Measuring the ambient light intensity using a photocell

    /* Photocell simple testing sketch. 
     
    Connect one end of the photocell to 5V, the other end to Analog 0.
    Then connect one end of a 10K resistor from Analog 0 to ground
     
    For more information see http://learn.adafruit.com/photocells */
     
    int photocellPin = 0;     // the cell and 10K pulldown are connected to a0
    int photocellReading;     // the analog reading from the analog resistor divider
     
    void setup(void) {
      // We'll send debugging information via the Serial monitor
      Serial.begin(9600);   
    }
     
    void loop(void) {
      photocellReading = analogRead(photocellPin);  
     
      Serial.print("Analog reading = ");
      Serial.print(photocellReading);     // the raw analog reading
     
      // We'll have a few threshholds, qualitatively determined
      if (photocellReading < 10) {
        Serial.println(" - Dark");
      } else if (photocellReading < 200) {
        Serial.println(" - Dim");
      } else if (photocellReading < 500) {
        Serial.println(" - Light");
      } else if (photocellReading < 800) {
        Serial.println(" - Bright");
      } else {
        Serial.println(" - Very bright");
      }
      delay(1000);
    }

Using the above code, I placed a photocell on a breadboard with 1 leg connected to 5v and the other leg connected to analog pin 0 as well as connecting the same leg to ground using a 10k pulldown resistor.

## Output

<pre>
Analog reading = 774 - Bright
Analog reading = 754 - Bright
Analog reading = 768 - Bright
Analog reading = 876 - Very bright
Analog reading = 960 - Very bright
Analog reading = 943 - Very bright
Analog reading = 906 - Very bright
Analog reading = 941 - Very bright
Analog reading = 694 - Bright
Analog reading = 677 - Bright
Analog reading = 673 - Bright
Analog reading = 677 - Bright
Analog reading = 669 

...

Analog reading = 62 - Dim
Analog reading = 63 - Dim
Analog reading = 62 - Dim
Analog reading = 62 - Dim
Analog reading = 63 - Dim
Analog reading = 63 - Dim
Analog reading = 62 - Dim

</pre>

With the room light on, the reading was *Bright*, and when I shone a torch light into the photocell it registered as *Very bright*. When I switched the room light off it registered as *Dim*, and when I cupped both hands over the photocell it registered as *Dark*.

## Resources

* Using a Photocell https://learn.adafruit.com/photocells/using-a-photocell

photocell light intensity sensor brightness