An add-on RS-232
Pelorus Display ("Compass Rose")
Note: While this page refers to the use of this display
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.
This compass rose is not the same one that comes
newer Montreal Doppler III. The one described here uses the
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")
Also note that since I have rewritten the software for the Montreal
III's compass rose display to use many of the features described below andrewritten
the software for the Montreal Doppler I, II and III units
adding the capability of the Montreal Doppler II to drive the Doppler
compass rose display) I have not updated the software described on this
For more info on the "SPI" based compass rose mentioned above, go
Compass Rose Firmwarepage. This page describes a
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
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
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
accessories would be a compass rose (also called a pelorus)
faux pelorus on the LCD is not easy to use. For
reasons mentioned above, it provides an intuitive display that
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
36 LED arranged in a circle providing a compass-like indication of the
bearing. Not wanting to feel left out, I designed a similar
The first question was "How many LEDs should it have?" Well,
seemed like a bit too few and 64 was simply too many. 32 or 36
seemed like a good number. In mobile use, it probably gives a
sense of the accuracy of the device - but one can always ignore a
extra information in this case. The question was then "32 or 36
36 LEDs seems like a good idea, as that means that each LED represents
10 degrees. 32 degrees represents 11.25 degree increments - which
like an odd number until one realizes that the traditional points of
compass are arranged as the power of 2 - and with 36 LED's you
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
is built so that the "North" LED is a different color to provide
in the dark, but the rest of them are of the same color - often
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
application of the appropriate polarity to the LED while Yellow is
by throwing AC (generated by alternating the polarity) at the
Since the pelorus uses a microprocessor as well, this was something
could be easily handled in software.
Having 3 colors to choose from, what does one do with it,
The most obvious answer is to indicate the quality of the
Green is a "good" quality bearing, Red would be a "poor" quality, and
would be somewhere in between.
Being under software control also allows the brightness of
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
Bearings with a quality of >= 7 are displayed in green
Bearings with a quality of >=4 and <7 are displayed in
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
bearing) are displayed, but much more dimly to allow clear
between the "current" and "past" readings.
The "North" (top) LED is dimly lit as green while the other 3
points are dimly lit as red.
This is the circuit of the
The microprocessor (a PIC16F876A) is the large IC. The empty
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
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
past readings are on a single LED, it will consequently be
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
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
been integrated into new software for the Montreal
Doppler III compass display:
"Flipping" the display. This would mirror the display,
pelorus to be set upon the dash of a vehicle. At night, the
of the LEDs in the windshield makes for a nifty "Heads Up"
I've tried it and it does actually work - if you can keep it from
around.) This is helpful if your DFing partner tends to get
while staring down at a display while you are driving. Also, it
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,
I'm just saying that it would be nice to make it easier to do so if you
Actually "average" the bearings within the pelorus itself.
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
on in the DF box, the update rate slows down. If the pelorus
were to do averaging, one could operate the 'EMM box at "full speed"
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
be limited by the amount of RAM that the device has, of course.
one is logging bearings, it would still be nice to have the data
out at full speed - and then one would have the option of doing
reduction in later analysis. (Remember: It's easier to
away extra data later than try to reconstruct it...)
Provide adjustable brightness. As it turns out, the
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
selecting red and green) do. While the 2-legged variety used in
prototype are about as bright as they come, they leave a bit to be
if one has the display in direct sunlight. 3-legged dual-color
may be used provided that one place a reverse-biased diode across each
of the LED segments (leaving the "common" anode - or cathode
The only requirement is that the current limiting resistor be changed
accommodate the extra 0.6 volt drop across the diode that is "reverse
across the currently "unused" color. If "killer-bright" LEDs are
used, they are then too bright to be comfortably used at night - and
a brightness control of some sort is required. While about "20
of brightness range may be accomplished in software, this isn't quite
to dim the LED enough at night and have enough range left over
a useful indication.
Integration with an electronic compass. I haven't quite
how I plan to do this: I'll either have this processor
with the compass, or have yet another processor do that. If one
in a vehicle, it is helpful to have bearings relative to the front of
vehicle. If one is logging or reporting bearings, a known
(to true north, preferably) is required for such data to be useful. (I'm
construction of the "yet another outboard box"
that will take the Agrelo readings and translate them to true-north
via the serial port.)
Because this pelorus is a "work in progress" I don't yet have a
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
of any of the above circuits is largely based on the skill and
of the operator. Your mileage may vary.
Do you have any questions on this or other DF-related
topics? Go here.