1-Wire Addressable Digital
for Environmental Monitoring
Meteorological data on rainfall,
wind speed and direction, solar radiation, relative humidity, and
barometric pressure can be collected, converted, and transmitted over the
same single twisted pair that powers the sensors.
Dan Awtrey, Dallas
Addressable digital instruments make it possible to measure
multiple environmental variables over a single twisted-pair cable.
The type and number of ADIs deployed determine the system function,
or "genome" of the system. The graphic illustrates a
weather-specific 1-wire net by way of comparison to a DNA strand.
1-Wire analog-to-digital converters (ADCs) have recently been introduced
that make it possible to measure a wide range of environmental properties
over a single twisted pair, and which open the way to a new generation of
transducers called addressable digital instruments (ADIs). An ADI consists
of a sensing element or elements, a 1-Wire chip that converts the input
into a digital format, and some peripheral and protection components. A
distinct advantage of ADIs is that they all interface to the master in the
same manner, regardless of the particular property being measured. Whether
the essential sensing element is voltage, current, or resistive based, all
communication occurs over the net using half-duplex 1-Wire protocol .
(This feature is in contrast to methods that incorporate a variety of
signal conditioning circuitry such as instrumentation amplifiers and
voltage-to-frequency converters, a design that necessarily makes their
outputs different and often requires a separate cable and power source per
sensor.) The unique ID address or serial number of each sensor is the key
for the bus master to interpret which parameter a particular ADI is
measuring. The ID also allows multiple ADIs to be placed on the same
cable, reducing installation and maintenance costs.
Since the science of meteorology is a familiar example of an
application requiring diverse sensors, several examples of 1-Wire
instrumentation designed for use in a weather station will serve to
illustrate their design simplicity and versatility. In addition to the
temperature and wind speed and direction that a basic weather station may
measure, environmental monitors commonly measure rainfall, humidity,
barometric pressure, and solar radiation as well. And many systems add
several other sensors to determine dew point and detect lightning strikes.
The circuits presented here not only show how to transform the specified
sensing elements into an ADI, but also illustrate the ease with which the
concepts can be extended to other instruments not described.
Note that at the center of every ADI is a chip that converts an input
or inputs into a digital format that communicates over the net with a
common protocol. For example, the DS2423 counter has inputs that respond
to logic level changes or switch closures, and includes a 1-Wire interface
front end that makes it suitable for various rate or event sensors.
events call for measurement of either a total, or a count per unit time
(rate). Examples include wind speed and rainfall or the number of times a
wheel has rotated, from which rpm and distance can be computed. A
magnetically actuated reed switch used as an input
to a DS2423
counter allows such events to be easily measured. A basic reed switch
circuit suitable for a rain gauge or wind speed sensor is shown in Figure
1. The dual diode BAT54S serves to protect the circuit from signals that
go below ground, and, with C1, provides a local source of power. While the
DS2423 has an internal pull-up resistor to keep the input from floating,
its high value (~22 M) can make it susceptible to noise. To avoid generating
spurious counts during turn-on and minimize noise pickup, an external 1
M pull-down resistor is substituted. Except for lithium
backup (not shown), this is the counter circuit used in the 1-Wire rain
Here, a small permanent magnet moves past the reed switch each time a
tipping bucket fills and empties. This momentarily closes the reed switch
that increments the counter, indicating that 0.01 in. of rain has fallen.
A similar circuit is used in the 1-Wire weather station to measure wind
The same circuit (also with lithium backup) has also been used as a
hub-mounted wheel odometer. Conveniently, the DS2423 also contains 4096
bits of user-accessible SRAM that can be used for temporary storage, or,
where lithium backup is provided, for calibration, location, and routine
Figure 1. This basic ADI circuit incorporating a
DS2423 counter with a reed switch input is used to measure
rate such as wind speed and rainfall. It may also be used as a
hub-mounted wheel odometer for counting tire rotations.
knowing wind speed, the direction from which the wind is blowing is of
particular interest and may be measured with an ADI. While the original
1-Wire weather station used a DS2401 silicon serial number to label each
of the eight magnetic reed switches in its wind direction sensor, a single
DS2450 quad ADC can perform the same functions .
As in the DS2423 counter circuit, a dual-diode BAT54S protects the circuit
from signals that go below ground, and, with C1, provides a local source
of power. Note that C1 has been increased from 0.01 to 10 µF to ensure
that the voltage across the resistor network remains relatively constant.
As shown in Figure 2 , a single D2450 replaces the eight DS2401s
originally used with five fixed resistors.
A wind direction sensor based on the DS2450 quad ADC differs from
the previous design in that no initialization is required; each
compass point generates a unique digital code.
As the wind rotates the wind vane, a magnet mounted on a rotor that
tracks the rotation opens and closes one (or two) of the reed switches.
When a reed switch closes, it changes the voltages seen on the input pins
of U1, the DS2450. For example, if the magnet is in a position to close S1
(North), the voltage seen on pin 7 changes from Vcc to
1/2 Vcc, or approximately 5・.5 V. Since
all 16 positions of the wind vane produce unique 4-bit signals from the
ADC, it is an absolute indicator.
accordingly no need to initialize the sensor or store a tagging code on
the board as was required by the original 1-Wire weather station. It is
necessary only to indicate North, or, equivalently, the direction the wind
vane is pointing. Table 1 lists the voltages seen at the ADC inputs for
all 16 cardinal points.
|Wind Vane Position vs. Voltage|
(seen on the
four DS 2450 inputs)
Because two reed switches are closed when the magnet is halfway between
them, 16 compass points are indicated with just eight reed switches.
Referring to the schematic and position 2 in Table 1, observe that when S1
and S2 are closed, 3.3 V is applied to ADC inputs A and B. The reason is
that pull-up resistors R2 and R3 are placed in parallel and the pair is
connected in series with R1 to form a voltage divider with 0.66
Vcc across R1. Notice that this also occurs twice more at
switch positions 4 and 16.
The DS2438, a Versatile
Originally designed to measure the condition of a
battery pack, the DS2438 contains two ADCs and a temperature sensor. The
main ADC performs 10-bit conversion on a 0・0 V input, or 9-bit conversion
on a 0・ V signal with an internal multiplexer that allows it to read the
voltage applied to its power supply pin. The other ADC was intended to
measure the voltage developed by large battery currents flowing across an
external 0.05 resistor with signed 10-bit accuracy at a full-scale
reading of ｱ250 mV. The DS2438 also contains a 13-bit temperature sensor
similar to the DS18B20. Among other additional features such as a
real-time clock, the part provides 40 bytes of nonvolatile memory that is
useful for storing calibration, location, and function information. The
introduction of this part simplifies the design of many ADIs, as
illustrated by the circuits described below.
of sunlight and its duration are additional parameters that meteorologists
and horticulturists are interested in measuring. The amount is a measure
of air and sky conditions; duration is related to the length of the day.
Although the mechanics of mounting and filtering tend to be complex, as
shown in the following two examples, the electronics can be easily
implemented to form a DS2438-based ADI. Figure 3 illustrates a solar
radiation sensor using a photodiode; Figure 4 uses a photovoltaic cell. In
each case, a dual-diode
Figure 3. The photodiode in an ADI solar radiance
sensor can be provided with optical filters or selected to be
sensitive to a particular portion of the spectrum.
protects the circuitry from signals that go below ground, and with C1,
provides a local source of power.
Figure 4. In a photovoltaic cell-based solar radiance
sensor, R1 and R2 form a voltage divider to keep the voltage
seen by U1 within its maximum range.
In Figure 3, a sense resistor is connected in series with a photodiode
and between the two 田urrent・ADC pins. Light striking the photodiode
generates photocurrents that in turn develop a voltage drop across the
sense resistor that is read by the ADC. In commercial units, optical
filters are typically added to match the wavelength and bandpass to the
human eye response (the CIE or photopic curve). More sophisticated units
add other desirable features such as a translucent hemisphere that
collects light to enable the sensor to view the sky from horizon to
horizon. In this case, the sensor actually focuses on the inside of the
hemisphere to obtain its reading .
An interesting variation of a solar radiation sensor can be constructed
using a standard LED in reverse bias mode. An LED is selected that
generates acceptable photocurrent levels when exposed to the sun at high
noon on a clear day. The resistor is sized to develop 250 mV maximum using
Rsens = E/I
E = 0.25 V
I = maximum photocurrent generated
One example is the EFA5364X from Stanley (Irvine, CA). This is a
super-bright orange AlGaInP LED with a peak response at 609 nm and a
narrow (15°) spectral field of view. A 4.7 k sense resistor provides acceptable outdoor performance,
which may be increased to 100 k if the circuit is to be used with indoor lighting. LEDs
made from other compounds will have their peak response in a different
portion of the spectrum, which can prove useful in certain
A solar radiation sensor can also be based on photovoltaic cells, which
generate electricity when exposed to light. As shown in Figure 4, a
suitable solar cell is connected to the 田urrent・ADC input of the DS2438
through voltage divider R1 and R2. The divider is necessary to limit the
typical 0.45 V generated by a single solar cell to the 300 mV absolute
maximum allowed across pins 2 and 3 of the DS2438. Resistor values for the
divider are chosen such as to avoid unduly loading the cell痴 power
capacity. One advantage of this approach is that several cells can be set
to face different sectors of the sky for horizon-to-horizon coverage. The
cells are connected in parallel and R2 is sized so that maximum sunlight
on the sensor(s) develops no more than 0.3 V across it, as described
above. If signal filtering is required in a particular application, it may
be done in hardware as recommended in the DS2438 datasheet, or by
averaging in software.
only is humidity an important factor in many processing and manufacturing
operations, but it also directly affects our own comfort and well-being.
Too low, and we must deal with static electricity and ESD problems; too
high, and mold, condensation, and mugginess affect us. With the proper
sensing element, humidity can be easily measured with an ADI over the
1-Wire net. The Honeywell capacitive sensing element specified here
develops a linear voltage vs. relative humidity (RH) output that is
ratiometric to the supply voltage. That is, when the supply voltage
varies, the sensor output voltage follows in direct proportion. This
necessitates measuring both the voltage across the sensing element and its
output voltage. In addition, calculation of true RH requires knowledge of
the temperature at the sensing element. Because it contains all the
necessary measurement functions to do the calculations, the DS2438 is an
excellent choice for an ADI humidity sensor.
5, the analog output of the HIH-3610 humidity-sensing element is converted
to digital by the main ADC input of a DS2438. As with other ADIs, a
dual-diode BAT54S protects the circuit from signals that go below ground,
and, with C1, provides a local source of power. In this case, a value of
0.1 µF for C1 is sufficient to handle the 200 µA operating current
required by U2, the HIH-3610. The RC network on the output of U2 is a
low-pass filter that removes the low-level clock feed-through from the
sensing element痴 signal conditioning circuitry. If averaging is done in
software, however, R1 and C2 may be omitted and the sensing element output
connected directly to the Vad pin of U1. In operation, the bus
master first has U1, the DS2438, report the supply voltage level on its
Vdd pin, which is also the voltage across U2, the sensing
element. Next, the master has U1 read the output voltage of U2 and report
local temperature from its onchip sensor. Finally, the master calculates
true RH from the three parameters supplied by U1 .
Since the bus master identifies each ADI by its unique serial number, many
humidity sensors can be placed on the line. This is particularly
convenient in applications such as greenhouses where it is desirable to
know the humidity at multiple locations within the enclosure.
Figure 5. In a humidity sensor based on the DS2438,
R1/C2 form a low-pass filter that may be omitted if averaging
is done in software.
Figure 6. A barometric pressure sensing element
requires special power management circuitry (not shown), or an
external source of power must be provided.
Atmospheric, or barometric, pressure is a valuable
indicator of imminent weather change when a front moves past the
instrument. This meteorological parameter can also be measured over a
1-Wire net using an ADI. Selecting a pressure sensor that contains
comprehensive onchip signal conditioning makes the circuit in Figure 6
very straightforward. As was the case with the humidity sensing element,
the suggested pressure sensing element is ratiometric, which requires that
both the output voltage representing atmospheric pressure and the supply
voltage across the element be known in order to accurately calculate
As is typical for an ADI, a dual-diode BAT54S protects the circuitry
from signals that go below ground, and, with C1, provides a local source
of power. In this case, because U2, the MPXA4115 pressure sensor, may
require as much as 10 mA at 5 V, special power management circuitry (not
shown) is required, or else an external power source is needed. Notice
that the external power is also connected to the power pin of the DS2438,
allowing the circuit to measure the supply voltage applied to the
pressure-sensing element. In many installations, supplying external power
is not a problem because the barometer will be mounted inside near the bus
master and a power source. Flexible tubing can then be routed to sample
the outside air pressure and avoid unwanted pressure changes (noise)
caused by the opening and closing of doors and windows or elevators moving
inside the building.
introduction of 1-Wire ADIs makes it possible to measure a variety of
environmental parameters, convert the signals locally, and transmit the
digital data over a common communications link. Multiple ADIs measuring
the same or different variables can be placed on the same twisted pair
that also supplies power for the sensors. For example, wind speed and
rainfall may be measured by an ADI counter; wind direction by a ADI DS2450
ADC; and humidity, barometric pressure, and solar radiation by
DS2438-based ADIs. Each ADI can also measure two variables such as
humidity and solar radiation. The ability to mount a variety of
instruments on a single cable reduces installation and maintenance costs.
Each sensor has a unique ID address by which the master keeps track of it
and the environmental parameter it is measuring. Sensor-specific or
calibration information may be stored in the ADI痴 memory, or, eventually,
by downloading from the Web using the sensing element痴 ID address as a URL
1-Wire is a registered trademark of Dallas Semiconductor.
1. Dan Awtrey. Feb. 1997. ・A
HREF="/articles/0297/onewire/index.htm" TARGET="_top">Transmitting Data
and Power over a One-Wire Bus,・Sensors:48-51.
2. Dan Awtrey. Dec. 1999. ・A
HREF="/articles/1299/56_1299/index.htm" TARGET="_top">A 1-Wire Rain
3. Dan Awtrey. June 1998. ・A
HREF="/articles/0698/wir0698/index.htm" TARGET="_top">The 1-Wire
4. Walter Butler. June 1999. ・A
Optical Sensing of Changing Environmental
5. Dan Awtrey. Aug. 2000. ・A
HREF="/articles/0800/62/index.htm" TARGET="_top">A 1-Wire Humidity
Awtrey is a Staff Engineer, Dallas Semiconductor, 4401 S. Beltwood
Pkwy., Dallas, TX 75244-3292; 972-371-6297, fax 972-371-3715, firstname.lastname@example.org.
information about available 1-Wire sensors contact John Compton,
CEO, Point Six, Inc.,
383 Codell Dr., Lexington, KY 40509; 859-266-3606, fax