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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 internal digital value between 0 an 1204. The Arduino literature refers to these as “analog” inputs. The process of converting a real analog signal to a digital number is called analog to digital conversion (ADC).

Consult the documentation for your specific Arduino board to detmine which pins support analog input.

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.

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 this for an example).

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

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