Basic analog input questions

Forum 2005-2010 (read only) General Frequently-Asked Questions
1

hello!
I have the following questions regarding analog inputs in arduino
first of all, which im guessing its normal, but i'll ask anyways: is it normal for the values from a sensor to fluctuate randomly near a number (meaning the sensor is working, but the value goes randomly up and down)?
the second, and more important thing.
i had a basic ldr (http://www.olimex.cl/pdf/ZX-LDR.pdf) setup connected to analog input 0 . all was fine till I added a second sensor to the mix, leaving me in WTF state. Thing is, values keep affecting each other, also , it seems the analog 1 gets a lower value than the 0, and both are the same sensors, using the same resistors under the same light conditions (both are taking power from the same source and go ground to the same outlet , which I guess its the part im getting wrong)
any suggestions? I guess its a pretty basic thing but its really annoying... I read using resistors from sensor to ground solves the problem but in my case it didnt seem to do anything at all (using 300 ohm resistors, too low maybe?)

yes, i've used the search function.

thanks a lot to whoever replies, bye!

2

yes it is normal for unconnected pins to fluctuate. these pins are very high impedance and they will trigger on almost anything if unconnected to a voltage source.

Re the 300 ohm resistors, those are very low values. Try the same thing with 4.7K or 10K. Also, flip the sensors and see if the results are reproducible.

D

3

Another thing to note is that LDR's like all resistors come with a build in tolerance.
For som LDR's this tolerance is quite high, menaing that two LDR's that are the same type, and look identical, might have quite different resistances given the same amount of light.