is rotated in the earth’s field.
In any of the compasses, it is nec-
MAILBAG
Dear Russell,
I have not been successful in
getting the turn signal reminder from
July to work, even after re-checking.
It won’t shut off. Did you
build the unit and check it
out? If so, do you have
any changes to the circuit
in the October issue?
— FMC
Response: Thanks
for writing. I did not
build it but I have
simulated the circuit and
see the problem. The
second 555 trigger is left
low when the blinker
stops, which makes the
output stay high. The
solution is to put an R-C
between the two 555s as
in Figure 8. Mea culpa!
essary for the sensor to be horizontal
or aligned with the earth’s field.
Compensation for declination is done in
better compasses but near the north
magnetic pole, the declination is so near
90 degrees that a gyrocompass is needed to supplement the magnetic sensor.
For those wanting to build a
good digital compass, I recommend
Microchip’s application note AN996,
which you will find at http://ww1.
microchip.com/downloads/en/AppN
otes/00996a.pdf. The compass is
designed around the PIC18F2520 and
you can download the schematic and
program. There is also a map of the
earth’s field which will be useful to
anyone using a compass. NV
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26
February 2008
