The simple pully arrangement could be built quite inexpensively as shown in the picture below. I show the thread in red and it traverses in a horizontal manner over the top side of the lower two pullies. These two pullies are mounted in a fixed position on the bracket assembly. The third pully can move up and down freely in a slotted hole in the bracket and is set to ride on top of the thread. Tension in the thread would keep the movable pully toward the upper end of the slotted hole. The material used to make the movable pully can be selected so that it is compatible with the heft of the string and the tension. If the movable pully is relatively thin it can be arranged so that it can slide down into the slot of an off-the-shelf slotted aperture optical detector. This can be mounted below, with an adjustable hole so that it can be positioned so that the edge of the movable pully will break the beam at loss of tension or passage of thread through the pully system.



I think this arrangement is much like one someone else tried to describe earlier in this thread, but I thought the picture could get across just how simple this could be. (Note I would make sure to have the movable pully be made of an opaque material so the slotted aperture optical interrupter works correctly.

Michael Karas