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mistergreen 11-07-2010 04:45 PM

DIY PAR meter, Yeah you heard me
*Edit 5-10-13
NOTICE - Firmware has been updated for outdoor lighting.

Final product list:

Photodiode: VTB8441BH
price: $4.660

Cosine diffuser: 1/8" thickness, 2447 white plexiglass
price: varies, get the sample or buy a big sheet of it.

Basic 16x2 Character LCD - Black on Green 5V
price: $14 but you can get it cheaper on e b a y

Arduino Uno
price: $30 (newer model)


CptanPanic 11-07-2010 05:06 PM


Originally Posted by mistergreen (Post 1197502)
I made my order in for my DIY peristaltic pump and I bought this guy too

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.

Hoppy 11-07-2010 05:19 PM

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.

mistergreen 11-07-2010 05:21 PM

Good point.

I think taking the average of all of the readings per second would make it PAR?
All of the math can be done through my microprocessor.

Baadboy11 11-07-2010 05:57 PM

Someone send this man a par meter so he can see how close they are! STAT!

mistergreen 11-07-2010 09:03 PM

No need to send me a PAR meter although I wouldn't mind having one for a while :D

Once it's built I can post some standard readings like sunlight at noon and people with a real meter can confirm the readings.

mistergreen 11-08-2010 04:22 AM

I found some neat µmol m-2 s-1 conversions and some standards like full sun is 2000 PAR on Apogee's website.

Posting for posterity.

1 µmol = 6.02 × 10^17 photons

Jim Miller 11-08-2010 01:01 PM

Different light sources have different spectral output characteristics. I think to make this work you'd need to put a filter which combined with the silicon sensor response would approximate a PAR curve.

Sounds like an interesting project.

Good luck!


mistergreen 11-08-2010 01:39 PM

Here's the spectral sensitivity of this photodiode. As you can see, not the best match :)

I did find a photodiode that fits the bill though

It's $17 , I think. Maybe I'll get it one day... I couldn't pass up a $1.5 photodiode.

Jim Miller 11-08-2010 02:58 PM

Try getting a light modifier for theater lights, a piece of gel, with a spectrum near what you are looking for. I bet you can get a "goodnuf" match.


mistergreen 11-08-2010 04:05 PM

Is there a gel that blocks InfraRed?

Well, hopefully, calibrating the output based on Hoppy's chart might be 'goodnuf'.

I went ahead and contacted Hamamatsu on how to get their photodiode.

Jim Miller 11-08-2010 04:37 PM

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!


keithy 11-08-2010 04:47 PM

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.

mistergreen 11-08-2010 06:26 PM

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:

And spectral sensitive is another term for photo sensitivity ( another unit of measurement like inches & centimeters ).. I assume they measure the same thing.

mistergreen 11-09-2010 10:04 PM

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.

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