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I want a sensor to measure the thread length , how much it is withdraw from a retractor. retractor can rotate both clock and anticlock wise, its if u pull the thread it will come out from the retractor , if u leave the thread it automatically return to the retractor, so i want to measure how much length of the thread withdraw from the retractor, with high level accuracy,

and it the measurement device must to be small if it so its better .pls

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You probably should edit the question and add the minimum and maximum distance you wish to measure. Plus an idea of the maximum acceptable size for the sensor would be useful. Maybe you also have a photo of the device you can add? – PeterJ Feb 20 '13 at 4:52
Kinda hard for anyone to help without more specific info. "small" and "high level accuracy" don't mean much until you state some numbers. Can you put up a photo of this "retractor"? – DarenW Feb 20 '13 at 4:57

In theory, you can use a rotary encoder to measure the length of thread withdrawn from the retractor.

The encoder needs to be interfaced to a microcontroller and the microcontroller can deliver various kinds of outputs including, graphic LCD, char LCD, 7segment displays, etc.

The device can then be calibrated by measuring the pulses outputted by the encoder vs the actual length of thread withdrawn from the retractor.

There can be other ways to achieve this and passing by experts will be chiming in after this :)

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You can purchase a string potentiometer (or just "string pot") commercially. There are many models for different applications.

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