LDR lumens

[bobobano]

Greetings,

For my degree project I'm controlling the light in a room as the outdoor light increases and decreases, I'm having an issue calculating the amount of lumens I'm actually measuring and I'm hoping someone hear can help clear it up.

Using the formula from the datasheet of the LDR Ladyada provided:
http://pdf1.alldatasheet.com/datasheet-pdf/view/237784/ADVANCEDPHOTONIX/PDV-P8001.html

Log(R100)-log(R10) / log(L100) - log(L10) = 0.6

And this is the first issue I have, shouldn't that be an inverse relationship? (ie -0.6)

I know that lumens can be found by lux*Sa, and for these LDRs I calculated it to be L = lux*2.188x10^-6

So i rearranged the equation to be

Log(R@100/R@10) = log(L@100/L@10)^0.6
R@100 = R@10*(L@100/L@10)^0.6

Pluging in the given R@10 and L@10 (and calculated L@100)

 I get R = lumen^0.6 * 6.37x10^-3 * (3K to 11K)

A linear relationship.  And thus, the cause of my problems, by experimentation I should be getting an inverse relationship.
 Help me Obi-Wan Kenobi, you're my only hope.

[Soeren]

Use the Force, Luke...


If you're just trying to keep the light level in a room steady, you don't need to know the Lumen number, as a comparator would just maintain a set level of whatever Lm.

[bobobano]

theoretically yes, but I have to controls the blinds to open and close, and turn a lamp up and down.  I can find the measured voltage on my sensors vs blind angle and lamp PWM control, but this exponential and I can't simulate it.  Since this is for my degree project I've been told to simulate everything that I can.

[Soeren]

Hi,

theoretically yes, but I have to controls the blinds to open and close, and turn a lamp up and down.  I can find the measured voltage on my sensors vs blind angle and lamp PWM control, but this exponential and I can't simulate it.  Since this is for my degree project I've been told to simulate everything that I can.

OK, do you have a light meter handy to make some reference lightning to compare with?
(If not, try to sweet talk someone in a photo shop to help you out).
Don't count on being able to get the exact 2856K (that's a K, not a °K as they write) light temperature (it's a very warm/reddish light).

The exponential function shouldn't worry you, but since the LDR is spec'd rather broadly, you'll have to measure the one you have got and derive the precise formula from that.

And the resistance goes down when the light increases - It's not proper of course, but they may argue that a change is a change (when they say 0.6 Ohm/Lm), but yes, it really should be -0.6 Ohm/Lm (but you wanna check/calculate the exact number too after measuring).

Sure, schools are fond of simulating, like the teachers like to simulate that they know any current electronics - Then, after you graduate, you enter Real  Life, where people know that sims are very misguiding (if not worse) sometimes, well, anytime you get beyond simple circuits actually.


BTW. if you want a linear resistance(light) function, here's a way of achieving it with a photo diode...

[bobobano]

BTW. if you want a linear resistance(light) function, here's a way of achieving it with a photo diode...

That was the original plan, but I couldn't find a photo diode operating within the visible light spectrum, usually either just below or just above.
Thanks for the advice, I did try and get a light meter from the physics department here but they didn't want to let us use one.  I'll have to find a workaround to get it running.

Thank you for your help everyone.

[Soeren]

Hi,

That was the original plan, but I couldn't find a photo diode operating within the visible light spectrum, usually either just below or just above.

Oh, lots of them do, sometimes with reduced output, but I wouldn't think that was important in your situation (consult their data sheet so see the full specter), as they would still perform relative to intensity, but you can find photo diodes which (more or less) matches the human eyes sensitivity to different wave lengths.
If they have peaks outside the visible range that you are worried about, a filter (talk to photo shop) can be used to cut (dampen) UV and/or IR.

Also check out "ambient light sensors" like Vishay TEMD6200FX01 and OSRAM makes some too.

I did try and get a light meter from the physics department here but they didn't want to let us use one.  I'll have to find a workaround to get it running.

They couldn't even help you setting up two light levels for reference?
What a lousy attitude!

Perhaps if you know a photo buff, or... If you could get just a single known source, you could use photographic NDF's (Neutral Density Filters) to dampen the output for the second level - eg. an NDF8 will reduce light 8 times.

If you want to throw a little money after it, www.Velleman.be makes some fairly cheap light meters that almost any high street electronics shop either carry or can order for you.

[Ricfy]

Hi,

Anybody know how to convert the ohm at the LDR NSL 5520 to lumen or candela?

[Ricfy]

Hi all,

This the circiuts to compare R1 and R2 by using opamp.

Assume the R2 is LDR how i know the LDR value convert to candela?

[Soeren]

Hi,

Anybody know how to convert the ohm at the LDR NSL 5520 to lumen or candela?

Please post new topics as new topics!
Please read your posts before sending them!
When asking about specific components, please provide a link to them!

So, try again and make it comprehensible, none of your posts makes any sense as is.