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how to make a tripwire thats not a tripwire?

hello everyone its been a long time since i have asked a question here but with curiosity staring me in the face i have no choice... so as the question title states im trying to make a tripwire thats not a tripwire and what i mean by that is im trying to make a type of sensor that you pull the string from it (which when you let go it will retract) and as soon as you pull the string the sensor turns on and knows how far the string is out...  im hoping to make this part of bigger project and this is definitely a big part of the project but i dont generally make things myself even though i want to... im hoping to get a small program that will just register the sensor since it will be connected to the computer and program while its on ill need to program many of them for different things but the main issue is just telling it what to do when a certain amount of the string is out or put back in from retracting... you know? it needs to be precise aswell... as in "sensor 1 pulled 1 inch out" (brain then moves piece 1) "sensor 2 pulled 8 inches and sensor one retracted 1 inch"  (brain moves piece 2 a few inches and piece one back 1 inch) do you see what i mean? it needs to be precise and all the sensors need to work together in one big network... there will be probably... 100 or more sensors working with there strings in a huge layout and i just when it comes to the sensor i dont know where to start or how to do this do i use Arduino some other parts and thinner paracord? you know so if anyone could help me out with this or what they would do that would be awesome

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iceng1 month ago

Wind the string on a pulley and use a low cost shaft encoder. Now the pulley diameter will set the resolution to whatever pleases you and the micro can keep track....

If this needs more turns then a spring can handle or another string wound on shaft with a weight to return and take up slack... Try using a small toy motor at 1/4 voltage to rewind..

iceng iceng1 month ago
Rotary-encoder-kit_1.jpg
seandogue1 month ago

for precise scientific metrics, you could use LVDTs, but it'd be costly.

ShinigamiTentetsutou (author)  seandogue1 month ago

well i mean even if its as basic as a extended range dog leash that retracts i just need basically a sensor that knows when the string was pulled how much and how much of the string it retracted...to operate occordingly

bwrussell1 month ago

I'm not sure what this is officially called but you need some sort of constant spring-force (this type of spring might not be necessary but regardless you will need
all the springs to be identical if distance pulled is a critical measurement) linear motion sensor.