robotic sensor for green line detection

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

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.

--
  /-----------( Brendan Gillatt )----------\
<---( http://www.brendangillatt.co.uk  )---->
  \---(brendan[at]brendangillatt.co.uk )---/

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

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