DIY PAR meter, Yeah you heard me - Page 27 - The Planted Tank Forum
 
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post #391 of 731 (permalink) Old 06-05-2012, 02:20 AM
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Originally Posted by O2surplus View Post
I'm gonna do some experiments with acrylic for a new, water proof sensor design. I just scored a nice, new 6' x 4' x 3/8" sheet of acrylite GP for FREE! I'm going to see how much work it will take to embed the sensor diode and diffuser between two fused pieces. Since the acrylic is 3/8" thick, I'm pretty sure that I'll be able to grind out a small recessed area for the diode/diffuser and still be able to fuse the two pieces together. I'll post some pictures later, after I've had a chance to try it out. Worst case scenario - I see a few 12"x12"x12" rimless tanks in my future. Oh yeah, I've got the next batch of twenty pretty much done, I'm just waiting for some LCD's to show up in the mail. MISTERGREEN's gonna send me his diodes and I'll get to work on the sensors. Once they're done- IT'S OFF TO THE SWAP-N-SHOP.
Yes that's what i like to hear!!!
This time no more bathroom breaks ill piss my self first lol!!!!!!

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post #392 of 731 (permalink) Old 06-05-2012, 02:40 AM Thread Starter
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Wow, I finally read the whole thread. Amazing.

Just one suggestion, or maybe I missed it. Why don't you guys charge a little bit more per unit, say $75 and take pre-orders so that you can get yourself new soldering irons, etc. etc. to make it a bit easier. Then you just have to do the one batch and it's over.
Pre-order is bad business practice and is against forum rules. If we make it too expensive, I think we lose the hobby side of things.

Beside, we make it when we have free time and when we want to. Nobody to bug us when their order is done. It's a pretty good plan.

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post #393 of 731 (permalink) Old 06-05-2012, 03:22 AM
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Pre-order is bad business practice and is against forum rules. If we make it too expensive, I think we lose the hobby side of things.

Beside, we make it when we have free time and when we want to. Nobody to bug us when their order is done. It's a pretty good plan.
Right. Forgot about that rule on tpt. Oh well, it's still good for everyone and hopefully, eventually, everyone who wants one can get one.

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post #394 of 731 (permalink) Old 06-06-2012, 03:51 AM
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Maybe we can luck out and both make the cut on the next batch and save on shipping 2wheels (neven btw)

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post #395 of 731 (permalink) Old 06-06-2012, 04:04 AM
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I got a big box of stuff from MISTERGREEN in the mail today. Looks like I'll be busy working on sensors this weekend.
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post #396 of 731 (permalink) Old 06-06-2012, 05:40 AM
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Maybe we can luck out and both make the cut on the next batch and save on shipping 2wheels (neven btw)
Sounds good. I can even pick up in Pt. Roberts so there is no customs.

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post #397 of 731 (permalink) Old 06-06-2012, 06:05 AM Thread Starter
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I got a big box of stuff from MISTERGREEN in the mail today. Looks like I'll be busy working on sensors this weekend.
Good thing you're making them. I'll be on the road for a little bit.

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post #398 of 731 (permalink) Old 06-08-2012, 06:07 PM
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Looks like a lot of progress since I last checked. Cool.

But so far as I can tell, these are still using the Excelitas VTB8441BH sensor.

As I understand, a PAR meter has spectral response like an "M" - sharp cutoff of UV/IR, rejection of green in the middle, and strong sensitivity to red/blue.

But the VTB8441BH's peak sensitivity is 580nm, which is yellow; and it tapers off towards red/blue. That resembles the response of a LUX meter more than PAR.

Granted the results may "close enough", but then wouldn't a $20 commercial LUX meter, adjusted by the closest correction factor, be only a little further off?

Back on page #9 I suggested a RGBW sensor, which is available in single quantity from Mouser and Digikey. Combined with a bit of cheap IR filter material, this would provide a better PAR reading by summing red/blue (and possibly subtracting some amount of green/white). By displaying raw RGB values, it would also allow a user to determine spectrum, and shift over time, of their lighting; a capability which would set this PAR meter apart even from commercial ones, and provide additional info to help users decide when a bulb replacement is really necessary. It even costs less than the Excelitas part. I was surprised that no one even commented on it.

And it's not the only sensor that would have been superior in the same price range, like this one from Sparkfun. Either one would interface to the Arduino with the CAT5 cable you're already using.

Seems a darn shame to have put so much time and effort into a PAR meter, when it could have offered more accuracy and capabilities; with no increase in hardware or price.
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post #399 of 731 (permalink) Old 06-08-2012, 06:40 PM Thread Starter
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As I understand, a PAR meter has spectral response like an "M" - sharp cutoff of UV/IR, rejection of green in the middle, and strong sensitivity to red/blue.
Not necessarily true. As long as the diode is sensitive to 400-700 spectral range, it'll work. Scientists have made a similar DIY sensor so I'm going off their research. It is entirely possible to take a lux meter and do some math to find the PAR value. The big question is what is the photodiode in the lux meter reading exactly and can vary from manufacturer to the next. It's a big question.

The color sensor you showed is awfully hard to work with. It's tiny. It might work though.

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post #400 of 731 (permalink) Old 06-08-2012, 07:30 PM
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While I agree with you on your points, I think the sensor from Sparkfun is a little out of the reach of the general hobbyist. Even though it's a ridiculously simple BGA (ball grid array) part, it's still a BGA part and I don't think a lot of people possess hot air rework units to solder that properly (yes, I suppose you could do it with an iron but you can't guarantee that it will seat evenly and correctly). I also think that the technicians and electrical engineers that would have had experience doing any amount of miniature SMT soldering are in the minority in the general hobbyist population, even in this forum.

Despite the sensor selection and limitations, and the Arduino-based nature of the project (I'm not a fan of Arduino for my own endeavors), I'm still pretty impressed at how good of an approximation it turned out to be compared to "the real thing".

[EDIT]
I don't mean to accidentally relegate anyone involved in the active development of this project to a "mere hobbyist". I was just trying to make a point. I don't intend any offense.

Quote:
Originally Posted by DarkCobra View Post
Looks like a lot of progress since I last checked. Cool.

But so far as I can tell, these are still using the Excelitas VTB8441BH sensor.

As I understand, a PAR meter has spectral response like an "M" - sharp cutoff of UV/IR, rejection of green in the middle, and strong sensitivity to red/blue.

But the VTB8441BH's peak sensitivity is 580nm, which is yellow; and it tapers off towards red/blue. That resembles the response of a LUX meter more than PAR.

Granted the results may "close enough", but then wouldn't a $20 commercial LUX meter, adjusted by the closest correction factor, be only a little further off?

Back on page #9 I suggested a RGBW sensor, which is available in single quantity from Mouser and Digikey. Combined with a bit of cheap IR filter material, this would provide a better PAR reading by summing red/blue (and possibly subtracting some amount of green/white). By displaying raw RGB values, it would also allow a user to determine spectrum, and shift over time, of their lighting; a capability which would set this PAR meter apart even from commercial ones, and provide additional info to help users decide when a bulb replacement is really necessary. It even costs less than the Excelitas part. I was surprised that no one even commented on it.

And it's not the only sensor that would have been superior in the same price range, like this one from Sparkfun. Either one would interface to the Arduino with the CAT5 cable you're already using.

Seems a darn shame to have put so much time and effort into a PAR meter, when it could have offered more accuracy and capabilities; with no increase in hardware or price.

I like creepy things, vines included.

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post #401 of 731 (permalink) Old 06-08-2012, 07:54 PM
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Yeah, I call that sensor "a little small" for even people with advanced SMT soldering skills!

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post #402 of 731 (permalink) Old 06-08-2012, 08:05 PM
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I agree the BGA part would be a major PITA. It was included just to show that such RGBW sensors are available from multiple vendors. Sparkfun does have a pre-soldered breakout for it, though for more $$$ of course. And the bare part may be available in other packages, I haven't checked.

But don't forget my original recommendation from page #9, the Taos TCS3200D, which is more hobbyist-friendly:



Even for the SMT-phobes (I'm one of them), SOIC-8 is easy.

As for the current sensor being used, I didn't see a spectral response graph in the data sheet. We don't know how rapidly response drops off at red/blue, only that it detects yellow best. I'd be curious to see how it performs against a real PAR meter with a more extreme case - light sources like fluorescent/LED grow lights (with primarily red/blue and lacking green/yellow); rather than sunlight or bulbs which approximate it, against which it appears to be software-calibrated. If it still comes close, it's earned my trust.
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post #403 of 731 (permalink) Old 06-08-2012, 08:48 PM
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Shame about the SMT phobia. I hate through hole components, too much wasted land area.

Since we intend to convert the output into a digital format anyway, why not just get a digital part? TCS3471, also from TAOS (now ams), provides a digital output over I2C. Removes the frequency to level conversion requirement, and has the same spectral responsivity. In fact, it's also cheaper and has built in gain control (though admittedly, it isn't as configurable as a full on analog solution but neither is the light-to-frequency component.

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post #404 of 731 (permalink) Old 06-08-2012, 11:30 PM Thread Starter
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You know what, if you want this to be more accurate, put a green light gel above the photodiode we have now. It'll now capture red and blue only and you'd need to change the code a bit because it'll restrict the light intensity.

I don't think it's necessary but for those of you who want to play with the notion.

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post #405 of 731 (permalink) Old 06-09-2012, 02:34 AM
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Since it was calibrated with a PAR meter and white light, I'm satisfied. I don't plan on anything other than mostly white light over my planted tanks. Even if the thing was 50% off on some frequencies, it's a lot closer than my eyeballs and chart guesstimates that I would otherwise end up doing.

Thanx

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