robotic sensor for green line detection

I want to build a simple mobile robot which can follow the green line (the floor is white).
what kind of sensor is well apply with this task by condition that the
light will can change by environment but not much.
thanks in advance
ps Is there has any useful information like www ?
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a wrote:

Is there a reason why the line has to be green? It would be a little easier to follow a black line (higher contrast). How dark is the green? If it's dark enough you can use a couple of photo resisters looking down at the line and measure the change in resistance with light reflecting (or not) from the floor. I suppose you could put a colored filter in front of the photo resisters to darken the green line, but I have never tried that. it might filter out too much light. You want pretty consistent lighting in general. Also, a wide opaque stripe, say 2-3 inches works well...depends on how close to the floor you can get with the photo resisters, how far apart they are, etc. The further away they are from the floor, the more likely it is that they will capture unwanted light reflection. There's a trade off, too close and no reflection, too far and too much reflection....you'll have to experiment a little with whatever parts you use. There are other types of line following techniques, but this one is pretty simple. Have fun.
Shawn
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a wrote:

You could use RED led's and photo transistors. The red would make the green line look black to the photo transistors and so would have a large contrast. There are lots of data sheets around for various LEDs and photo transistors. Try to match the spectral response of the photo transistors as closely as possible to the LEDs to get a good read.
Another possibility would be to use a butchered barcode scanner. Although quite expensive it provides a nice RS232 interface suitable for a PDA controller with suitable software. This way would provide a much higher level of accuracy because the software could match the line to its position of the line on the sensor compared to matching it with found or not found.
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thank all Shawn and Brendan .I use a green line because there is a robotic competition in my little provice. This is the condition from the commitee.By the way now I really exicite to try the both way :filter and red led ,and to see the result what happen.
thank
a
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Brendan Gillatt wrote:

I was going to suggest the use of colored LEDs to control contrast, but I assumed you wanted to keep the power usage down. Maybe I assumed wrong though.
Shawn
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