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:

• This compass rose is not the same one that comes with the newer Montreal Doppler III.  The one described here uses the "Agrelo" formatted RS-232 serial data for display while the one for the Montreal Doppler III uses the two-wire synchronous serial port (e.g. "SPI") instead.  Also note that since I have rewritten the software for the Montreal Doppler III's compass rose display to use many of the features described below and  rewritten the software for the Montreal Doppler I, II and III units (including adding the capability of the Montreal Doppler II to drive the Doppler III's compass rose display) I have not updated the software described on this page.
• For more info on the "SPI" based compass rose mentioned above, go to the New Compass Rose Firmware page.  This page describes a similarly-functioning display with more features than the one described here.
• This is NOT an official page of VE2EMM:  Please direct questions about the contents of this page to the link at the bottom of this page.
  

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:

• Bearings with a quality of >= 7 are displayed in green
• Bearings with a quality of >=4 and <7 are displayed in yellow
• Bearings with a quality of >1 and <4 are shown in red
• Bearings with a quality <= 1 do not update the display
• A "rolling history" of the past 16 bearings (not including the current bearing) are displayed, but much more dimly to allow clear differentiation between the "current" and "past" readings.
• The "North" (top) LED is dimly lit as green while the other 3 cardinal points are dimly lit as red.

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:

• "Flipping" the display.  This would mirror the display, allowing the pelorus to be set upon the dash of a vehicle.  At night, the reflection of the LEDs in the windshield  makes for a nifty "Heads Up" display (yes, I've tried it and it does actually work - if you can keep it from sliding around.)  This is helpful if your DFing partner tends to get carsick while staring down at a display while you are driving.  Also, it can make it less dangerous if you happen to be DFing "solo" if you can see the bearing in the windshield without having to take your eyes directly off the road. (No, I'm not recommending that you do "solo" DFing, but I'm just saying that it would be nice to make it easier to do so if you had to...)
• Actually "average" the bearings within the pelorus itself.  At the moment, the update speed of the display is a direct result of how fast bearings are being spat out by the DF box:  If the averaging is turned on in the DF box, the update rate slows down.  If the pelorus itself were to do averaging, one could operate the 'EMM box at "full speed" (i.e. no averaging) and display a "sliding average" of the past "N" bearings - allowing for a relatively fast update rate and a "smooth" transition from one bearing to another.  The amount of averaging will actually be limited by the amount of RAM that the device has, of course.  If one is logging bearings, it would still be nice to have the data spitting out at  full speed - and then one would have the option of doing data reduction in later analysis.  (Remember:  It's easier to throw away extra data later than try to reconstruct it...)
• Provide adjustable brightness.  As it turns out, the available versions of "2-legged" dual-color LED (those in which the color is determined by polarity reversal) don't come in quite as high brightness of models as the "3-legged" ones (those with a "common" lead, but two other leads for selecting red and green) do.  While the 2-legged variety used in this prototype are about as bright as they come, they leave a bit to be desired if one has the display in direct sunlight.  3-legged dual-color LEDs may be used provided that one place a reverse-biased diode across each of the LED segments (leaving the "common" anode - or cathode floating.)  The only requirement is that the current limiting resistor be changed to accommodate the extra 0.6 volt drop across the diode that is "reverse biased" across the currently "unused" color.  If "killer-bright" LEDs are used, they are then too bright to be comfortably used at night - and thus a brightness control of some sort is required.  While about "20 db" of brightness range may be accomplished in software, this isn't quite enough to dim the LED enough at night and have enough range left over for a useful indication.
• Integration with an electronic compass.  I haven't quite figured out how I plan to do this:  I'll either have this processor interface with the compass, or have yet another processor do that.  If one is in a vehicle, it is helpful to have bearings relative to the front of the vehicle.  If one is logging or reporting bearings, a known reference (to true north, preferably) is required for such data to be useful. (I'm actually leaning toward construction of the "yet another outboard box" that will take the Agrelo readings and translate them to true-north reference via the serial port.)

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.

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!