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In the Midwest, the main topics of
conversation may include the local football team or how
the crops are doing. Inevitably though, the conversation
always leads to the weather. Most of us take the weather
for granted and don�t give it a second thought. But, in
states like Kansas, Oklahoma, and Texas, weather is
spelled with a capital �W� and is closely monitored by
both individuals and the media. Weather is
thunderstorms, tropical depressions, tornados,
blizzards, squall lines, stationary fronts, cold fronts
from Canadian, warm moist Gulf air, and hurricanes. The
onset of any one of these events can be detected by
monitoring a few basic conditions.
When you check the weather on the
television or the radio, it�s always what conditions are
like at the airport or some other remote location. But,
what are conditions like in my backyard? To address
this, I embarked on the design of the Wireless Weather
Station. I had meant to do this project many years ago
but two bits of technology finally spurred me into
action: the first was the development of low-cost, solid
state humidity sensors and the second was the
availability of small, monolithic RF transceivers.
The Wireless Weather Station is composed
of a remote station and a base station. The remote
station is solar-powered and wakes up once a minute to
collect and transfer data. The base station receives and
buffers the incoming data and then transfers it via an
RS232 connection to a PC for processing. Within each of
the stations is a dedicated circuit card as well as a
separate, RF circuit card. We�ll start our discussion
with the remote station design.
The Remote Station consists of four
functional sections: the sensors, the PIC16F873
microcontroller, the RF circuit, and the power supply.
The schematic, shown in Figure 2, shows the sensor and
microprocessor areas.
For humidity sensing, I opted for the
Humirel HS1101 capacitive sensor. This device, when
combined with a CMOS 555 timer operating as an astable
multivibrator, produces a signal with a humidity
dependent frequency. To minimize temperature effects, it
is important to use the Texas Instruments TLC555 device
in this design. Refer to the HS1101 datasheet if another
device is used. Also note that care must be taken at the
node of the HS1101 and the 555. Stray capacitance values
will lead to erroneous and unpredictable measurements. I
chose to solder this node above the circuit card.
The relationship between the output
frequency of the 555 and the relative humidity can be
seen in Figure 1. A first order equation that relates
relative humidity to frequency is:
R.H. = 565.1 � 0.0767 * f
A second order equation can be used for
improved accuracy,:
R.H. = -6.4790E-06 * f2 + 1.0047E-02 * f
+ 2.7567E+02
Figure 1
Temperature sensing is very
straight-forward with the LM335. This output of this
device is equal to the absolute temperature in degrees
Kelvin divided by 100 or:
Vout=Temperature(K)/100
To determine the temperature in degrees
C, use the equation:
�C = 100 * Vout � 273
To determine the temperature in degrees
Fahrenheit, use the equation:
�F = 1.8 * �C +32.2
= 1.8 * (100 * Vout � 273) + 32.2
= 180 * Vout - 459.2
At room temperature, this device outputs
about 3volts.
Pressure sensing is provided by a
Motorola MPX5100A, which operates from 0 to 16 PSI.
However, we�re interested in only a very small part of
that range. Barometric pressure readings fall between 28
and 32 inches of mercury. This translates to 13.75 to
15.72 PSI. To increase the dynamic range of the output,
I added an amplifier circuit (U4), which subtracts about
3.7 volts from the sensor output and then multiplies the
difference by 4. Since the MPX5100 can require as much
as 10mA, Q1 was added to provide
microprocessor-controlled switching.
I chose the Microchip PIC16F873 because
it had the right mix of program and data memory, a
10-bit A/D, and three timers. Timer2 is used to measure
the period of the humidity signal. The A/D is used to
measure the temperature and pressure sensors as well as
to monitor the battery voltage. To maximize accuracy I
used an external 4.096 volt 0.1% reference from National
Semiconductor. With the 10-bit A/D, this provides a
resolution of 4mV per count.
The interface to the RF link consists of
an enable line and a data output. Since the transmitter
circuit operates at 3.3 volts, I used analog switches to
translate from the five-volt outputs of the
microprocessor. Although not shown, pin 14 of the
74HC4066 needs to be connected to +5 volts while pin 7
needs to be grounded. JP1 is a 6-pin header that
connects to the RF circuit card.
Click
Here to View Circuit
The power supply for the Remote Station
is shown in Figure 3. When the sun is shining on the
solar panel, enough power is generated to drive the 50mA
current source formed by Q1, U1, and R1. This current
acts as a trickle charger for three AA NiCAD batteries.
The batteries power U2 - a switchmode regulator that
provides the 5 volts for the microprocessor and sensors.
This is followed by U3, which is a linear regulator that
provides the 3.3 volts for the RF circuitry. L2 and C5
were added to reduce the switching noise from U3. D2 is
used to isolate the solar panel from the rest of the
circuit when it is dark. Note that C3 and C5 should be
6.3V low ESR capacitors.
Click Here to
View Circuit
The RF section of this design, shown in
Figure 4, was built around the TX5002 and RX5002 chips
from RF Monolithics. Due to the footprint of these
devices, it was necessary to design a small printed
circuit board. Fortunately, the chips have a pin-out
that allowed a single board to be used for both the
transmitter and the receiver. The completed layout can
be seen in Figure 5.
I was able to get all the traces on a
single layer so fabrication was greatly simplified. The
four corner holes were drilled out to allow mounting
with 4-40 screws. The antenna is connected via a BNC
jack on the left side. However, a length of coax can be
connected to this pad in order to use a panel-mount
connector.
It is necessary to use solder paste to
connect the RFM modules to the PCB. Carefully apply the
paste to the pads of U1 on the board. Position the
module on board and heat with a hot air gun until the
solder melts. Except for C2, the remaining components
are in 0603 SMT packages. C2 is a tantalum capacitor in
an �A� package.
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