Hi all,
I'm thinking about a project which would involve light sensors detecting changes in ambient light through the day - that is, quite subtle changes: sunlight to cloudy to overcast etc. I get the impression that CdS sensors aren't sensitive enough for this. Is that right, and if so, is there something more suitable?
thanks,
TimH
The most sensitive and widest range sensor that I've come across is the device sold by Sparkfun. It requires more complex software to take advantage of it's wide range but there are examples in their offering.
Lefty
Thanks for that - I had come across it, and this tutorial:
Would it be possible to connect one of these directly to an XBee - i.e. without an Arduino board at all? I'm not clear whether the "more complex software" requirement would rule that out.
Sorry, I have no experience with the XBee product. Perhaps others might be able to answer that.
Lefty
I have a light sensitivity system running now using cheap LDRs I bought on ebay. They are just in a voltage divider circuit with a 10k resistor. They are more sensitive to light change than my eyes. The live results are available here :
http://pluggy.is-a-geek.com/index.html
The light graph is near the bottom of the page.
There are graphs for up to the last month.
The Resistance varies between a few tens of ohms in bright light and over a megaohm in darkness. The 10 bit resolution of the arduino works nicely. On a nice sunny day with a wind the graph goes up and down a fair amount with clouds obscuring the sun at intervals. The sensors are inside and read the light levels in rooms. When the reading gets down to 200, its time to turn a light on......
Thanks for the link - and I like your recycle-re-use approach 
Interesting that you report those LDRs detecting such small changes - I'd definitely got the impression from various demo videos on youtube/flickr that they only responded to quite dramatic changes.
I guess the logical next step is to get hold of some and start experimenting.
TimH
Not too badly priced:
Product Description
The Precision Light Sensor measures human perceptible light level in lux; its measurement range is from 1 lux (Moonlight) to 1000 lux (TV studio lighting)
I get my stuff from HERE.
Don't be scared of the plug end - its the standard Parallax / Servo output so Gnd/+5V/Sig
I recently removed 1 of the 11 watt CFLs from a light fitting that had three. With three lights the reading is just over 600, with 2 its just under 500. I do use a smoothing algorithm because there is a certain amount of sensor jitter. (not many analogue circuits don't have jitter).
The LDRs I have are very similar (if not the same) as these. Not exactly a pricey item 
The LDRs I have are very similar (if not the same) as these. Not exactly a pricey item Wink
http://cgi.ebay.co.uk/LDR-Light-Dependent-Resistor-Lot-of-5_W0QQitemZ12034378255...
I ordered that very item about 2 minutes before reading your post!
Just to round this off: having run a few tests, those sensors linked to above by pluggy (around 50p each inc. p+p!) seem to be more than adequate for the job.
Thanks everyone.
I had a check on the TI website and it looks like the tsl230 (which is the IC being sold by sparkfun - mentioned above) is now obsolete .. does anyone know what it's been replaced by?