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Telescope mounted KA7OEI Optical Receiver

The following describes an optical communications receiver project initiated by a small group of Knights inspired by one of our most recent members (Sir Rye, K9LCJ). As a result, we can now proclaim that we're involved in everything from DC to light.

Sir Rye provided a demonstration of his LED communications system when we were first introduced at our August, 2008 QRPigOut. He described some of the work and communications achievements that have been published. He described how he was inspired by a group of hams in Tasmania and then provided his choice list of  websites to visit, and we were suddenly swept off toward another fun and challenging adventure.

Sir Rye provided the following list of reference url's which I'm posting here as must review sites since they continue to provide much useful and valuable insight and guidance for us. In Sir Rye's own words:

There are about five pivotal web sites for this optical/LED comms stuff:

 The  Radio and Electronics Club of Southern Tasmania is the most interesting and their activities are  quite well documented.  This site takes days to wade through.

 Clint KA7AOI holds the US (world??) record for  LED comms of 173 miles.  The linked page has a couple of interesting pictures.  One shows a LED emitter from 14 miles and there are a couple of comparison pictures of LED versus LASER signals showing the fading characteristics of each.    There are links to a bunch of good things at the bottom of this page including Understanding the Performance of Free Space Optics,   Decibels versus Dollar that I mentioned last night (presented by the VK7s), and a whole bunch of other really interesting stuff.  Clint seems to be another of the hyperactive hams to which we all aspire.

 F1AVY Yves  has a really nice web presence, but it’s a little hard to navigate.  It appears as if he has been messing around with this stuff for a long time and has a good background in optics etc.  His focus  seems to be more on LASERs.  He has a good presentation on Cloud Bounce as well as a lot of other activities centered on optical comms.  The graphs of path loss are quite exciting.

 W1VLF did (does??) some interesting cloud bounce stuff using massive arrays of smallish  IR LEDs.  I think this was all before the advent of the big visible light LUXEON diodes. 

 Alberto I1PHD is the author of the SPECTRAN software that I demonstrated last night.  He’s another one of those fine guys who can’t seem to stop producing really good technical tools for us Hams. 

 There are links on these pages that reach out to almost everything else on the topic.  It is interesting to note that most of this work is being done overseas.  I was glad to find the KnightLites as you guys seem to be one of the last bastions of Ham homebrew in the area.  The TAR robotics guys also seem to be into soldering iron burns…

It wasn't long before we realized that this was a convergence of our shared interests in QRP, and amateur astronomy, and the hook was set. As we shared ideas and insights, we discovered that we shared other interests as well... such as circuit board building processes. That led us to a recent revelation that Staples makes the best darn paper a PC board maker would ever want. The story is long to be told, but the path to where we are today has been fast and furious, and that brings this discussion to where my story begins.

I am easily inspired by the enthusiasm and excitement of others, and can't resist getting myself involved. That was the lure that drew me into this project. Sir Chris (KD4PBJ) first met Sir Rye at the Cary hamfest in July, and learned that he was in search of a group of spirited hams that still liked construction projects. Sir Chris instantly recognized that he was one of us, and wasted no time inviting him to join us at our August QRPigOut. In the meantime; he was becoming intensely integrated in the work that Sir Rye was doing, and enthusiastically sharing his insight with me. Again the story becomes long, but soon I was learning about high power LEDs, optical communications systems, experiments, and communications link records. That quickly led to my own pursuit of a system, and with a slight nudge from Sir Rye, I found myself gathering parts for a system of my own. Having not yet acquired any optical hardware, I suggested that I use my telescope for the task, and decided to start with a receiver. After a brief period of brainstorming and digging through my junque... or rather "jewel" boxes, I discovered that a copper plumbing coupler was a perfect fit into the eyepiece holder of my telescope, and the idea of  an optical eyepiece was born.

Once again, with Sir Rye's enthusiastic influence, I was introduced to Cadsoft's EAGLE Light Freeware PCB layout software. I learned that Sir Bob (AE4IC) was already consumed in using it, and also on track toward building an optical system as well. Sir Bob encouraged me to give it a try, and that the learning curve would go quickly. Indeed; all that was being shared was just as advertised. Within a day I had one of  KA7OEI's optical  receiver schematic entered, and was manipulating the parts placement on the board. Within 3 days, I had the layout completed and I was ready to etch a board.

Oh yes... Did I fail to mention that we needed to somehow get that nice EAGLE Light file onto a circuit board? Well... That's another fun part of this project... That story is long as well, and all the important details can be found at:

We (Sirs, Chris, Rye, Bob, and myself) took nothing at face value, and its in none of our personalities to delegate such matters as proving a concept or that a process works as advertised, so we all set off independently to pursue improving the process. We all failed miserably on that agenda, but much to our delight, we all came to the same crisp conclusion that the process is well polished, is easy to reproduce, and is cheap and efficient. We even compared two different etchants, and found both equally safe to work with, and easy to use.

Our conclusions on the board photo resist process we investigated and now unanimously endorse are once again best summarized in Sir Rye's own words:

Upon looking at Tom Goottee’s web page (which is where this all started) I find that it has everything you need to know about the process in well written detail.   In many respects we just reinvented the wheel, but it was a good proof of concept effort.

About the only thing that I have found that probably isn't emphasized enough on Gootee’s page is that you have to use a good copier.  Some of the smaller office type machines don’t give good results.  Staples, Office Depot, and Office Max all have big Xerox copiers that do a good job of laying down toner.

Gootee emphasizes that “it’s the paper” and I believe we have proved that again.  Use the Staples Paper unless you want to waste a lot of time before you finally go to Staples and get the right stuff….

 Staples Photo Basic Gloss for Inkjet printers  Item #648181  

Accept no substitutes!

The etchants we compared and agree are equally safe, cheap and effective are:

Ferric Chloride
Muriatic Acid and Hydrogen Peroxide:

Its important to note that obtaining Ferric Chloride may be a tad more difficult, and should probably make it your second choice if you're just starting to consider producing your own high quality printed circuit boards.
The history and technical details for the KA7OEI optical receiver I adopted for this project can be found at the following urls:
Discussion:  http://www.modulatedlight.org/optical_comms/optical_rx1.html 
Schematic:   http://www.modulatedlight.org/optical_comms/ka7oei_optical_rx_ver310.gif

The receiver shown in the following photographs is an electrical clone of the Simplified Version of KA7OEI's "Version 3" Optical Receiver. I abandoned his reverse voltage power protection scheme in favor of my own less sophisticated approach however, by eliminating his shunt diode (D1) and series self-resetting thermal fuse (TH1), and inserting a series connected 1N4148 diode (anode to battery positive terminal) to achieve the intended result.

I also substituted an MPF-102 on the front-end in lieu of the 2N5457  identified in his schematic, and 2N3904's for both of his MPSA18 NPN transistors. I also used 47 uF for all of the electrolytic bypass capacitors throughout with the exception of C4. I actually used a much larger value of 100 uF for C4 whereas this is much smaller (2.2 uF) in his schematic. I would have to say that changing C4's value so dramatically is the most significant deviation I made from his published design, but it hasn't surfaced as an issue so far, and I'm aware of this difference if I discover an elusive anamoly down the road. Since this serves as a DC bias current bypass, I suspect the impact  is principally excessive dampening of the DC bias response time due to large changes in signal interference levels.

I entered the schematic into the free version of the EAGLE PCB layout program, and designed a board which would allow me to insert as an "eyepiece" into my telescope with the detector diode optimally centered and positioned at the focal point of the primary lens.

The schematic shown here is what I produced in EAGLE Light:

Revised KA7OEI Schematic

...  and the EAGLE layout editor enabled me to effortlessly design the following PC board layout:

Telescope mountable Optical Rx board


This schematic (and layout) shown here are both in error (the inputs of IC1A are reverse connected). Refer to KA7OEI's schematic instead... at least until this disclaimer is removed. Its an easy cut and jumper fix if you leaped into this with exuberance and anxious enthusiasm before reading this, but it just won't look quite as pretty. 

The following is a photo of the inside of the Altoids tin used to house the 9V battery, The lid serves to contain the battery and doubles as an additional EMI shield to protect the otherwise exposed power and output signal wiring.

OEI Rx rear view

The following shows the completed receiver assembly with its lens cover fashioned from a pill bottle. The pill bottle has a slot cut into one side that enables the o-ring shown to function as a keeper that prevents the lens cover from falling off easily. The circuit board is completely housed and mounted inside the dual diameter copper coaxial assembly, with the narrow end inserted into the smaller coaxial section that was fashioned from a standard 3/4 inch copper tubing coupler used in house plumbing applications. The O.D. of the coupler serendipitously matches the standardized 0.96" eyepiece diameter, making it a perfect match for this receiver.

OEI Rx with Lens cap

The following reveals the "business end" of the receiver assembly which becomes the "lens" that gets inserted in the eyepiece holder of the telescope. Thankfully, the detector diode is easily seen in this photo. I was reluctant to use my flash for this view out of fear of damaging the detector. I wasn't going to take that risk for a photo shoot, and I'm glad the results without a flash came out so well.

OEI Rx detector view

The following photo shows where the receiver interfaces with the outside world. There are several noteworthy points to observe in this photograph that are obvious, but perhaps not immediately apparent:

    1. The gain control switch is mounted such that its wiring remains inside and never exits the coaxial receiver chamber.
    2. The gain control switch is installed within and thus shielded by the Altoids tin when the lid is closed.
    3. The battery used to power the receiver is contained within (and under the cover) the Altoids tin when operating.
    4. 4 wires (battery +, battery -, Audio, and ground) enter/exit the receiver through a single 1/4 inch hole in the bottom of the Altoids tin.

The boundary between the receiver and the outside world provided by maximizing the integrity of the Altoids bottom, compartmentalizing the power system, and minimizing the lead lengths of the essential power and signal output leads entering/exiting the coaxial receiver chamber, results in a receiver that is nearly perfectly immune to EMI. 
OEI Rx wiring view

The following shows where one might find light at the end of the tunnel.

OEI Rx in Telescope

The finished telescope mountable version of KA7OEI's optical receiver is now mounted and ready for service.