Hi!
1, Raghunathan R, Sorry i didnot tell that i have to sense 400 threads. Actually i wanted to know all the possible option.
2, Terry Lingle, Each thread comes from seperate spool but spools are fixed. Also i want an idea which works with all qualities/colors of threads. So, far mechanically moving a pully to sense a thread movement is cheap and may work for all which i understand from your last post. Still i didnot get the picture of setup which you was talking about. If you are using 2 or 3 pair of light emitters and sensors to detect the movement of pully, just like in the encoders found at the back of heavy induction motors found in industry then it is still applicable.
3,Kai Klaas, Your idea is also very good but i am not sure what kind of results it will give when i use it on black color thread. Perhaps i should also experiment with this kind of setup.
4, Fred Flintstone, Your idea seems to get best possible results but it requires to hire 400 Fred Flintstones to see the trajectory on 400 diffrent threads. And also it need a large quantity of 8051s.