Reading A Photo Resistor using ADC

[jessey]

Hello,

I'm looking to write some code to detect a varying light level using a photo resistor. I've tried using the pot command but it doesn't seem to be suitable for my application. I would like to be able to detect dawn (sun just beginning to rise) and dusk. Using the pot command works somewhat but not with any kind of repeatable accuracy.

I read in the archives about using the ADC to read a photo sensor but I'm not sure how to get started doing that. I won't have any problems to set up and get the ADC working but what I'm not sure about is how to wire the photo resistor to the adc input. Once I get the photo resistor wired properly and see what the photo resistor is inputting to the ADC then I shouldn't have any problems writing the code for that.

Could anyone here suggest a wiring scheme and possible considerations for accomplishing this? Or another method that would be suitable.

Thanks
jessey

[malc-c]

I used a LDR to detect when the main lights on my fish tank went out so it would turn on the night lights for a set period of time. This used a 12F675, and the LDR simply connected between a pin on the PIC and GND.

As I was simply using it for "logic" (ie is it dark - yes / no) rather than actually measuring the level, the ADC function of the PIC was disabled.

If its any help the project files can be downloaded from my website via the link in my signature

[Bruce]

RCTIME works pretty well for something like this. Here's an example with
simple schematics http://www.rentron.com/Micro-Bot/CDS_IR_Navigation.htm

You could replace the 0.01uF caps with resistors forming a voltage divider
with your CDS photocell then read the output with an A/D pin just as easily.

[DynamoBen]

I have a related question. What if you wanted to sense the frequency that a light was turning on and off? (nothing more than 60hz)

[Bruce]

You would want a sensor with a much faster response time than a CDS
photocell. Check here http://www.taosinc.com/index.asp

They have a lot of nifty optical gadgets.

[DynamoBen]

makezine.com had a post about using a solar cell. Do you think that might work?

You can find the instructable here:
http://www.instructables.com/id/E5KC...FO02Y?ALLSTEPS