Sketch

/*
Conditionals - If statement

This example demonstrates the use of if() statements.
It reads the state of a potentiometer (an analog input) and turns on an LED
only if the potentiometer goes above a certain threshold level. It prints the
analog value regardless of the level.

The circuit:
- potentiometer
Center pin of the potentiometer goes to analog pin 0.
Side pins of the potentiometer go to +5V and ground.
- LED connected from digital pin 13 to ground

- Note: On most Arduino boards, there is already an LED on the board connected
to pin 13, so you don't need any extra components for this example.

created 17 Jan 2009
modified 9 Apr 2012
by Tom Igoe

This example code is in the public domain.

http://www.arduino.cc/en/Tutorial/IfStatement
*/

// These constants won't change:
const int analogPin = A0;    // pin that the sensor is attached to
const int ledPin = 13;       // pin that the LED is attached to
const int threshold = 400;   // an arbitrary threshold level that's in the range of the analog input

void setup() {
// initialize the LED pin as an output:
pinMode(ledPin, OUTPUT);
// initialize serial communications:
Serial.begin(9600);
}

void loop() {
// read the value of the potentiometer:
int analogValue = analogRead(analogPin);

// if the analog value is high enough, turn on the LED:
if (analogValue > threshold) {
digitalWrite(ledPin, HIGH);
} else {
digitalWrite(ledPin, LOW);
}

// print the analog value:
Serial.println(analogValue);
delay(1);        // delay in between reads for stability
}