POT_PIN);
  Serial.println(reading);

  for (byte i = 0; i < 4; i++) {
    if (reading < 400) {
      digitalWrite(LEDS[i], LOW);  // on
    } else {
      digitalWrite(LEDS[i], HIGH); // off
    }
  }
  delay(100);
}

Multi-Function Shield: Potentiometer

An analog input on the shield