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Optical Fiber Pressure & Motion Test Circuit
By: Dave Johnson

Optical fibers have been used for detecting motion and pressure for quite a few decades now.  If you would like to experiment with this interesting technique, you can use the circuit below.

The circuit uses an off-the-shelf bright red LED light source, designed for plastic optical fibers.  It also uses a matching phototransistor light detector.  The devices are designed for plastic fiber.  I would suggest you keep the total fiber length less than 50 feet.  Bending or stepping on the fiber will produce a detectable output signal.
This method relies on the phenomenon of “microbending.”  Light is routed through the fiber more efficiently when the fiber is perfectly straight.  When it is forced to move through small angles, a bit of the light is lost through the sides.  When the fiber cable is moved or when pressure is applied, small changes in the intensity of the light detected can be observed.  This technique has been used to monitor ground motion caused by humans and vehicles.

The circuit is powered by a +5v source.  A single resistor, R1, controls the LED current. The Amplifier circuit has a low DC gain but a high X100 gain to AC signals.


Please send comments to me

January 2011

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