[...] the objects are of two types ,one is white and the other is Grey and the robot needs to knock down the grey ones.so i m going to use ir proximity sensor for identifying objects and i m planning to use a color sensor to identify the type of the object.so is it foolproof to use color sensor to differentiate.
White and grey are the same "color"!
If you have an opportunity to "read" one of each immediately before your run, you could use a LED + light detector setup to find the amount of "greyness" (light reflected) and differentiate from a percentile median calculated from that.
It would be so much easier to detect the difference between eg. red and green (still without a color sensor).
i had a link of a tutorial on the PCB layout of the 50$ board.it will be gr8 help if someone can give the link of it .i can't somehow find it( don't know why)
Perhaps the one by Webbot in this post: http://www.societyofrobots.com/robotforum/index.php?topic=3292.msg25198#msg25198
i hav also thought of mechanism of knocking down the objects.
What shape, size, weight, material etc. are the objects?
[...] instead of gears i will use velcro.
Sounds a bit weird to me, but whatever works...