Convert Lux meter to PAR meter? - Page 3 - The Planted Tank Forum
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post #31 of 175 (permalink) Old 06-18-2012, 05:08 AM Thread Starter
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Quote:
Originally Posted by mistergreen View Post
It won't necessarily be a match. The voltage output may vary from diode to diode so the readings will be off even though they read the same spectrum.
Since I have to calibrate it anyway, that doesn't bother me. I was concerned that it simply wouldn't work at all. I notice that the "cathode" is identified on the diode case. Is that the one that the negative polarity wire is soldered to? And, by convention, is the white or red wire the negative one? One last question, if it is connected backwards does anything happen other than it just not working?

End of today's photo electronics lesson for me

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post #32 of 175 (permalink) Old 06-18-2012, 08:27 AM
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Power flows from anode to cathode, just like a LED. So to get a positive voltage out, you ground the anode, then measure the voltage on the cathode.

General convention for low-voltage wiring is that when a colored wire is paired with a colorless (white or black) wire, the colored one is positive.

I just checked my LUX meter with a voltmeter to be sure, and it does follow the convention.

Hook one up backwards, chances are nothing bad will happen; apart from it just not working.
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post #33 of 175 (permalink) Old 06-18-2012, 05:53 PM Thread Starter
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Well, it is good to know that I have a 50% chance of hooking up the photo diode correctly the first time! And, if I can see the tiny identifier dot well enough I can improve the odds. Thank you DarkCobra. When I assemble an electronic device now days it feels like walking a tight rope with no net.

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post #34 of 175 (permalink) Old 06-18-2012, 06:55 PM
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On the photodiodes, there's a dot on the plastic that indicates the positive side.


Nothing bad will happen unless you run current through it.

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post #35 of 175 (permalink) Old 06-18-2012, 09:03 PM Thread Starter
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This shows the dot marked "cathode". Is that the positive side?

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post #36 of 175 (permalink) Old 06-18-2012, 10:29 PM
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yup

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post #37 of 175 (permalink) Old 06-18-2012, 10:42 PM
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Even if you can't find the dot, you can still identify the cathode by putting a voltmeter on it. The bare photodiode (with no filters) from mine produced 0.235V in room lighting during last night's test.
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post #38 of 175 (permalink) Old 06-19-2012, 08:36 PM Thread Starter
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I just visited Tap Plastics and bought a 6 foot long 3/4" OD by 1/16" wall acrylic tube and some 3/4" diameter acrylic half spheres. (less than $6) I plan to use fine sandpaper to frost a surface of the half sphere, probably the flat surface. Then glue that to the end of the tube, and cut off an inch long piece, to be the housing/cosine "filter" for a new PAR sensor. My photodiode will be here late tomorrow.

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post #39 of 175 (permalink) Old 06-20-2012, 11:25 PM Thread Starter
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Gluing the tube to the half sphere has been difficult. My first effort leaked badly at the joint. This is the second effort, using a new tube of Weld On #16.

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post #40 of 175 (permalink) Old 06-21-2012, 03:14 AM Thread Starter
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The photo diode, beside a penny. Just barely big enough for me to solder the lead wires to it. This is the same one Mistergreen used in his pioneering PAR meter build, DIY PAR meter, Yeah you heard me

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post #41 of 175 (permalink) Old 06-21-2012, 09:49 AM
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How do you prevent the solvent glue from melting and clearing up the frosted surface of the half sphere?

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post #42 of 175 (permalink) Old 06-21-2012, 04:44 PM Thread Starter
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How do you prevent the solvent glue from melting and clearing up the frosted surface of the half sphere?
That was a problem. Weld On #16 is pretty viscous, so it doesn't flow like the almost pure solven glues do. I put a bead of the glue over the end of the tube, working as fast as I could to beat the time it takes for it to skin over. Then I grabbed the half sphere and quickly put it in place (about 1/64" off, but that isn't a problem). The glue didn't run, and grabbed the pieces quickly. After about 5 minutes I ran a bead of glue around the joint, and let that cure for 30 minutes, then ran another bead. After an hour I tested it by weighting it with a fingernail clipper and stuck it into a glass of water. I checked for water inside every half hour or so. No leaks!

I knew this part was going to be difficult, so I bought 3 of the half spheres (45 cents apiece). I was lucky to make it work in two tries.

It is interesting to see what the optical effect of a half sphere is. It seems to act as a lens, as it has to, but it collects light over a large area and concentrates it into a circle about half the diameter of the sphere. With the flat side frosted it diffuses the light so there is just barely an image visible through it.

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post #43 of 175 (permalink) Old 06-21-2012, 06:38 PM
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This shows the dot marked "cathode". Is that the positive side?
Cations are + eg K+, Ca++ etc........metals.........good way to think about it for some.

Regards,
Tom Barr
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post #44 of 175 (permalink) Old 06-21-2012, 08:12 PM
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You might need a semi-opaque acrylic material to make it a real cosine diffuser.
The parabolic curve (output) may be too steep with your frosted semi-sphere. Calibrating that won't be fun. I found out that the light box acrylic I used works pretty darn well. It doesn't have to be a curve/lense.

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post #45 of 175 (permalink) Old 06-21-2012, 11:12 PM Thread Starter
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This is how the calibration of the DIY meter looks, for the first version, with the sensor head inside an acrylic jar. This should be repeatable if anyone else does one this way.

The next test is the one it will probably fail - submerging it in water to see if it leaks.

EDIT: First a 5 minute dunk, then a 15 minute dunk. No leakage that I can see! Next will be to compare the readings in water to the PAR meter, to see if the calibration changes.

EDIT: Tested in aquarium, 2 feet high (2 feet head of water pressure on sensor). The data point falls on the same line as when calibrated in the air. This definitely works. However, the sensor is still too buoyant, floating up if your fingers slip at all. And, it needs a "wand" to be able to use it effectively without your hand getting in the way.

I think the sensor-from-scratch that I'm making will be a big improvement.

Hoppy

Last edited by Hoppy; 06-22-2012 at 07:41 PM.
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