I have a photo cell at the end of about 50 feet of wire. Its not working to well the values don't change as dramatically as they did when I used a foot of wire. Does it make a difference if the pull down resistor is at the arduino end of the wire now its at the photo cell end of the wire
Thanks
Hi,
The wire will have some resistance and that will affect the reading, but I would not think it should be dramatic. Can you post a schematic? How many cores does this wire have (I'm guessing 3 given your description).
Paul
You really need - as so often happens here - to describe your situation.
What is the wire? Where is the wire? What is the purpose of your arrangement? What is the value of the pulldown resistor? Why are you using a pull-down rather than normal engineering practice of having a pull-up resistor and your LDR to ground - or do you actually mean a photo-voltaic cell? What is the specification of the cell?
Where are the pictures of your setup? (Focused and size no greater than 1024 by 768.)
Paul__B:
normal engineering practice of having a pull-up resistor and your LDR to ground
Why is that, by the way?
PaulRB:
Paul__B:
normal engineering practice of having a pull-up resistor and your LDR to groundWhy is that, by the way?
Paul__B explains in Reply#8 of this thread.
The wire is 2 pair phone wire, the wire is running from the basement up one floor into the attic and then outside. At the photocell there is a 10 k resistor between the ground and the side of the cell going back to the arduino. The sketch and cell worked fine when I had it 10 feet from the arduino. So I'm guessing from the replies it does not matter if the resistor is at the arduino or the cell end of the wire.
JimboZA:
Paul__B explains in Reply#8 of this thread.
Thanks, that makes obvious sense.
Trickyrick:
The wire is 2 pair phone wire, the wire is running from the basement up one floor into the attic and then outside. At the photocell there is a 10 k resistor between the ground and the side of the cell going back to the arduino. The sketch and cell worked fine when I had it 10 feet from the arduino. So I'm guessing from the replies it does not matter if the resistor is at the arduino or the cell end of the wire.
another word for 50 feet of wire is ... antenna
4 conductor, phone wire is not usually shielded.
if your reading go hay-wire when the neighbor mows the lawn, you can figure out that a lot of smoothing will be needed.
PaulRB:
Thanks, that makes obvious sense.
But it seems we will not be getting any more relevant detail from the OP. 
Trickyrick:
The wire is 2 pair phone wire, the wire is running from the basement up one floor into the attic and then outside. At the photocell there is a 10 k resistor between the ground and the side of the cell going back to the arduino.
You need to tell us what the 'photocell' is, because it is ambiguous.
You also need to draw your current circuit. At the moment it sounds as if you have a resistor in series with the photocell, which doesn't seem sensible.
Take a pic of the photocell and post it. The forum may be able to identify it.
Not entirely sure what I'm looking at there! What is that large item at the bottom with the conical cover and the metal bracket?
However, on the small board with the relay at the top of the picture, I can see a component standing up from the board with insulation on its legs and a square-shaped zig-zag pattern on the top. Is that what you mean by "photo sensor"? If so, its exactly what I expected, what's known as an LDR (light-dependant resistor).
Connect the LDR to a multimeter set to measure resistance, then get the it into as dark a place as you can find, perhaps wrap it in some thick cloth. What resistance do you measure? Then hold it right next to a switched on light bulb or point it towards the full sun. What do you get then?
Trickyrick:
Ok thanks
here is the pic
The original circuit is of no interest - it's the component you removed from it we're interested in. Can you get any pictures showing more detail of the component?
OK next to a 60 watt light bulb i get about 220 ohms
completely dark I read nothing (infinity)
Please try to answer all our questions. We dont ask them because we enjoy typing. Was I right in my description of the item you are calling "light sensor" in my last post?
Your readings do sound like an ldr though. Did they drop quite smoothly as you moved it away from the light? I'm surprised you could not measure the dark resistance, even on the highest range on your multimeter? Most ldrs have a dark resistance of a few Mega Ohms, but some are 20M, which would be hard to measure.
Anyway, I'm convinced it is an ldr. So try the suggestions given earlier. Change your circuit to have a pull up fixed resistor at the Arduino and the ldr to ground. Thus you need only 2 conductors to connect the remote ldr. This means you can double-up the cores in your cable, halving its resistance. This should reduce its influence on the reading (which I'm still surprised was so noticeable - could the cable be damaged?)
Ok thanks PaulRB. You are right on did what you said and I got it working. I moved the resistor from the end of the wire to the arduino. Not sure y. There is no problem with the wire at least no shorts between conductors anyway.
I do have another question about photo cells.
I have a second one the surface of the cell is allot bigger in diameter. Does this mean that its better at sensing less light.
What would be the difference
Thanks
Rick
Its possible. Take a look on this page:
http://www.rapidonline.com/FFSearchResults.aspx?query=ldr&filterSearchScope=1
As well as the "ordinary" ldrs, you can see a range from a specialist manufacturer that all look identical, but have different ratings. Imagine a graph of light level versus resistance. An LDR has a certain curve, and there is a point on that where a small change in light level gives the largest change in resistance. The different models have that point in a different place, I think. So, yes, some LDRs are more sensitive (meaning a larger change in resistance to a small change in light) at low light levels, others at high light levels, some in the middle. But I doubt you can tell by just its physical size.
As an experiment, you could measure the resistance of both ldrs at 1cm, 2cm, 4cm, 8cm.... 64cm... 256cm away from the same light bulb, and plot the values on a graph (paper or Excel). The light level will be falling away corresponding to the distance squared.
Thanks Paul