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I was thinking of attaching a sensor to the rope but there are so many types I'm not sure which one to use.

Pizeoelectric, rolling ball, vibration, tilt and tension sensors could all work.

I need the lowest cost, most durable solution. Anyone have any ideas here?

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    \$\begingroup\$ Can you further describe your application? What is "the rope"? What is the rope connected to? \$\endgroup\$
    – Samuel
    Nov 20, 2012 at 21:04
  • \$\begingroup\$ Do you mean a rope lying in a pile? One that's tightly strung between two posts? Why? \$\endgroup\$
    – Tony Ennis
    Nov 20, 2012 at 21:05
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    \$\begingroup\$ Sorry, yes a rope strung between two posts. I could add a spring to one side to increase the vibration sensitivity and duration as well. This is for a kids game. They have to maneuver around the ropes, touching one sets off a buzzer. \$\endgroup\$
    – Thom
    Nov 20, 2012 at 21:14
  • \$\begingroup\$ Sensitivity and environmental noise needs to be defined; wind, sun, music, shouting, touching the post. Blocking false positives & detection accuracy is what adds cost, which is why I suggested a tiny bell tied to middle of the rope. Sensing is cheap. \$\endgroup\$ Nov 21, 2012 at 18:38

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I'd recommend an opposed-beam optical sensor. PCB-mount varieties can be purchased for around $0.25 in low volumes. Use an extension spring in series with one end of the rope to provide a little compliance (to make it kid proof, keep a length of rope in parallel with the spring so that the spring can only be stretched so far until the rope takes the load).

Use a "flag" or other object attached to the spring to block the sensor when the rope isn't being touched/pulled, and not block the sensor when the spring is stretched a little bit due to rope tension. You'll need to work out a mechanical solution so that the spring doesn't rotate, which would prevent the flag from returning to its position in the middle of the sensor when the rope isn't being touched.

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  • \$\begingroup\$ Dean, How would you adjust the sensitivity? I don't want kids to bump the wall and set off the sensors. I also don't want kids bumping the rope without the sensor going off either. Thoughts? \$\endgroup\$
    – Thom
    Nov 21, 2012 at 22:35
  • \$\begingroup\$ The coarse way to set sensitivity would be to adjust the spring rate. You'd probably do this once and then settle on a particular spring. The fine way to set sensitivity would be to adjust the size of the flag or the position of the flag relative to the sensor such that a certain force on the rope is required for the flag to leave the sensor. This could be accomplished by making the sensor axial position adjustable relative to the spring+flag. \$\endgroup\$
    – HikeOnPast
    Nov 21, 2012 at 22:37
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The easiest thing to use is a proximity sensor. These are commonly used as safety mechanisms on machines. You will need to use thin copper wires instead of a string and isolate them from ground. Anyone touching the wire would cause the system to trip. These are frequently used in security to protect products on shelves.

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