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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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#686 · (Edited)
For those of you intending to build one of the CEPs...There are a couple of important things I learned when building the sensor that you might find helpful.

I had done what I thought was a really nice job of encapsulating the entire bottom part of the photo diode, from the side rim of the photo diode, all the way down to where the cable begins, making absolutely certain not to get any caulk on the face of the photo diode, so as to not obscure in any way the area where the light is collected through the photo diode surface.

I thought this would seal everything up really well, but the caulk must have acted as a conductor, causing the leads to short, and readings to be way off on the meter. So, I pulled the caulk loose, and cleaned everything up (once the caulk was dry). Readings came back to normal, and the meter starting working perfectly again. Maybe it's the brand of caulk I used...who knows? I used a caulk made by Locktite that comes out white and dries clear. Perhaps another brand, such as GE Silicone, would not have caused this problem...not sure. If you do caulk the leads, make sure you allow several days for it to dry. Perhaps that would have avoided the issue of the leads shorting. But, after waiting at least two days for the caulk to dry, I still experienced the issue with the short, and the meter returning bogus readings. I also tried caulking where the jar lid meets the jar, on the inside. But, drying time was so lengthy, that I abandoned that idea, and cleaned all the caulk up that was on the inside rim of the jar rim and lid.

I finally decided just to caulk the outside surfaces of the jar rim where it meets the lid, also caulking around where the cable meets the cable connector. I also completely caulked the cable connector where it meets the outside top portion of the jar lid where the cable enters the jar. Actually, I made two applications of caulk, allowing the first application to completely dry before moving on to the second application. It seems to have sealed very well.

So, caulking the leads of the photo diode doesn't really seem to be necessary. But, if you do it, I would recommend caulking each lead separately, and trying not to get any between the leads, allowing it to dry a long time, thereby avoiding potential for a short between the two leads of the photo diode, assuming mine wasn't sufficiently dry after two days. When I soldered the leads, I also used heat shrink on each lead, which I thought would sufficiently insulate the leads from a short by the caulk. But, I suppose that enough caulk migrated from one pin entry point into the diode to the other pin entry point, and created a short.

Also, if you want to layer the Rosco Filters the full diameter of the acrylic diffuser, like I did, this may be of interest. I requested some sample sheets from Rosco Filter Company, requesting a sample of each of the specific types that are outlined earlier in the thread. I requested them in the size 6" x 10", so I would have plenty to work with, since I have several PCBs, and figured I might want to build more at a later date. I never received an answer from them after making my request with their customer service department via email. But, low and behold, a sample of each showed up in the mail today, in the size I requested, 6" x 10", and free of charge.

Also, just so you are aware, the face of the photo diode is pushed all the way flush with the Acrylic Diffuser/Rosco Filter Sandwich on the inside of the jar. I then painted the jar because I figured that mine looked better that way, and I figured it would also limit light entering the jar from the sides/lid of the jar, thereby directing light only through the surface of the diffuser. I doubt that it's functionally necessary to paint the side walls/outside lid of the jar, but it can't hurt.
 
#687 ·
Mistergreen,

Can you advise where you obtained the jars/lids? I recall from reading early in the thread that you had some difficulty at first finding a jar that did not have an obstructed bottom area where there was an embossed area, printed matter, or an uneven surface area, which would distort the light coming through the bottom of the jar, and that it needed to be perfectly flat to achieve the best results.

Also, can you advise your source, and/or the specifications for, the acrylic diffuser material. Some have PM'd me requesting this information so they can build a sensor to the proper specification. I didn't want to advise them to obtain just any acrylic from Lowes or Home Depot, not knowing if there is a particular scientific standard or charactertistic to the acrylic you used.
 
#688 ·
I found the paint container at Hobby Lobby I think. It might be easier to buy off their online store because you never know what they have in stock at the stores. I'm sure you guys can find another way to build the housing though.

As for the diffuser, it's 1/8" thick 2447 white acrylic. I've only found them online. I've posted the links on the first post.
 
#689 ·
Do you think that a white opaque acrylic sheet from someplace like Lowes or Home Depot would suffice, something they sell as a flat sheet shade material for flourescent light fixtures for example. Those are probably about 1/16" thick.

Would you consider that to be the wrong type of material, and/or too thin?
 
#691 ·
No, I wouldn't change a thing. I was just curious for those that might find it cheaper to obtain the sheets from a local source, rather than pay the shipping costs, which are often as expensive as the product itself. Besides, it sounds like changing the code would be more of a hassle than it's worth.
 
#693 ·
#697 ·
I think that checking the light intensity as a function of frequency was the intent...

That said, if all you're looking to do is make relative comparisons of spectral intensity, the diffraction grating suggestion I made earlier is all you need to do things like this: (Finnex Planted+ 660nm LED vs 7000k white LED)



In theory you could post-process an image like that into an intensity graph.

That said, with a spectrum-aware probe, you could use some software and actually create a PUR meter instead of a PAR meter... a bit tricky calibration wise, but doable...
 
#698 · (Edited)
New Thread relating the CEP...

In case anyone is interested in following an important thread that relates to the CEP. Hoppy is testing CFLs with a commercially produced PAR meter to establish whether CFLs are consistent enough within the same model to rely on them as a bench mark in order to be able to calibrate your CEP to a particular model CFL.

http://www.plantedtank.net/forums/sh...d.php?t=875977

Once several different bulbs are tested, and the data is gathered and recorded, CFLs may be able to be used to calibrate your CEP yourself, without having to obtain a commercially produced PAR meter to calibrate to, or having to send your CEP to someone with a commercially produced PAR meter for calibration.
 
#703 ·
I wanted to bring this discussion back to the CEP thread, so as not to hijack Hoppy's CFL testing endeavors.

I took some readings off of the PAR meter that Mistergreen calibrated to his commerically manufactured PAR meter, and the CEP is calibrated according to the readings on the POT as follows:

By the way, the 15K resistor on the calibrated CEP reads exactly the same as my other uncalibrated CEPs, 11.94K, not 15K.

CEP #1 (Calibrated by Mistergreen):

POT 1st Pin = 11.94K
POT 2nd Pin = 15.56K
POT 3rd Pin = 15.56K

Actual Calibration = 27.5K

I took readings as Mistergreen recommended, one probe on the top pad of the 15K resistor, and the other probe taking individual readings from Pin #1, #2, and #3 of the 10K POT. Just to make sure I took accurate readings, I took them numerous times, and the readings repeated each time.

I adjusted the other two CEPs that haven't been calibrated as follows, mimmicking as closely as possible the readings I took off of the CEP that Mistergreen calibrated using his PAR meter. However, the POTs are very touchy, so I got as close to the POT readings from the calibrated CEP as possible.

CEP #2:
POT 1st Pin = 11.94K
POT 2nd Pin = 15.52K
POT 3rd Pin = 15.52K
Calibration = 27.46K (Adding together the values from the 1st and 3rd Pins of the POT).

CEP #3:
POT 1st Pin = 11.94K
POT 2nd Pin = 15.56K
POT 3rd Pin = 15.56K

Calibration = 27.5K (Adding together the values from the 1st and third pins of the POT)

The center pins on all the CEPs always read the same as the 3rd pin of each of the POTs, even on the one calibrated by Mistergreen. I was very careful not to touch the POT on the calibrated CEP, other than to take readings off the pins, in order to make sure to preserve the calibration done by Mistergreen via his commercially manufactured PAR meter.

Now, with regard to the discrepency between mathematical calibration of 22.53K, versus the 27.5K reading taken off the calibrated CEP, I suspect that it may be because the 15K resistor isn't really 15K. But, even at that, the resistor is only off by 3.06K, not 4.97K, which is the actual discrepency between what should be the mathematical calibration (22.53K) versus the actual calibration (27.5K) based on Mistergreen's PAR meter calibration. So, I'm not really sure how to interpret these findings as to why there is this discrepancy between the mathematically calculated calibration number of 22.53K versus the actual number calibrated by Mistergreen of 27.5K.

My next step will be to compare how the other two CEP meters perform based on setting them according to the calibrated CEP, by taking actual light readings from each of the CEPs to see what numbers they return, to see if they all correlate.

I want to use the same sensor from CEP #1 that Mistergreen had in his possession when he calibrated CEP #1 if possible, rather than build a new sensor to test CEP #2 and #3. But, the sensor from CEP #1 has a 2.5mm plug on it, and rather than mess with that, I plan to use a 2.5mm jack to plug that sensor into, which can then be connected to CEP #2 and #3 with a wire extension off the jack.

The problem is, my local Radio Shack, as well as all the Radio Shacks in St. Louis, have closed down, effective last week, and there is only one remaining in the area, and it's about 50 miles away. Our Industrial Electronics Distributors in the area are also 50 miles away. So, until the 2.5mm jacks that I have on order with China arrive, there may be some delay in making the comparison between CEP #1 (calibrated via PAR meter) and CEP #2 and #3 (mimmicked calibration).
 
#707 ·
In order to reach that position, because it's in the enclosure and those pins are face down, and the POT is all the way against the surface of the PCB, I'm working kind of blind, and having some difficulty reaching with a probe underneath the board and being sure that the probes are in the right position. I can take more time to attempt to confirm it later today if this number doesn't make sense.

It looks like .896K, or could be 8.96K. I didn't stop to verify if the meter was on the proper decimal setting. This is on the CEP that you calibrated to the PAR meter.

Let me know if you want me to make a more definitive attempt at it, and I'll be happy to get it to you later this evening.

Hey quizcat, can you measure the pot pin 1&2. That should give the resistance of the pot setting.

http://www.instructables.com/id/How...a-Potentiometer/step7/Varying-the-resistance/
 
#714 ·
Too bad...Well, maybe by measuring the resistance of the one that was calibrated to the PAR meter, and plugging in those same numbers into the POTs of the uncalibrated ones, or as closely as possible, we'll find that the other ones will calibrate pretty closely based on those same settings of the resistance showing on the PAR meter calibrated CEP.

I'll experiment, and compare the readings I get from light sources using the one calibrated via the PAR meter, against what PAR readings I get off the ones that were calibrated using the same resistance settings as the one calibrated to the PAR meter, and see what I get.

But, if not, then I at least I have the one calibrated via the PAR meter, and can set the other two to the PAR levels returned off the one calibrated to the PAR meter.
 
#715 ·
I've got an idea that should make calibrating the new meters against the known calibrated unit really easy. You can use the voltage signal on A0 and the PAR number output to the Lcd to calibrate the units. It's a simple matter of turning the sensor circuit into a voltage divider by using a resistor instead of the photo diode.

1.) Start by disconnecting the photo diode from the known good calibrated unit.

2.) Install a 10K resistor across the photo diode connection points at the CEP PcB.

3.) Power up the CEP and take note of the PAR number displayed on the Lcd. This PAR number will be your reference.
4.) Reinstall the photo diode and set this CEP meter aside.

5.) Grab one of the new units and install the same resistor across the photo diode connections.

6.) Power up the unit and adjust the pot until the Par reading is the same as the original calibrated unit.

7.) Perform the same steps above on any remaining uncalibrated units.

Following these steps should set up all your CEP's to be identically calibrated. The only variable left will be slight electrical variations between the photo diodes.:hihi:
 
#720 · (Edited)
I'll try this as soon as I can. Because I live in a rural area, it may take a few days to come up with a 10K resistor. They just closed our Radio Shack for good. I'll check my stash to see if I have one, but if not, then I'll have to order one from Mouser. So, it may be a few days before I can get back to you guys. But, if it works, it's a wonderful stroke of genious!

Now, one question, regarding my calibrated CEP, the PCB is permanently stuck to the back wall of a plastic cabinet via double stick tape, and can not be removed, it's like concrete. The photo diode is connected to the PCB via a power jack/plug via wire extensions between the jack and the input for the sensor at the PCB. See the white wire on the left? The sensor connects to the PCB through the jack. Power, is the Red/Black wire on the right, connected to power on the PCB through a power plug/jack.

Can I just unplug the sensor, unplug power, and put the 10K resistor across the center pin and ground at the sensor's jack terminals, and be good to go, or will having the jack in the mix throw things off with respect to the jack and wire extenstions to the PCB adding resistance? And, if accessing the sensor terminals through the jack is a problem, technically speaking, then it just makes it a little more difficult to access the sensor terminals inside the cabinet to make the connections, due to the confined space inside the cabinet, and because the CEP can't be removed. It can be done, no problem, but it would make it easier and quicker to connect at the jack if it won't throw everything off.

Audio equipment Electronic device Wood Blue Product


What are the anticipated electrical varitions (ie:tolerances) between the photo diodes with respect to photo diode specs? Anybody know the percentages?
 
#717 ·
heh, crazy enough to work!

LOL- I knew there had to be a dirt simple solution......Just had to stare at the circuit for awhile to see it.:hihi:
 
#721 · (Edited)
Here are the results...

I was very fortunate, in that I had an Electronics Rainbow kit for my grandson that I was able to find a 10K ohm resistor.

So, I wrapped black electrical tape around the leads of the resistor so that my holding it did not change the resistance.

First, I tested the CEP that had been calibrated to Mistergreen's commercially produced PAR meter, we will call it CEP #1, and got the following result on the LDC screen of the CEP:

Indoor = 3813
Outdoor = 3458
(Note Mistergreen's reading was 4357, an almost identical reversal of the 1st two numbers above on my Outdoor reading)

I tested several ways, from the soldered pins of the jack, and also from the two terminals of the green block where the Sensor wires connect, holding the resistor with my fingers, making sure to only hold the resistor through the black electrical tape I applied to the resistor. I also took readings holding the resistor with a set of tweezers grasping the black tape area. But, I don't have a set of plastic tweezers, which would have been preferable to perform this test. So, I hope that the black electrical tape did the job to isolate the resistor from any effects from my touching it. The readings were taken multiple times, and the results were always consistent.

I also tested the two CEPs that have only been calibrated by me using the resistance method, and not through a light emitter via a commercially produced PAR meter. These CEPs were calibrated by putting a probe on the top pad of the 15K ohm resistor, and touching the other probe to pin #1 and #2 on the 10K POT. You may recal that CEP #2 calibrated
to 27.46 ohms, and CEP #3 calibrated to 27.5K ohms using this method, and were calibrated to closely match the results using this method from CEP #1, which had been calibrated using a PAR meter by Mistergreen.

When using the 10K ohm resistor touched to the input of the sensor terminals, the CEPs returned the following values on the LCD screen:

CEP #2 previously calibrated to 27.46K using the resistance method between the 15K resistor and terminals 1 & 2 at the POT:
LCD Reading =
Indoor = 3813 using 10K resistor at the input terminals of the sensor
Outdoor = 3458 using 10K resistor at the input terminals of the sensor

CEP #3 calibrated to 27.5K using the resistance method between the 15K resistor and terminals 1 & 2 at the POT:
LCD Reading =
Indoor = 3819 using 10K resistor at the input terminals of the sensor
Outdoor = 3463 using 10K resistor at the input terminals of the sensor

CEO #2 and CEP #3 are consistent with the results returned from CEP #1, the one calibrated to Mistergreen's PAR meter, but slightly off from one another because the POT is pretty touchy, and it's hard to get that fine of an exact adjustment using a screw driver.

I did not change the POT adjustments of CEP 2 & 3 because they appeared to mimmick the adjustment I made via the top resistor pad and the POT terminals previously made, versus the 10K resistor method, so I left them alone, right where they were.

I don't know why the values I obtained versus the values obtained by Mistergreen on his CEP using the 10K resistor at the sensor terminals are so far off from one another.

Could it have anything to do with my 15K resistor value being measured at 11.94K versus 15K? If so, then perhaps a reading of the 15K resistor would be in order before calibrating using the 10K resistor method, and some kind of conversion factor could be applied to acheive actual calibration based on the actual 15K resistor value of any random CEP.
 
#722 ·
I think that O2Surplus envisioned that any calibrated CEP, once calibrated to a PAR meter, and the values known, will calibrate exactly the same way each time via the 10K resistor method, the 10K resistor eliminating the variability of the sensor itself. Then, because of the standardization of the sensor input to 10K, only one calibrated CEP would be necessary for anyone to calibrate their individual CEP using the 10K resistor method.

But, having taken some readings from my own CEP, the theory does not seem to correlate to readings Mistergreen obtained from his calibrated CEP.

I confirmed the value of my 10K ohm resistor that was put across the sensor terminals, and it is 9.97K ohms. Close enough, tolerance of about 3%, but it occurs to me that there may be variabilites in all the components within the circuits of individual CEPs to make universal calibration through a mathematical formula almost impossible as these variabilities add up...unless there is some kind of conversion factor that can be used to acount for the actual values of components used in individual CEPs. Take my 15K ohm resistor for example. Mine reads 11.94K, and not 15K.
 
#723 ·
hmm, interesting. The only thing I can think of is the variance on the voltage regulator and resistors in that chain to the atmega that would give us different results. So it would vary from batch to batch.

The regulator is outputting to 4.99V (middle & right pin)and the resistor by the atmega is 10.05K.

Yeah, there might be too many variance to get an exact number. I guess the only way to calibrate is to use another calibrated CEP with light sensor or a PAR sensor.
 
#726 ·
so in the morning I will make it more permanent but I have 3 colored ronco disk cut to fit the jar and just dropped in then I will glue the photo optic to the diffuser and drop that in and dab a bit of silicon on the edges to hold it in place few hours drying time I should be almost done.
 
#727 ·
I believe that the diffuser goes into the bottom of the jar first, then you put the filters between the acrylic diffuser and the photo diode, so that light passes first through the diffuser, then through the filters, where it hits the screen of the photo diode. Correct me if I'm wrong Mistergreen.
 
#730 ·
*Edit 5-10-13
NOTICE - Firmware has been updated for outdoor lighting.

Final product list:

Photodiode: VTB8441BH
price: $4.660
Excelitas Technologies Sensors - VTB8441BH - SILICON PHOTODIODES - Allied Electronics

Cosine diffuser: 1/8" thickness, 2447 white plexiglass
price: varies, get the sample or buy a big sheet of it.
Plexiglass White & Black acrylic sheets
Acrylic 1/8" Tinted & Colored Sheeting | U.S. Plastic Corp.

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
Dear Mr. Green,
I see you've been through a long journey of building a PAR sensor. May I ask if you have complete instructions on hand? If yes I'd really appreciate if you could share them.

If not, could you give me few pointers. I have few questions here. What is the purpose of Cosine diffuser and how do you use it? Is the Photodiode VTB8441BH submersible? I was thinking to put the sensor into the aquarium for constant monitoring. Will this work with Miniature Solar Cell - BPW34? I'd prefer to use the latter one since it is much smaller.

Also, what is the algorithm to calculate the PAR value from the sensor reading?

Thanks a lot,
Ruben
 
#732 ·
This has been a long thread, hopefully it can still garner some activity.

What would need to be done to make this code work with an Arduino Mega 2560 and a ITDB02 or Nextion TFT screen?

I'm wanting to building my own LED lighting controller and I think having a PAR meter built in would be a great feature. This way it could display the PAR value during the different light cycles like sunrise/sunset etc.
 
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