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[on:]

I'm going to use a bunch of LDR Photoresistor for sensing light brightness in rooms/outdoors (modest/slow changig values). What specifications should I look for?
I mean, I've already used a 25K/500KOhms one with Arduino succesfully, and I've seen there are the 10K/1MOhm and even 200K/10MOhms out there. Which (and why) do I choose? What am I going to expect from each of them?

I still don't know if I'm going to attach them to Arduino itself or use some external 8bit i2c ADC. I already had a bad experience with multiple analog reads from Arduino (there's a fair amount of cross interference I cannot get rid off). I'm thinking about a PCF8591. Can you suggest some other cheap IC?

Btw, I'm not going to build high precision stuff here, I'm willing to accept some error in readings.

Cheers :-)

[Reply #1 on:]

Really high impedances won't be able to drive the ADC input without significant error.  For instance the Arduino analog inputs want 10k or less input impedance for best performance (both for accuracy and to avoid cross-talk on reading multiple channels - though the double-call-to-analogRead technique will help).

There are integrated light sensing modules available if accuracy is important.

Also an LED can be used as a light sensor (but alas high-impedance).

[Reply #2 on:]

Really high impedances won't be able to drive the ADC input without significant error.  For instance the Arduino analog inputs want 10k or less input impedance for best performance (both for accuracy and to avoid cross-talk on reading multiple channels - though the double-call-to-analogRead technique will help).

I did experience very bad cross interference on two channels with 10KOhm pots :-(
I successfully minimized the problem (but not solved) by attaching the pots on two pins on two different ports. That helps but does not eliminate the interference.
The double-call-to-analogRead() was no use at all in my case, even with long delays in between (I stopped testing at delay(100); over 100 it would have caused too much delay on my code).

Anyway, back to ldr. You are telling me that using GL5528 (datasheet: http://www.google.it/url?sa=t&rct=j&q=GL5528&source=web&cd=2&ved=0CDIQFjAB&url=http%3A%2F%2Fmdfly.com%2FDownload%2FSensor%2FPD0001.pdf&ei=ku1JT6vKIMGbOur-uPoN&usg=AFQjCNEgEqXlr-LgsF_01YA5AopD5M_oeg&cad=rja) would be useless, right? I have to search for some LDR that have a 10K impedance in the dark, right?

[Reply #3 on:]

I'm just currious. Is it not possible (for more precission and faster readings) to use small solar cells (from low cost calculators) and connect these to your analog input?

[Reply #4 on:]

I'm just currious. Is it not possible (for more precission and faster readings) to use small solar cells (from low cost calculators) and connect these to your analog input?

I can't imagine how a solar cell could be more precise or fast. But I'm open minded, if you have a point, which is ... ?

[Reply #5 on:]

The response time of solar cells is amazingly fast + they are very sensitive. They use them for welding goggles where the solar cell with the slightest spark makes the LCD screen black.
http://inverterwelder.en.ecplaza.net/welding-helmet-welding-glasses--178046-1591646.html

Switching Time 　Light - dark: 0.0008s (0.8ms) at room temperature, 0.0003s (0.3ms) at 131 ℉ /55 ℃

[Reply #6 on:]

That's interesting, really :-)