Adaptation of a Low-cost Wireless Sensor for Freshman and Outreach
Programs
Mike Fortney and Jeff Frolik
University of Vermont
Underrepresented Groups in Engineering
Abstract
This paper details the development of new CricketSat designs and education programs at the
University of Vermont (UVM). UVM first explored the use of this wireless sensor in Summer
2002 after attending a NASA Starting Student Space Programs workshop. Work at the university
has since involved improvements to the design to expand functionality and facilitate successful
student circuit assembly. High school, undergraduate and graduate level students are involved
with CricketSat sensors and systems, design and testing. Collaborative and outreach programs
involve other institutions.
Introduction
The CricketSat wireless temperature sensor was originally designed in 1999 at Stanford
University's Space System Development Laboratory as part of the NASA Space Grant "Crawl,
Walk, Run, Fly" student satellite program1. The purpose of this NASA program is to instruct
students into methods of space hardware development. Student satellites range from the simple
balloon-borne CricketSat to the more complex earth-orbiting CubeSat. To assist colleges and
universities in developing their own programs, "Starting Student Space Hardware Programs"
workshops are held frequently at the University of Colorado campus in Boulder2. The workshop
covers the range of student satellite designs, with emphasis on the BalloonSat program.
Representatives from UVM attended workshops and a CricketSat program was
implemented at the University in 2002. UVM CricketSat objectives involve improvements to the
original design, its use as an educational tool, and outreach activities. This paper discusses the
following activities which take place within the program:
1.
2.
3.
4.
CricketSat development and testing
Collaborative work with other colleges and universities
High school outreach
Freshman introduction to engineering course
The CricketSat System
The CricketSat system (Fig. 1) is composed of a single wireless sensor and a receiving
station. The CricketSat transmitter contains a simple, 555 timer-based circuit that produces an
audio tone that changes frequency in response to changing temperature. This tone amplitude
modulates a 434 MHz carrier. Calibration of the sensor is performed by measuring the tone
Proceedings of the 2005 ASEE New England Section 2005 Annual Conference, Copyright © 2005
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frequency taken at various temperatures. From the calibration, graphs are produced for
converting frequency to temperature during use.
Figure 1. CricketSat Wireless Temperature System
For flight, the CricketSat device can be attached to a helium balloon as small as 2 feet in
diameter. During flights, the 434 MHz signal is received by the ground station. The ground
station consists of a Yagi antenna, a UHF radio receiver and an audio frequency measurement
device. The frequency of the tone is measured with a frequency counter or audio-spectrum
software. Flights have been tracked for 90 minutes before the signal becomes too weak to
measure reliably. During this time, the balloon may travel a distance over 150 km, reach an
altitude of 10 km and experience temperatures less than –70 C. The sensor and balloon are
seldom recovered.
BalloonSat is a much larger system, toting a payload of several pounds, and a price in
excess of ~$500. This system contains a GPS device and a radio transmitter used to broadcast
the position coordinates for tracking during flight. Sensor data is usually collected and stored
during the flight. Recovery of the payload is necessary for the expensive equipment and data.
Complexity, cost, and logistics for flight preparation and tracking may make this system
undesirable. Flights may achieve altitudes of 30 km in 100 minutes before the balloon bursts and
the payload parachutes back to earth.
In comparison to BalloonSat, the CricketSat system has benefits of low cost ($10), low
weight, and live data telemetry. It is also simple to understand, easy to assemble, and simple to
use. Drawbacks include single-sensor operation, and requirements for calibration and frequency
conversion. Work at the UVM is involved with improving the performance and flexibility of this
device.
UVM CricketSat Development and Testing
Development work at UVM includes improvements to the original design, adaptations for
new sensing capabilities, and the design of multi-sensor systems. Improvements have also been
made to better the electrical performance, system reliability, and the likelihood of successful
assembly. Concerning the latter, component outlines and designations have been added to the
board, and a protective layer to minimize electrical shorts. A prototype area has been expanded
Proceedings of the 2005 ASEE New England Section 2005 Annual Conference, Copyright © 2005
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to support student adaptations to the CricketSat design. The most recent UVM CricketSat design
is shown in Fig. 2.
Figure 2. UVM CricketSat Wireless Temperature Sensor
One objective of the UVM CricketSat work is to extend the circuit’s capability beyond
simply measuring temperature. Common sensors for air pressure, humidity, light level, and
acceleration can be now accommodated in the design. In addition, easily built sensor circuits3
will also interface with this design. With this added flexibility, the CricketSat may used as a
platform for a wide variety of wireless sensor applications. To date, multi-sensor CricketSat
designs have also been developed by both university and high school students. Eventually, the
improved circuitry will lead to the development of a low-cost student radiosonde (CricketSonde)
containing meteorological and other scientific instruments. Such a design may enable
community-based, meteorological measurements at a much finer spatial resolution than those
currently available using the current network of National Weather Service radiosonde stations.
This work toward this end is detailed in the following sections.
High School Outreach
The HELiX (Hughes Endeavor for Life Science Excellence) Program at UVM
HELiX/EPSCoR4 is a NSF funded outreach program supporting area college and high
school students. Among the HELiX activities is a summer workshop for high school students
that is designed to provide students insight into the "real world" of science. Teams, consisting of
a teacher and a few students, conduct a research project, assisted by scientists at the university.
At least one of the students must be female. A HELiX-sponsored CricketSat workshop titled
"Building and Launching Cricket Satellites to Measure Various Atmospheric Conditions" was
conducted during the summers of 2003 and 2004 (one is also planned for June 2005). The oneweek session, outlined in Table 1, involves lectures and hands-on activities for the students and
teachers. Classroom instruction includes an introduction to the earth's atmosphere and operation
of the CricketSat sensors. Hands-on activities involved the assembly, soldering, calibration, and
Proceedings of the 2005 ASEE New England Section 2005 Annual Conference, Copyright © 2005
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flight of these sensors. Students fly balloons, collect data and analyze the results. School teams
must then conduct a related research project to be conducted over the following year.
Table 1 . HELiX CricketSat Workshop Schedule
Day
Monday
Tuesday
Wednesday
Thursday
Friday
Activity
Temperature profile of the atmosphere
Introduction to the CricketSat sensors
Practice soldering
CricketSat assembly
CricketSat testing
CricketSat calibration
Balloon flights and data collection
Spreadsheet data entry
Analysis and results
The high school teams have provided an important role of testing and evaluating the
CricketSat sensor designs. These teams have also developed and evaluated more complex multisensor CricketSat designs. Each balloon flight strives to surpass previous results, contributing
towards advancing the system. Parameters analyzed for each flight are duration, distance,
minimum temperature and maximum altitude.
2003 – 2004 HELiX Team
This initial workshop (2003) involved a team from the Waldorf High School in Charlotte,
Vermont, consisting of a female science teacher and three female students. The June flights
involved the initial testing of the newly developed pressure and humidity sensors and ground
station receiving system. Balloons were released over the water from a causeway on Lake
Champlain. The results were not very encouraging. Frequency measurements using a meter
became unstable less than 10 minutes into the flight. The meter did a poor job of measuring the
signal in the presence of background radio noise. Expected qualitative variations were observed
for the temperature, pressure and humidity sensors.
In short, the system worked satisfactory for close-range work, but not for balloon flights.
As such, for their long term research project, the team decided to build a wireless weather station
consisting of temperature, pressure and humidity sensors. The goal was to make automatic
measurements at periodic intervals. A frequency measurement meter was connected to a
computer for data collection. Since all of the CricketSat sensors share the same radio frequency,
a rotary switch was added to the station to provide power to the CricketSat sensor to be
measured. The system worked for three-hour intervals before the meter would turn itself off.
This appeared to be caused by the meter’s inability to properly track the changing signal.
A new method was devised for performing the frequency measurements during balloon
flights. Several audio spectrum analyzer programs were investigated. These programs allow for
individual frequencies in the audio signal to be “seen” on the computer and measured. The
CricketSat signal was easily identifiable, even when far away, allowing it to be measured
reliably. The SpectraRTA5 software was selected at the time due to its data logging capability.
Spectrogram6 is now recommended due to its low cost.
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2004 – 2005 HELiX Teams
This improved platform was utilized for the second HELiX workshop (2004) in which
two high schools participated. The first team was from the Milton High School located in
Milton, Vermont. The team was composed of a female science teacher and two female students.
The second team was from the John D. O'Bryant (JDOB) School of Mathematics and Science
(Boston Public Schools) located in Roxbury, Massachusetts. This team consisted of a female
science teacher and two students: one female and one male.
Day and evening CricketSat flights (MHS-1) to monitor atmospheric temperature profiles
were conducted. A CricketSat humidity sensor was also flown along with an experimental audio
alarm device attached. Collectively, the flights were a remarkable success. The SpectraRTA
software performed well, allowing measurement of the tone signals for a much longer period of
time than the previous method using a frequency
meter. The shortest flight was tracked for 45
minutes and the longest for 91 minutes. This
allowed for measurements much higher in the
atmosphere. Accordingly, the lowest measured
temperature was -41 F (-40 C), and the highest
recorded altitude was 26,732 feet (8.1 km).
Factoring in the velocities of the upper-air winds
obtained from the National Weather Service
(NWS), the longest CricketSat altimeter flight
was 144 km.
For their follow-on project, the JDOB
team prepared a detailed presentation of the
MHS-1 flights, placing second at the 2005 Boston
Regional Science Fair in March 2005 (Fig. 3).
They will go on to compete at the Massachusetts
State Science Fair to be held in May at MIT.
Figure 3. The John D. O'Bryant School
CricketSat presentation of the MHS-1 flights.
The team received a 2nd-place finish at the 2005
Boston Regional Science Fair.
For the Milton High School team, their
work was just beginning. The school hosted two
BalloonSat flights in July 2005 for students from
Medgar Evers College (MEC) of the City
University of New York (CUNY). CricketSat
sensors were flown as payload to provide realtime flight support data for the MEC team. These
flights also allowed CricketSat sensors to achieve
altitudes and conditions not normally experienced
with the smaller balloon flights.
For the first BalloonSat flight (MHS-2),
the MHS team monitored the temperature inside
the BalloonSat instrument flight bag (Fig. 4). The
temperature was measured for 125 minutes and
never dropped below 63 F (17 C). For the MEC
team, this validated the use of the insulated lunch
Figure 4. Medgar Evers College (MEC) team
preparing for a BalloonSat launch at the
Milton High School. The CUNY school
provided flight support for the MHS-2
payload.
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bag for holding instruments during BalloonSat flights. This experiment also demonstrated the
compatibility between CricketSat and BalloonSat payloads concerning radio co-interference.
With the successful results of the single CricketSat sensor on the MHS-2 flight, the team
was now presented a challenge of measuring data using several CricketSat sensors. Unlike the
earlier weather station, this system would need to sequence through the sensors automatically.
The problem was presented to the Milton team, and with a little guidance, they devised a
sequential timing algorithm for segregating and identifying sensors during flight. In addition, the
design needed to be light weight for the balloon application. A circuit was designed for the
students using a BASIC Stamp controller. One ambitious student assembled the circuit, wrote a
PBASIC program employing the timing algorithm, and tested the controller. The CricketSat
Array System (CAS) assembled for flight is shown in Fig. 5. During the BalloonSat flight
(MHS-3), the CricketSat flight bag and external temperatures were measured, as well as altitude
(air pressure). The system worked very well, properly segregating the data from the various
CricketSat sensors, as seen in Fig. 6.
Figure 5. The Milton High School CricketSat
Array System flown on the MHS-3 flight. This
project allows the measurement of several
CricketSat sensors over 50 miles away.
Figure 6. Properly segregated data collected from
the Milton High School developed CricketSat Array
System. Raw frequency results are shown.
New levels of performance were achieved. The flight was tracked for 134 minutes, to an
altitude of 85,781 feet (26 km), and with a bone-chilling external temperature of –92 F (-69 C).
The CricketSat altimeter worked properly below 32,000 feet (10 km), meeting expectations.
External temperature versus altitude data correlated with NWS sounding balloon data. The
CricketSat altimeter data agreed well with altitude data provided by the onboard GPS. The
results were presented at the Northeast Regional Space Grant Conference held in October 2004
in South Burlington, Vermont by the author and the two Milton High School students.
Design changes to the CricketSat are necessary for improvement to the temperature and
pressure measurements. As can be seen in Fig. 6 (Ext Temp 1 & 2), for very low temperatures,
the CricketSat frequency is very low and difficult to measure. In addition, the CricketSat
pressure sensor only works properly up to altitudes of 10 km. To completely characterize the
environment experience during these large balloon launches, the sensors need to perform
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measurements to altitudes of 30 km with temperatures as low as –90 C. As such, thesis work is
in progress by the author towards the development of a linear frequency response CricketSat
temperature sensor for use in extreme cold environments. This may lead to the development of a
family of linear response CricketSat sensors for radiosonde experiments (i.e., CricketSonde).
The linear response provides benefits of uniform sensitivity, and simplified calibration and
conversion methods. The sensors will be evaluated on future HELiX and BalloonSat flights.
College Freshman Engineering Course
As a result of the above successful programs, the CricketSat was chosen in Spring 2004
as a project platform for UVM’s freshman design course for electrical and mechanical students
(instructed by the co-author)7. In this course, students first fabricate, test and calibrate the basic
wireless temperature sensor. Then, working in teams, the 60 students adapted over a six week
period this sensor for an application of their own choosing. The project requires electrical
modification of the circuit and mechanical design requisite of the application. Student projects
from the first offering included a wireless wind-chill instrument (Fig. 7-left), a wireless
synthesizer and a wireless alarm system (Fig. 7-right). We view the breadth of these designs as
being indicative of the flexibility and simplicity of the CricketSat platform to accommodate a
variety of introductory-level student projects. The course is currently in its second offering to 70
students and utilizing the improved CricketSat design illustrated in Fig. 2.
Figure 7. Example student design projects: Wireless Wind-Chill Instrument (left) and Wireless
Door Alarm (right)
Conclusions
UVM’s CricketSat activities have in addition enabled collaborative meteorological
studies with the University of Alaska along with the aforementioned work with Medgar Evers
College. Like UVM, the University of Alaska is involved with CricketSat development and
testing. Designs and methods are being shared between the two schools towards a common goal
of improving this design. With this wide range of “customer” input, we view the UVM
CricketSat design as rapidly migrating towards a simple, flexible and yet a powerful platform
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upon which meaningful projects can be developed for a wide range of wireless monitoring
applications. We hope that with this paper along with additional material (schematics, project
ideas and kit information) available online8 will provide a resource that other institutions may
utilize to develop their own entry level sensor programs. The author encourages interested
institutions to contact him9 should they have any questions.
Acknowledgements
The authors would like to acknowledge the Vermont and Colorado Space Grant
Consortiums, and the UVM HELiX outreach program for their support in the development of the
CricketSat program at UVM. The authors would also like to acknowledge Dr. Shermane Austin
of Medgar Evers College and Dr. Neal Brown of the University of Alaska for their flight and
technical contributions, respectively.
BIBLIOGRAPHY
1
NASA, Learning to Fly on Mars, online:
http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Learning_to_Fly_on_Mars.html
2
NASA Space Grant Consortium, StudentSat Workshop, online: http://spacegrant.colorado.edu/studentsat/
3
Mims III, Forrest M., 1988-2000, Engineer’s Mini-Notebook Series
4
UVM HELiX/EPSCoR, online: http://www.uvm.edu/~helix/
5
Sound Technology Corporation, online: http://www.soundtechnology.com
6
Visualization Software LLC, online: http://www.visualizationsoftware.com/gram.html
7
J. Frolik and T. Keller, Wireless Sensor Networks: An interdisciplinary topic for freshman design, 2005 ASEE
Annual Conference, Portland, OR, June 12-15.
8
UVM CricketSat website: http://www.uvm.edu/~cricksat
9
Mike Fortney, email: mfortney@uvm.edu
BIOGRAPHIES
MIKE FORTNEY received the B.S.E.E. degree from the University of Vermont (UVM) in 2000. He is currently a
candidate for the M.S.E.E. degree at UVM with an expected October 2005 graduation date. He has been active with
the HELiX outreach program since 2002, and Vermont Space Grant projects since 1999.
JEFF FROLIK received the B.S.E.E. degree from the University of South Alabama, Mobile in 1986, the M.S.E.E.
degree from the University of Southern California, Los Angeles in 1988 and the Ph.D. degree in Electrical
Engineering Systems from The University of Michigan, Ann Arbor in 1995. He is currently an Assistant Professor
in the Electrical and Computer Engineering Department at the University of Vermont (UVM). He is the recipient of
the ASEE Southeastern Section New Teacher Award in 2002 (while at Tennessee Technological University).
Proceedings of the 2005 ASEE New England Section 2005 Annual Conference, Copyright © 2005
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