The 'Tension Switch' is device that grew out of the need to detect when a high altitude latex balloon had burst and the payload was falling. It was used to cut away the balloon shards and activate a freefall experiment. Devices of this type had been built before, and used successfully, but the time and equipment required was beyond the schedule and budget available. This device was assembled from inexpensive parts from the local hardware store with minimal machining required.
Basically, a small microswitch is mounted in the center section of a turnbuckle (with the left hand threaded eyebolt cut off). The Right hand threaded eyebolt is filed or ground down and the hole widened so it can move freely in the turnbuckle. A locking nut and spring finish the device so when tension is removed between the two eyes, the free eyebolt hits the microswitch. See the attached figure and pictures.
The unit works reliably, although adding a little heat shrink tubing on the sliding eyebolt might make it better, or cause it to stick. Adjustment of the required tension to hold the switch off, can be made by switching out the spring and/or tightening or loosening the locknut.
There may be a small 4.7kOhm resistor as a pull up/down for computer input. Removing the center conductor on the bottom of the microswitch makes it fit better.
The old part numbers are on the diagram, but the prices have all changed by now anyway. I wish I could find the original word file to fix the typos in the pdf.
Last Revised: 18MAY2015