Voltage division

Most sensors are detected via a changing voltage that's fed into the analog inputs using voltage division.

 There are some good introductions to analog voltage on the Arduino including this one.

And here's the Arduino tutorial about the analog pins.

LDR, our first sensor

Some good code and information here.
Light dependent resistor circuit. See http://www.ladyada.net/learn/sensors/cds.html

Two LED program

/*

Blink2

Turns on a LED on for half a second, then off for half a second, repeatedly. It also does the same to an off-board LED connected to pin 12 so that when one LED is on the other is off.

The circuit:

* LED connected from digital pin 13 to ground via resistor.

* second LED connected from digital pin 12 to ground via resistor. I used 330 ohms.

* Note: On most Arduino boards, there is already an LED on the board

connected to pin 13.

Created 1 June 2005

By David Cuartielles. Adapted by Peter Brook

based on an orginal by H. Barragan for the Wiring i/o board

*/

int ledPin = 13; // LED connected to digital pin 13

int redLedPin = 12; // LED connected to digital pin 13

int del =500;

// The setup() method runs once, when the sketch starts

void setup() {

// initialize the digital pin as an output:

pinMode(ledPin, OUTPUT);

pinMode(redLedPin, OUTPUT);

}

// the loop() method runs over and over again,

// as long as the Arduino has power

void loop()

{

digitalWrite(ledPin, HIGH); // set the LED on

digitalWrite(redLedPin, LOW); // set the LED on

delay(del); // wait

digitalWrite(ledPin, LOW); // set the LED off

digitalWrite(redLedPin, HIGH); // set the LED on

delay(del); // wait

}