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DIY PAR meter, Yeah you heard me

299K views 740 replies 110 participants last post by  henrybri 
#1 · (Edited)
*Edit 5-10-13
NOTICE - Firmware has been updated for outdoor lighting.

Final product list:

Photodiode: VTB8441BH
price: $4.660
http://www.alliedelec.com/search/productdetail.aspx?SKU=70219652

Cosine diffuser: 1/8" thickness, 2447 white plexiglass
price: varies, get the sample or buy a big sheet of it.
http://www.eplastics.com/Plastic/Plexiglass_Acrylic_Sheet_Black_and_White
http://www.usplastic.com/catalog/item.aspx?itemid=23681

Basic 16x2 Character LCD - Black on Green 5V
price: $14 but you can get it cheaper on e b a y
http://www.sparkfun.com/products/255

Arduino Uno
price: $30
http://www.sparkfun.com/products/10356
http://www.sparkfun.com/products/11021 (newer model)


Diagram
 
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#2 ·
I made my order in for my DIY peristaltic pump and I bought this guy too

http://www.sparkfun.com/commerce/product_info.php?products_id=9541

cost $1.50

It's sensitive to visible light and a little IR as well so I hope it won't throw the
reading too much. So yeah, it's not super accurate but maybe good enough
for the regular hobbyist.


I've already started building the waterproof body to hold the photo cell. The case
doesn't have a diffusor like a real PAR sensor but hopefully it'll be fine.


It a little plastic paint jar and wires are threaded through the tubing. The signal
will be read with my handy all purpose ardiuno...


Thanks to Hoppy's PAR chart, I can calibrate the signals to a rough guessimate.




Compared to a $300 PAR meter & sensor, this might be a deal.
Neat project, but that is a light sensor not just PAR, so would measure lumens.
 
#3 ·
The little photodiode might sense PAR or it might sense nearer to lumens, or anything in between. Considering the cost, that slight disadvantage isn't that important. It could work well for adjusting the height of T5 lighting, checking how much shading the plants are doing, noting bulb deterioration, etc. But, for getting data that can be compared to other's data, it wouldn't be that great.
 
#12 ·
If you're measuring underwater I don't think you'll need to worry about infrared. In a dry tank you probably need something but I'd try the green gel first before working on the infrared part. In an otherwise subdued light room you'd be measuring the output of flourescents which don't have any significant IR output to measure. If you're measuring incandescent or MH that's a completely different story with lots of heat output.

Good luck!

Jim
 
#13 ·
mistergreen, what's the difference between relative spectral sensitivity and photo sensitivity? If they're the same, the trend of the graph looks similar however, the peaks are very far off(1.0 and 0.3). If there is a scaling factor/calibration, it would be worth looking into before spending $17 bucks.

Sorry if my questions/comments sounds dumb, I am no light expert, just want to contribute and point out what I though to be a little bit of inconsistency. Overall, I think you have got brilliant idea rolling!

Edit: I looked at it again and seems like the photo sensitivity/peak of photo sensitivity = relative spectral sensitivity. Is this correct? If it is, it may fit pretty well.
 
#14 ·
Oh, I'm just looking into the Hamamatsu diode... Haven't bought it yet until I played around with the cheaper diode.

photo sensitivity is a electrical engineering term referring to photodiodes:
http://en.wikipedia.org/wiki/Photosensitivity#Interpretation_in_electronic_engineering

And spectral sensitive is another term for photo sensitivity ( another unit of measurement like inches & centimeters ).. I assume they measure the same thing.
 
#15 ·
I received the photodiode yesterday. Got to play with it a little. In this picture, I'm trying to find the right resistor for this set up.


A 4.6K resistor seems to work. I'm calibrating the max output to sunlight.. 1024 (5V) is the max reading and sunlight through a window gives a reading of ~900. So I think I got it.

Mounted the photodiode on a electrical cap


Assembled the jar... This nub in the jar is a problem. It casts a shadow on the photodiode.


I moved the diode to the side and gave the plastic a little frost with fine grit sandpaper to scatter the light a little.



Next step is to calibrate these numbers to the PAR readings.
 
#18 ·
I don't think I showed you how I'm reading the values so far.



It's all through USB... It's supplying the ardiuno power and a way for the ardiuno to communicate to my computer...
This is my serial monitor outputting the values.


The values jump/dance around like it's being hit by photons, kinda cool. So I wrote the code to find the average within every second...

NON-PAR code...(NOT FINAL)
Code:
unsigned long average = 0;
unsigned long time;
int counter = 0;

void setup() {
  Serial.begin(9600);
  time = millis();
}

void loop() {
  int sensorValue = analogRead(0);
  average += sensorValue;
  counter++;

  //every second or 1000 millis
  if(millis() > time+1000) {
    average = average/counter;
    Serial.println(average);
    
    //reset timer & counter to get ready for the next second.
    time = millis();
    counter = 0;
    average = 0;
  }
}
Oh, I double checked the readings by going outside... Full sun will give me the maximum value of 1023.. In the shade, around 100-120.
 
#21 ·
Yeah, I'm mapping out the values now to see the relationship...

Hoppy, I have a question about your PAR - distance chart.
Your graph implies a parabolic curve, meaning the closer you get to the bulb, the more it gets to infinity or to the max value (the sun 2000 PAR). This is assuming your bulb is outputting at the level of the sun? It doesn't seem right.

I'm definitely getting a fixed value no where near the maximum with my DIY meter.
 
#24 ·
Ok,
I found the equation... There were some guessing going on because I didn't know what hoppy's chart start value is.

It looks like these bulbs output are bell curves rather than parabolas.

f(d) = A * (11e^(-d^2/(2*35^2)))
A is the value from my DIY sensor, d is distance.

I'll graph this out to see how close they fit.
 
#26 ·
The inverse square relationship of PAR and distance only works within a relatively small range of distances. When you get close to the bulb it breaks down. I determine relationships like this by just plotting the data on log-log graph paper. If the points form a straight line, it demonstrates that one variable is a function of a power of the other variable, with the slope of the straight line being the power.

It is very hard to measure distances very accurately with tests like this, and PAR meter readings can fluctuate as you read the meter, and vary from hour to hour. So each data point can be assumed to have a pretty large uncertainty. Considering all of that, if a line with a slope of minus 2 will fit the data points reasonably closely within the distance range that is relevant for aquariums, it is a good approximation to assume the inverse square relationship. Then, when you toss in values for multiple bulb fixtures, using the PAR number divided by number of bulbs, and those data points follow the same line, you can see that it is a good approximation to say that n bulbs give n times the PAR of one bulb.

It makes no difference to us, when we are looking for an aquarium light, whether or not the PAR varies with square of distance at distances of 1 inch to 4 inches, so I never bothered to measure it at those distances, except with single LEDs. Single LEDs are so near a point source that the inverse square relationship will hold true at even 3-4 inches from the single LED. It gets much more complicated when you have an array of LEDs.
 
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