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How long a wire can i connect to a LDR before the data i am receiving is irrelevant? and what should i do in such a case
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OK, first of there is no definitive answer to this as there are so many variables.
But first off I would say 2', which is about as meaningful as your question.

Best to say how long you want to send the data at then see what you have to do to get it.
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i want to send it 100m! i know im being stupid thinking i can use a single wire, i would just like to know what type of aplifier to use
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It is not a question of just amplifying it, all that does is to increase the amplitude, this will do nothing to get over the interference and line capacitance.
 
With that sort of distance you need to look at different techniques. One way is to feed the LDR into a timer like a 555 and have that produce a pulse train who's width is proportional to the reading. Then transmit that using a digital driver, perhaps even a differential driver. At the other end have a differential receiver and measure the input pulse.

As I said that is just one way to do it there are lots of others.
« Last Edit: June 16, 2009, 02:40:14 am by Grumpy_Mike » Logged

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Ciao,
the solution I suggest is to place Arduino near the sensor,
so you don't introduce noise by running long cables,
and then use a usb-to-cat5 extender,
that can run up to 50-100m without noise problems.

The only downfall is that with such a long distance you can have a little delay,
you just have to think if it's a critical parameter for your application.

For an installation a couple of years ago I used these Lindy USB-to-CAT5 extenders, they're rated up to 50 meters,
but they say that for low-traffic devices you MAY achieve up to 100m distance. They cost around 50€, I found them for 10€ on ebay and they worked PERFECTLY.

You just need to power the "remote extender" from the arduino side with 5V,
then plug the USB from arduino into the "remote extender",
connect a 50meters LAN cable to it,
and at the other side you just get back from LAN to USB with the "local extender".
Really easy, cheap and stable.

Here's the link:
http://www.lindy.co.uk/usb-extension-cat5-usb-extender-premium-up-to-50m/42801.html

And if you google "usb to cat5 100m" you will find other options:
http://www.blackbox.com/Store/Detail.aspx/Single-USB-rarr-CAT5-Extender-100-Meters/IC245A-R2

These up to 100m can even cost 500€,
depending on how much money the manufacturer wants to steal you,
as they're quite simple circuits...

Hope this helps,
bless,
kk
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If you're going to pull Cat5,  why not just use a serial-based Arduino,  and put an RS-422 interface on it?

But,  if there's no need for an Arduino to actually do anything out at the sensor end,  Mike's idea of a simple "digitizer" like a 555,  feeding an RS-422 transmitter chip,  is very cheap,  and should be much more reliable than trying to get a USB interface working.

Ran
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Yes LAN-duino would be TECHNICALLY the best solution,
though judging from the number of posts
I suppose Apophis have not enough experience to build such s system, that is not difficult at all for an expert but that can be reall frustrating for a novice user... that's why I suggested a ready-made solution, though obviously it's much more expensive...

Another overcomplicated but easier-to-accomplish solution be to use a laptop or netbook to make signal acquisition near the sensors and then send the data via WIFI using a protocol like OSC or whatever-you-prefer. Really easy to do with Processing or PD/MaxMsp.

I definetely would not suggest the BLUETOOTH Arduino,
as they are a nightmare as far as signal stability is concerned,
and doubt they can go up to 100m.

Just my 0.0002 cents,
bless,
kk
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Hi,

You can add RS485 to your arduino for only 1 euro=dollar with a 75176 trasnceiver. You can achieve long distances.

http://www.maxim-ic.com/appnotes.cfm?appnote_number=763&CMP=WP-1



You can follow this link with an example => http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1245694725


Regards,


Igor R.
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Quote
You can add RS485 to your arduino for only 1 euro=dollar with a 75176 trasnceiver

So how is this going to transmit the output from an LDR sensor then.  :-?
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You can install the arduino near the sensor and send the info with RS485 bus to computer or other Arduino.

It´s easy to build USB-RS485 converter for the PC with a FTDI chip. It has a TXDEN pin to enable Tx/Rx automatically. Or you can use an MAX232 + 75176 and control Tx/Rx with RTS.

I´m not sure what exactly is the application, this is only an idea.  smiley-wink


Igor R.


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