Reading A Photo Resistor using ADC

[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

[TonyA]

Hi,

I do this all the time. I have found that using adcin is best. Use the adc pins of your pic like the PIC 18F452.

TRISA = %11111111 'Set PORTA to all input
ADCON1 = %10000010 'Set PORTA analog and right justify result

adcin 1, photoVar 'read analog input of the photocell wired as a voltage divider input on pin 3 of Pic 18F452: result is stored in variable "photoVar"

use the result stored in the variable "photoVar" to do something

From what I understand, from the literature, adcin is a much better choice if your PIC has built-in analog to digital conversion such as the RA pins of Pic 18F452

This is how you would wire your photocell:
Scroll down the page and see the photocell, which is wired as a voltage divider.
http://www.phidgetsusa.com/tutorials...ue_sensors.asp

Oh, I also just wanted to say that using a solar cell would be cool but solar cells are more costly than photocells. If you wanted to store energy I would say use a solar cell, but from what I understand about your application a photocell, which costs maybe $1.00 at the most would be very effective.

The Hot wire to +5
the green to the pic pin
the black to ground

I make MIDI controllers that use a photocell to detect changes in light and dark, and in response produce MIDI notes. MIDI notes (messages) need to be very fast, the photocell and ADCIN are very fast. It's all you need for your application.

If you need help let me know.

[DynamoBen]

Bruce felt that a photocell would be to slow to measure frequency. That is why I asked about a solar cell.