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I am trying to read a quantum light sensor (spec sheet: http://goo.gl/R7mfm). Since the photodiode already has a built in resistor of 604 Ohms, I think I am limited to measuring the forward voltage (i.e., I cannot reverse bias the diode without modifying the sensor cables), correct?

When I hook it up to measure the voltage by reading one end of the sensor through an analog pin (as in Fig 1 here: http://goo.gl/7kL5H), I get values which jump around and do not respond to changes in light. I have tried adding additional resistors (up to 500k) across the sensor but this does not make any difference. Any assistance on how to correctly read this type of sensor would be appreciated. My code is below.

Code:
int lightPin = 5;  //define a pin for Photo resistor

const int reads = 4;
float light[reads]; 
float lightTot = 0;
float lightAvg = 0;
 
void setup()
{
  Serial.begin(9600);  //Begin serial communication
   
  for (int i = 0; i < reads; i++){
   light[i] = 0;
 } 
   
   
 for (int i = 0; i < 19; i++){
   pinMode(i, OUTPUT);           
digitalWrite(i, HIGH);       // turn on pullup resistors
 }
}

void loop()
{
lightTot = 0;

 for (int i = 0; i < reads; i++){
   
 light[i] = analogRead(lightPin);
 Serial.print(light[i]);
 Serial.print(", ");
 lightTot = lightTot + light[i];
 delay(500);
 }

lightAvg = lightTot / reads;
 Serial.println();
 Serial.println("AvgDio: ");
 Serial.println(lightAvg);

}
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The sensor is NOT an LDR or photoresistor.  It's a photovoltaic (generates current) detector.  It typically generates 5 microamps at 1 milimole per second per square meter.  Since the Arduino measures voltage and not current  Your sensor should have come with a calibration value to to replace the 'typical' value of 5.  To convert from current to voltage multiply by .604.

Measure the the voltage across the sensor, divide that by 0.604 to get millivolts, divide that by the calibration value to getmillimoles per second per square meter.  You will probably want to use a the lowest available analog reference (Aref) value to get a reasonable useful range of voltage readings.
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Did you get this sensor working?

If so: could you explain how? I have all the parts I need for a PAR sensor with a readout so that I can save $200 or so but I'm not quite sure what would go where (why 3 wires on this? I'm used to 2 with arduino).

Did the advice given above provide values that looked sane?

Any help would be appreciated,
Thanks
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Hi,

I found all this informations very useful! However as I am new to arduino, I am struggling finding what I should get for my remote sensing system.
I have this quantum sensor http://www.walz-usa.com/products/light/ls-c/introduction.html which has a BNC connector instead of classical wires, How do I connect it to an arduino Uno? And do you think I need an amplifier to read it?
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