Azhar: Mikes picture showes the basic idea but for 400 threads you need 1200 pulleys and assuming that you can group the moving pulleys say 25 as a set that trip one flag sensor you 400/25=16 flag sensors this would work and be cost effective. if you need to identify the exact thread then cost and complexity will rise. The slack boxes that I made were multiple pass versions like a rope block they prevented breaks by keeping the tension on the product constant at the weight of the moving side divided by the number of moving pulleys. No springs that change as they extend or retract. The product was kept in the active regeion by motor drives on the input and output. a driven system is neither practical or needed for your project because the threads must already have some form of tension control. Terry