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As an example, a rope tied to a tree when pulled with Y newtons would illuminate an LED but wouldn't when only pulled with X newtons.

I guess what I'm looking for is a basic force meter.

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    \$\begingroup\$ "what I'm looking for is a basic force meter." Which means you need a force sensor. Just use that search term in e.g. Digi-Key I can't tell you the rest of the circuit as that would be a design service which we do not supply here. \$\endgroup\$
    – Oldfart
    Commented Jul 14, 2019 at 19:20
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    \$\begingroup\$ A position sensor (or limit switch if you just want thresholding) with a linear spring to provide the counter force. Or a strain gauge if you want to measure the force more directly but this is much trickier. \$\endgroup\$
    – DKNguyen
    Commented Jul 14, 2019 at 19:35
  • \$\begingroup\$ a spring solidly attached to the tree ... rope attached to the other end of the spring ... an actuator attached to the junction between the rope and the spring .... the actuator operates a switch when the spring is pulled with Y newtons of force \$\endgroup\$
    – jsotola
    Commented Jul 14, 2019 at 23:45

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Not sure what your mechanical requirements are, but if you can afford some movement when the rope is pulled, go with DKNguyen's recommendation of using a linear spring with a contact at the spring deflection distance that indicates your "Y newton" point.

Green is insulation and orange is conductor (supposed to look like copper, eh) so you can trim your insulation at whatever length will give you the desired "d" distance based on the linear equation for springs:

\begin{equation} Force = Spring Constant * Distance \end{equation}

Force Meter

In actual implementation you'll want a way to reliably hold the wire against the rounded contact and prevent corrosion if left outside.

You can also measure it remotely with a water filled syringe. Attached the plunger to the rope/spring interface and the body of the syring to the fixed end of the spring. Run tubing to a safe area and use another syringe with a limit switch or an open vessel with a float switch.

Lastly, if you can't afford motion or need more accuracy, you can use a load cell. You could measure the pull of the rope directly using an S-type load cell like a cheap one scavenged from a hanging luggage scale. You could also use a flat load cell and compress it using a Class 1 lever, this would also give you flexibility to use the cheapest 1/2/5/10 kg cell you can find and adjust the lever arm for the force range you expect to measure. Either way, you could use a wheatstone bridge to set a threshold and trigger a response or use an ADC to read the load cell digitally with a microcontroller if you need to get actual numbers from the sensor.

For the variable resistance stretching cord that OP found:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ This is great, thanks. I can afford movement so could I use something like this (robotshop.com/uk/images-scientific-2inch-stretch-sensor.html) instead of the detector in your diagram? \$\endgroup\$
    – Sam
    Commented Jul 14, 2019 at 21:06
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    \$\begingroup\$ Good find, that makes the detection pretty easy and also gives you an analog value if you need to measure the force up to and past the LED illumination point. \$\endgroup\$ Commented Jul 14, 2019 at 21:29

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