| 1-Wire Barometer Design v.2 |
David Bray has designed a Barometer which connects to any Dallas 1-Wire
weather network. David has graciously offered this design to the Weather
mailing list community. David's design can be found here.
David's design is well tested and widely accepted. His design
presents a single power supply design with manual calibration, which can
be powered from the 1-Wire weather network by using spare wires in the
interconnect. David's design is designed to work well with the 1-Wire
Hub designed by Simon Atkin, and may be incorporated into future designs
of the hub.
After looking at David's 1-Wire Barometer Design, I had a few
ideas for enhancements.
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The Bray Barometer requires manual calibration for voltage offset and gain.
Although practical on the workbench, these devices are most likely mounted
in inconvenient locations making secondary adjustment difficult.
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The Bray Barometer uses potentiometers for offset and gain adjustments.
Potentiometers are notoriously temperature sensitive, and sensitive to
hostile environments.
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By only using a single 5V power supply, the Bray barometer cannot utilize
the full 1.5-10V range of the DS2438's Analog to Digital Converter. (ADC),
by using a higher voltage input we can improve instrument range or sensitivity.
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The Printed Circuit Board designs for the Bray Barometer are too big to
fit in most weather station thermometer pagodas.
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The second ADC in the DS2438 can be used to add other measurements, like
humidity.
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When powered remotely, the distant regulated 5 volt supply can be too noisy
for accurate measurement, given the ratiometric output of the Motorola
sensor.
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By using a dual voltage supply, instrument precision and/or instrument
range can be improved without adding significantly to the parts count.
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Fixed offset and fixed gain adjustments, given the improved instrument
range, can eliminate manual calibration, without affecting instrument precision.
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By regulating the 1-Wire network supply voltages locally, instrument accuracy
can be improved.
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Diodes can be added to protect the instrument, and the rest of the 1-Wire
network from reverse voltages.
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Higher precision OpAmps can be used with the dual voltage supply.
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Using widely spaced SMT components can allow a much smaller PCB construction,
while still being built by anyone with soldering experience.
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Addition of a Humidity sensor.
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Easier to mass produce.
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The output of a Motorola pressure sensor IC3 is filtered by a capacitor
C1, and fed to a high quality Operational Amplifier IC1B which is configured
as a differential multiplier.
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The other input to the differential multiplier is fed by a voltage divider
of R5 and R6, through a voltage follower made from the other half of the
dual OpAmp IC1A.
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The OpAmp can provide, via the differential multiplier's gain R3/R1 &
R2/R4, a full range 1.5V-10V input to the ADC of the Dallas DS2438 battery
monitor IC2.
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Stations above 1000M altitude will need to adjust component values.
This spreadsheet
can be used to calculate the ideal gain and offset for your desired instrument
pressure/altitude range.
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The PCB also has provisions for a humidity sensor, and an extra temperature
sensor.
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The output of the Humidity sensor is fed to the Current Sense input of
the DS2438 through a voltage divider R9 and R10.
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The sensors are powered by a 5V regulator VR1, while the OpAmp is powered
by 12V regulator VR2. Input and output capacitors C3-4 minimize power
supply noise. The DS2438 is powered from the 10V regulator, so that
it can test that voltage using its ADC.
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Both regulators can be powered from the local barrel type power connector,
or through pins 6 or 7 of the 1-Wire network, as selected by JP1.
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Dual Schottky diodes protect this circuit and the rest of the 1-Wire network
from reverse voltages.
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The PCB is smaller than 2"x2", which should fit in most weather pagodas.
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The PCB accommodates either the through hole or SMT version of the Motorola
sensor IC3A. Any of the through hole case styles of the sensor can
be accommodated.
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The PCB has provisions for voltage monitoring LED "on" indicators LD1 and
LD2. These are optional.
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Larger 1206 SMT components have been used, and those components have been
widely spaced to ease construction. Building this device should within
the grasp of most experienced electronic kit builders as a first SMT project.
I ordered the circuit boards for the v.1 prototype from Olimex, and
the other parts from Newark. The Dallas parts were special samples.
After the initial design, I'd discovered a 10 Volt regulator and decided
to use it. Resistor values were calculated accordingly. The
selection of resistor values from Newark was extremely limited, which made
value selection a problem.
By mistake, I ordered the vertically ported version of the sensor rather
than the horizontally ported version. It looks a little odd,
but works fine. The PCB is designed to support any of the MPX4115A
sensor types. I've ordered the un-ported SMT version of the sensor
for the v.2 prototype.
I soldered up the first board in less than 2 hours. I was surprised
how easily it went together, since this was my first SMT project.
The only parts which gave me problems were the SOT-3 diodes with their
short leads. After checking all of the solder joints with a volt
meter, I plugged in a 12V wall wart transformer and did some testing.
Photos 1 & 2 - The v.1 Prototype
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Testing with the TMEX iButton Viewer gave results as expected.
My only disappointment with version 1 was the amount of heat generated
by the onboard voltage regulators. This has been addressed in version
2 by replacing the zener offset voltage source, and the second LED is no
longer recommended.
This PCB was designed with the freeware
version of the Eagle layout editor by Cadsoft.
barometer2.sch
barometer2.brd
barometer2.erc
This is the Excel spreadsheet I
used to calculate gain and offset component values. If you live at
high altitude, or want increased precision from your 1-Wire barometer,
you can use this spreadsheet to experiment.
mpx4115a.xls
