An add-on RS-232 driven Pelorus Display
("Compass Rose")

Note:  While this page refers to the use of this display with the Montreal II doppler DF unit, it may used with practically any RDF unit that outputs a continous stream of bearing/quality information in the "Agrelo" (tm) format.

Additional notes:

A view of the 32-LED dual-color LED Pelorus.
A front view of the prototype pelorus "Compass Rose" display displaying an inputted bearing of 16 degrees.
Click on the image for a larger version.

Further discussion of the antenna drive circuit and the antenna arrays usable with this system may be found on the Doppler Antenna page at this web site.

A Pelorus (or "Compass Rose") display:

If you plan to do mobile operation, perhaps one of the most important accessories would be a compass rose (also called a pelorus)  for the faux pelorus on the LCD is not easy to use.  For the reasons mentioned above, it provides an intuitive display that indicates the bearing of the signal relative to the vehicle.  Several such pelorusus * have been devised - including one shown at the WB6EYV web site.  Pelorus displays typically consist of 16, 32 or even 36 LED arranged in a circle providing a compass-like indication of the bearing.  Not wanting to feel left out, I designed a similar pelorus as well.

The first question was "How many LEDs should it have?"  Well, 16 seemed like a bit too few and 64 was simply too many.  32 or 36 LEDs seemed like a good number.  In mobile use, it probably gives a false sense of the accuracy of the device - but one can always ignore a little extra information in this case.  The question was then "32 or 36 LEDs?"  36 LEDs seems like a good idea, as that means that each LED represents 10 degrees.  32 degrees represents 11.25 degree increments - which seems like an odd number until one realizes that the traditional points of the compass are arranged as the power of 2 - and with 36 LED's you can't have one LED indicating "Northwest" or "Southeast."  For whatever reason, I used 32 LEDs.

The next question was "What color of LEDs to use?"  Often a pelorus is built so that the "North" LED is a different color to provide orientation in the dark, but the rest of them are of the same color - often red.  I decided to take a different approach:  Use Bi-Color LEDs.

The use of the 2-color LEDs (Red/Green) actually allows three colors to be displayed:  Red, Green, and Yellow (Red+Green.)  Using the 2-lead bi-color LEDs allowed all three colors to be displayed - without complicating the circuitry:  The color is selected by the application of the appropriate polarity to the LED while Yellow is "created" by throwing AC (generated by alternating the polarity) at the LED.  Since the pelorus uses a microprocessor as well, this was something that could be easily handled in software.

Having 3 colors to choose from, what does one do with it, then?  The most obvious answer is to indicate the quality of the bearing:  Green is a "good" quality bearing, Red would be a "poor" quality, and Yellow would be somewhere in between.

Being under software control also allows the brightness of the LED to be determined.  This, too, could be used to tell something about the nature of the bearing - or past bearings.

While the software for the pelorus is still undergoing changes, the prototype (shown in the pictures) it currently works in the following manner:

The circuit-side of the Pelorus prototype.
This is the circuit of the prototype pelorus.  The microprocessor (a PIC16F876A) is the large IC.  The empty socket is for an MAX232 interface IC which was not installed (yet) for the prototype.
Click on the image for a larger version.

The effect of all of this is that one can see a history of the past several readings - and still see the current reading (it is the brightest.)  A side-effect is that if the past 16 readings dwell on just a few LEDs (that is, the bearing is fairly steady) then the brightness of those 16 "history" LED indications is cumulative.  That is, if 8 of the immediate past readings are on a single LED, it will consequently be brighter.  Furthermore, the color is also cumulative in that, for example, if half of those 8 past readings were shown in red - and the other half in green, the result would be yellow.

To be certain, this isn't the "final" version of the software.  A few more ideas to try.  Note that some of these have already been integrated into new software for the Montreal Doppler III compass display:

Because this pelorus is a "work in progress" I don't yet have a finalized schematic.  If you are really interested in it, contact me at the email address at the bottom of the page.

Note:  Neither the author or UARC officially endorse any vendors mentioned above.  The level and satisfaction of performance of any of the above circuits is largely based on the skill and experience of the operator.  Your mileage may vary.

Do you have any questions on this or other DF-related topics?  Go here.

Return to the KA7OEI ARDF Page.

This page updated on 20050228

Note:  This page (and other pages on this site) are not "official" pages of VE2EMM.  These pages are simply set up to aid those who have built or might build the described equipment.

Yes, the plural of "pelorus" is "peloruses" - rather than "pelori."  Look it up if you don't believe me!

Since 12/2010: