====== Analog Input ======

As we have seen, an Arduino digital input is one that have one of two states: HIGH or LOW. Most Arduino implementations also permit you to connect an input whose value can be any value between 0 volts and (by default) the power-supply voltage, and then convert the input level to an integer value between 0 an 1023. The Arduino literature refers to these as "analog" inputs. The process of converting a physical analog signal to a digital representation is called **analog to digital conversion** (**ADC**).

Consult the documentation for your specific Arduino board to determine which pins support analog input. On the Arduino UNO, inputs A0 through A5 can be used as analog inputs.

===== Night light =====

The following example uses an analog input and a digital output to build a simple night-light. A cadmium-sulfide photo-sensitive resistor (commonly called CdS cells) is used in a voltage divider setup to produce a voltage that changes with the amount of light incident on the sensor. If the voltage is above a certain threshold, the Arduino will turn on the LED.

<WRAP center round tip 60%>
CdS cells are notoriously tweaky devices. A lot of experimentation with your particular cell will probably be required to see what your circuit is actually doing---quite likely requiring the use of the serial communication to report the ADC values (see [[light_meter|this]] for an example).

There are more reliable and calibrated light-level sensors available that may be a better (but significantly more expensive!) choice if you wish not to deal with the tweakiness of Cds cells.
</WRAP>

FIXME We need a circuit diagram

In the following example, we output the light-level reading to serial communication. This should help you get a feel for the range of numbers that result from for your particular unit.

<file c cds2serial.ino>
/*
 CdS to Serial
 Ouptut the reading on the CdS pin to Serial.
 */

const int inputPin = A0;         // use analog 0 (A0) as input pin
const int ledPin = 13;           // connect the LED to pin 13

int lightLevel = 0;              // used to store the input value

void setup() {
  Serial.begin(9600);            // initialize serial communication
  pinMode(ledPin, OUTPUT);       // make LED's pin an output
}

void loop() {
  lightLevel = analogRead(inputPin);  // get an analog reading      
  Serial.println(lightLevel);         // print out the reading
  delay(500);                         // delay between reads
}
</file>

And here is the working nightlight. You may have to adjust ''lightThreshold'' so it works well with your particular CdS cell.

<file c NightLight.ino>
/*
 NightLight
 Turn an LED on when the ambient light is below a certain value.
 */

const int lightThreshold = 800;  // value below which LED turns on
const int inputPin = A0;         // use analog 0 (A0) as input pin
const int ledPin = 13;           // connect the LED to pin 13

int lightLevel = 0;              // used to store the input value

void setup() {
  pinMode(ledPin, OUTPUT);       // make LED's pin an output
}

void loop() {
  lightLevel = analogRead(inputPin);  // get an analog reading      

  if (lightLevel <= lightThreshold)
    digitalWrite(ledPin, HIGH);       // turn the LED on
  else
    digitalWrite(ledPin, LOW);        // turn the LED off
}
</file>