# Arduino Tutorial Bundle .:Arduino Experimentation Kit:. (ARDX)

## Step 11: .:Light:. (Photo Resistors) - CIRC09

What We're Doing:
Whilst getting input from a potentiometer can be useful for human controlled experiments, what do we use when we want an environmentally controlled experiment? We use exactly the same principles but instead of a potentiometer (twist based resistance) we use a photo resistor (light based resistance). The Arduino cannot directly sense resistance (it senses voltage) so we set up a voltage divider ( http://tinyurl.com/2sunta ). The exact voltage at the sensing pin is calculable, but for our purposes (just sensing relative light) we can experiment with the values and see what works for us. A low value will occur when the sensor is well lit while a high value will occur when it is in darkness.

The Parts:
• Photo-Resistor (x1)
• 5mm Green LED (x1)
• 10k ohm Resistor (brown-black-orange) (x1)
• 560 ohm Resistor (green-blue-brown) (x1)
• Wire

The Circuit and Plugging Everything In:
A Small Video of Everything Being Plugged in

The Code: - http://tinyurl.com/crdum6''
`/* * A simple programme that will change the  * intensity of an LED based  on the amount of * light incident on the photo resistor. *  *///PhotoResistor Pinint lightPin = 0; //the analog pin the                   //photoresistor is                   //connected to                  //the photoresistor is not                   //calibrated to any units so                  //this is simply a raw sensor value (relative light)//LED Pinint ledPin = 9;//the pin the LED is connected to               //we are controlling brightness so                //we use one of the PWM (pulse                //width modulation pins)void setup(){  pinMode(ledPin, OUTPUT); //sets the led pin to                            //output} /* * loop() - this function will start after setup  * finishes and then repeat */void loop(){ int lightLevel = analogRead(lightPin); //Read the                                        // lightlevel lightLevel = map(lightLevel, 0, 900, 0, 255);          //adjust the value 0 to 900 to  lightLevel = constrain(lightLevel, 0, 255);      //make sure the value is betwween 0 and 255                                                     analogWrite(ledPin, lightLevel);  //write the value}`

Not Working?
• LED is Remaining Dark - This is a mistake we continue to make time and time again, if only they could make an LED that worked both ways. Pull it up and give it a twist.
• It Isn't Responding to Changes in Light. - Given that the spacing of the wires on the photo-resistor is not standad, it is easy to misplace it. Double check its in the right place
• Still not quite working? - You may be in a room which is either too bright or dark. Try turning the lights on or off to see if this helps. Or if you have a flashlight near by give that a try.

Making it Better:
Reverse the response:
Perhaps you would like the opposite response. Don't worry we can easily reverse this response just change.
` analogWrite(ledPin, lightLevel); ----> analogWrite(ledPin, 255 - lightLevel);`
Upload and watch the response change.

Night light:
Rather than controlling the brightness of the LED in response to light, lets instead turn it on or off based on a threshold value. Change the loop() code with.
`  void loop(){     int threshold = 300;     if(analogRead(lightPin) > threshold){       digitalWrite(ledPin, HIGH);     }else{       digitalWrite(ledPin, LOW);         }}`

Light controlled servo:
Lets use our newly found light sensing skills to control a servo (and at the same time engage in a little bit of Arduino code hacking). Wire up a servo connected to pin 9 (like in CIRC-04). Then open the Knob example program (the same one we used in CIRC-08) File > Sketchbook > Examples > Library-Servo > Knob. Upload the code to your board and watch as it works unmodified.
Using the full range of your servo:
You'll notice that the servo will only operate over a limited portion of its range. This is because with the voltage dividing circuit we use the voltage on analog pin 0 will not range from 0 to 5 volts but instead between two lesser values (these values will change based on your setup). To fix this play with the val = map(val, 0, 1023, 0, 179); line. For hints on what to do visit http://arduino.cc/en/Reference/Map .

CIRC09-sheet.pdf(612x792) 28 KB
CIRC09-sheet.pdf(630x810) 42 KB
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