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    Wily Widget[Issue 1, September 2009]                                                                      Previous Issues     

Wave Length Indicator

Wily was in his back yard trying to get his beagle “Gizmo” to stop barking at a squirrel running along the fence when his cordless phone rang. 

The call was from a company, which installed optical fiber communications systems.  The caller described a simple battery powered device they needed designed and built for them.  The company installed several types of optical fiber systems and needed a quick way to spot check to see if a fiber cable was active with light and determine which one of 4 wavelengths was being used. 

Wily said he thought he could build such a device.  They agreed on a rough price for the work and Wily went to his basement lab to get started on the project.  Thus began the Optical Fiber Wavelength Indicator project.

The first thing Wily had to do was find some suitable light detectors.  As the drawing below illustrates, silicon detectors only worked to about 950n. The company wanted to detect an optical fiber carrying four different wavelengths. They wanted to detect 850nm, 1300nm, 1550nm and 1625nm.  Wily thought that with a bit of calibration, a single type of Germanium photodiode would be able to cover all the wavelengths. 

The concept was to have four germanium photodiodes along the front of a suitable plastic box.  Each photodiode would have a narrow band optical filter placed in front so it would respond only to light in that specific narrow wavelength.  A comparator circuit would monitor the light from each photodiode and light a simple LED and sound a beeper if the light from an optical fiber matched the selected band.  The user would just have to pass the end of the fiber over the four detectors to determine an active fiber and the wavelength of light being used.  Wily thought the whole circuit could be powered by 3 volts from two AA cells.  Wily ordered the necessary parts and when they arrived he built the device shown below.  It worked as expected and was shipped off to his client.  The client was very pleased with its operation.

Click Here to View the Schematic

Wily Widget, the Lone Inventor    Gadget & Gizmo

Issue #1:

September 2009  First Issue

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Good Idea
gone Badly
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What the World
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Wily Widget


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