A LOW-COST LINEAR-RESPONSE WIRELESS TEMPERATURE SENSOR FOR
EXTREME ENVIRONMENTS
ABSTRACT
This work will involve the development of a low-cost (<$15) wireless temperature sensor
that will perform wide-range temperature measurements in extreme environments. Work
at the University of Vermont (UVM) with the CricketSat wireless sensor has
demonstrated its capability for performing temperature measurements from -68 C to +40
C. Unfortunately, the nonlinear response of the circuit significantly reduces resolution
and accuracy for temperatures below -20 C. The work planned will explore methods to
mitigate this effect and to extend sensing capabilities of the CricketSat sensor, thereby
allowing reliable in situ measurements from -90 C to +60 C.
INTRODUCTION
Low-cost wireless sensors may be useful for networked and non-networked applications.
In a wireless network, the use of many simple, non-networked sensors may be used to
improve the spatial resolution of the system at a reduced cost. They also may serve a
purpose for single-use applications such as expendable, balloon-borne instruments. The
use of the CricketSat temperature sensor at UVM is an example of the latter case.
Balloon-borne CricketSat sensors have been used at UVM to perform temperature, and
air pressure measurements in the upper atmosphere. Flights were performed in
collaboration with the Medgar Evers College of New York City and EPSCoR funded
outreach with the Milton High School of Milton, Vermont. Upper atmospheric
temperatures as low as -69 degrees Celsius have been measured with the CricketSat
devices. Unfortunately, these measurements were far outside of the effective region of
operation for the sensor. Due to the non-linear frequency response of the temperature
sensing circuit, extreme cold temperature measurements suffer poor resolution and
accuracy.
SIGNIFICANCE OF THE PROBLEM
The limited functionality of the CricketSat temperature sensor significantly impacts its
usefulness in extreme cold environments. The sensor has many positive qualities that
justify an effort for improvement. These include its low-cost, long-range wireless
capability, educational benefit, circuit simplicity and adaptability.
Improving the CricketSat functionality would satisfy the present demands and provide
opportunities for new applications. How much improvement would be useful?
Atmospheric temperatures as high1 as +58 C and as low2 as -89 C have been recorded at
the earth's surface and to an altitude of 100 km. Redesigning the CricketSat sensor to
encompass this temperature range would extend its capabilities for worldwide
environmental applications.
Linearization of the frequency response would widen the temperature sensing range,
provide a constant sensitivity, and simplify calibration to one or two data points.
Improvements to this circuit may also apply to other sensor adaptations, providing a
benefit to them as well. Specifically, this would include the air pressure and humidity
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CricketSat sensors developed at the university, since they suffer from similar nonlinearity and sensitivity issues.
SUMMARY OF BACKGROUND MATERIAL
NASA Space Grant Program
The CricketSat wireless temperature sensor was developed to support the NASA National
Space Grant Fellowship3 "Crawl, Walk, Run, Fly" Student Satellite Program4. This
program exists to teach students the fundamentals of space hardware development.
Project complexity ranges from the very simple CricketSat, to advanced earth-orbiting
satellites. CricketSat, BalloonSat, CanSat and CubeSat are the primary student satellite
programs.
Student Satellite Programs
The CricketSat is the most basic and lowest cost ($10) of the student satellites. It is a
single-sensor telemetry design. It is typically flown on a small balloon. Temperature
measurements are made during the flight and transmitted live to a ground receiving
station. Balloon-borne CricketSat sensors typically reach altitudes of 10 km and travel a
distance in excess of 80 km before the signal is lost. Tracking and recovery of these
student satellites is difficult.
BalloonSat is a much larger and more expensive ($800) satellite than CricketSat. The
recoverable system is launched using a much larger balloon, required for the larger
payload. This usually includes a data-logging instrument, GPS, VHF radio, and a
camera. BalloonSat flights typically reach altitudes of 30 km before the balloons burst
and the payload descends by parachute. Payload recovery is essential for the collection
of the data.
The CanSat satellite is launched from a large amateur-type rocket or dropped from an
airplane. All of the electronics must fit inside a volume the size of a soda can, hence the
name. Data measurements are taken and results transmitted while descending by
parachute. Launch altitudes may reach 4 km. CanSat may be used as a development and
testing platform for CubeSat hardware.
The CubeSat satellite is designed for low-earth orbit. The cube-shaped payloads provide
education, industry and government low-cost access to space. Students, through handson work, develop useful skills required in the aerospace industry.
Space Grant Outreach
Each state has its own Space Grant Consortium of participating colleges and universities.
The Colorado Space Grant Consortium5 conducts "Starting Student Space Hardware
Programs" workshops at the University of Colorado Boulder campus. Educators from
various universities, colleges and high schools attend the one-week workshop to learn
methods for starting a student satellite program at their own institution. Dr. Mark Miller,
Dr. Jeff Frolik and the author have represented UVM at these workshops and have used
the material and training effectively at the university. Related work at UVM revolves
around the CricketSat program. This work involves improvements and adaptations to the
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original design, use in a freshman engineering design course and high school outreach
programs.
History and Use of the CricketSat Sensor
The CricketSat sensor was conceived at the Stanford University Space Systems
Development Laboratory6, directed by Professor Bob Twiggs. In 1999, David Joseph, a
mentor at the laboratory developed the CricketSat sensor, at the suggestion of Professor
Twiggs.
The primary application of the CricketSat is its use as an air-borne telemetry sensor as
described earlier. During the flight, students remotely record measurements of
atmospheric temperatures. Prior to flight, the CricketSat sensors are assembled and
calibrated by students.
The CricketSat Wireless System
The method of determining the remote temperature is straightforward. The CricketSat
produces an audio frequency tone that varies with temperature. An onboard radio
transmitter sends this tone on a commercial UHF radio frequency. A ham radio receiver
is used to receive the signal and recover the tone. The frequency of the tone can then be
measured with an instrument or computer software. Calibration charts are used to
determine the temperature from the measured frequency.
CricketSat Circuit Operation
The CricketSat contains simple circuitry to produce the temperature sensitive tone. The
heart of the circuit is the popular 555-Timer7 IC, configured as an oscillator, shown in
Figure 1 below. The frequency of oscillation is determined by resistive and capacitive
timing components. All component values are fixed except for the thermistor, R1, whose
resistance changes with temperature. Changes in this resistance affect the oscillator
frequency. The signal from the oscillator drives a display LED and the UHF transmitter.
The 434 MHz AM transmitter module provides 10 mW of signal output power that can
be received for tens of kilometers. A 9-Volt battery provides power for the circuit.
The nominal oscillator frequency is around 1000 Hertz (Hz) at 20 degrees C. This
frequency varies in a positive, non-linear relationship to the changing temperature as
shown in Figure 2. The circuit is overly sensitive for warmer temperatures and under
sensitive for colder values. This presents the primary problem affecting the resolution
and accuracy of extreme cold temperature measurements.
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Figure 1: The 555-timer astable oscillator.
Diagram courtesy of Doctronics8.
Figure 2: CricketSat non-linear frequency response
to temperature.
Extreme Temperature Electronics
Extreme Temperature Electronics9 (ETE) involves the operation of electronic
components outside their rated temperature range. For most components, this involves
temperature values below -55 C and above +125 C. Operation outside of the specified
temperature range may affect functionality, accuracy and reliability. These may occur as
electrical performance issues and mechanical packaging failures. There are many earth
and space-based applications where ETE operation is essential.
High Temperature Electronics (HTE) applications include petroleum and geothermal well
measurements, engine sensors and actuators, and Venus space probes. Low Temperature
Electronics (LTE) applications include sensitive astronomical instruments, satellites, and
the Mars rovers. NASA does a lot of work into investigating the use of commercial
components for ETE applications10. Operation of CricketSat at -90 C qualifies it as ETE.
Artwork Similar to the CricketSat
A similar 555-Timer based, temperature-sensing circuit11, named the "Electronic
Cricket", was published by the popular amateur scientist and author Forrest Mims III.
This circuit differs from the Stanford CricketSat, with the use of a speaker as an output
device instead of a radio transmitter. It was developed several years before the
CricketSat, possibly before 1990.
A couple of commercial electronic cricket kits are available on the Internet. These also
respond to temperature change, but are sold primarily for novelty purposes. These
circuits both drive a speaker, similar to the Mims circuit, but are also designed to sound
like a cricket. The first kit is the MK104 Electronic Cricket distributed by Velleman Kits
NV12. This circuit differs from the Mims circuit with the use of CMOS inverters to
create multiple oscillators, mimicking the cricket sound. The second kit is the ECS1
Electronic Cricket Sensor distributed by Ramsey Electronics13. This kit uses three 555Timer ICs to mimic a cricket sound.
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PROBLEM STATEMENTS
A couple of assumptions will be stated. The first assumption is that all of the work will
involve modifications to the CricketSat sensor. The second assumption is that all of the
CricketSat components will be exposed to the ambient temperature (in situ) that is to be
measured. A minimally insulated configuration provides the simplest implementation of
the CricketSat.
Performance and Cost Goals:
1. Measure temperature from -90 C to +60 C
2. Minimum resolution of 4 Hz per degree C
3. Frequency range limitations: 200 Hz to 4000 Hz
4. Components must be low cost, available through multiple sources, nonspecialized and have a long existing product cycle
5. Calibration: Convenient for students
6. Accuracy: Best possible with minimal calibration
7. Power: Minimal power drain, battery life > 2 hours
8. Maintain low cost: $10 to $15 unit cost
9. Lightweight: < 70 grams to allow single-sensor flights on a 2-foot. helium balloon
Scope of the work:
The scope of the work will involve background research, design, simulation, assembly,
calibration and evaluation of the CricketSat temperature circuits to meet the performance
and cost criteria. If linearization is possible, CricketSat adaptations for other sensor types
will be explored.
RESEARCH APPROACH
Below are the methods to address the boldface topics of interest:
1. Thorough evaluation of the latest-version UVM CricketSat:
 Perform tests to precisely determine frequency response to wide temperature
variations
 Perform tests to determine undesired sensitivities to temperature and supply
voltage
2. Minimization of undesired circuit sensitivities:
 Select cost-effective components to minimize undesired sensitivities.
 Apply circuit design changes as necessary.
 Perform Monte Carlo analysis to verify performance across wide temperature
range.
 Physical testing and verification.
3. Improvement of frequency response to temperature:
 Review types of temperature sensors, costs and methods of implementation
into the oscillator circuit.
 Strive for a linear-response circuit design
 Matlab and/or Excel will be used to demonstrate the sensitivity and the nature
of the mathematical relationships (linear, exponential, log, reciprocal) of the
various methods evaluated. The programs will also used to for selecting
component and parameter values.
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
Pspice will be used to verify the final results and to generate a schematic
diagrams
4. Effect of minimal calibration on measurement accuracy:
 Test effects of single and double-point calibration methods on the accuracy of
a linear-response CricketSat
5. Detrimental effect of operation outside the specified temperature range:
 Perform thermal cycling tests to evaluate the detrimental effect on circuit
performance
6. Application of a linear-response circuit for other types of sensors:
 Design and test implementations for pressure, humidity and light intensity.
WORK COMPLETED TO DATE
UVM Improvements to the Original CricketSat Design
Improvements to the general design of the CricketSat sensor include modifications to the
electrical circuit and the printed circuit board layout. Changes were made to improve
electrical performance, reliability, expandability, assembly process, and instructional
value.
 Electrical performance was improved with the addition of a voltage regulator to
reduce circuit sensitivity to power supply variation, additional capacitors to decouple
power supply noise, and printed circuit board (PCB) ground shielding enhancements
to minimize noise coupling.
 Reliability was improved with the addition of strain relief holes for the power and
antenna wiring, horizontal placement of components to minimize damage, and the
addition of a diode for the protection of reverse battery connections.
 Improvements for future expandability were made with changes to the prototype area.
The size of the area was increased to support more and larger components, and traces
added to compliment connectivity.
 Ease of board assembly and error reduction was achieved with the addition of a
silkscreen layer to aid in component placement and a solder mask layer to minimize
soldering errors. Mounting holes were enlarged to accommodate common #4 bolts.
 Labeled test points were added to the board for signal probing, aiding circuit
understanding and debugging.
UVM CricketSat Testing in low Temperature Environments
Low temperature testing was performed at the university with freezer calibrations and
high altitude balloon flights. Two freezers were used with temperatures of -25 C and -55
C for calibration of the sensors. Solo CricketSat balloon flights and larger BalloonSat
flights were held during the summers of 2003 and 2004.
The smaller balloon flights achieved minimum temperatures to -40 C. In this
configuration, the sensor boards were attached to the balloon with a string and no
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protection from the elements. The frequency produced was very slow at this temperature
and became difficult to measure.
For the final BalloonSat flight, a modification was made to the circuit to double its
frequency at room temperature. This would hopefully result in a higher frequency at the
coldest temperatures. On this flight, two external temperature sensors were flown. One
with the modification was located in an insulated flight bag, with only the sensor
exposed. The other unmodified sensor was taped to the outside of the bag, completely
exposed. Both achieved nearly the same results of -69 C. Amazingly, this agreed closely
with results from the nearest National Weather Service (NWS) balloon sounding station.
Again, the frequency was very low and difficult to measure.
The operation of the CricketSat at these low temperatures is intriguing. The commercialgrade 555 timer is specified for operation down to 0 degrees C. Five CricketSat devices
have flown unprotected at temperatures below this value and all appeared to have
operated normally.
Pressure and Humidity CricketSat Adaptations
Air pressure and humidity versions of the CricketSat were created at UVM. These active
devices control the 555 timer's frequency using a control voltage pin on the device. This
produces a non-linear response. The air pressure sensor, calibrated as an altimeter,
worked in good agreement with an onboard GPS during a BalloonSat flight. The circuit
was limited to an altitude of 10 km due to the sensor. Future work will be done to
improve its performance to altitudes 30 km. The humidity sensor has not been tested.
CricketSat Array Platform
For the final BalloonSat flight, a system was developed to allow multiple CricketSat
sensors to be operated on a single flight. An embedded control system was developed
with assistance with the Milton High School. The controller provided sequential power
to five CricketSat sensors, a camera, and a flashing strobe light. The data was well
segregated and correlated with known standards. Results were presented to the NASA
Northeast Regional Space Grant meeting held in October.
This success sets the groundwork for the development of a low-cost CricketSat-based
radiosonde (CricketSonde) for future development by the author. This educational
instrument would aid classroom instruction and compliment low-spatial resolution of
NWS atmospheric sounding data.
Investigation into a Linear Response Circuit
Of all the methods for modulating the 555-Timer circuit, one that exhibits a linear frequency response would be the most ideal. This behavior produces several benefits
stated earlier in the paper. The foundation for a linear response solution is based on a
modification of a linear ramp configuration of the 555-Timer oscillator.
In the linear ramp circuit, a high-side current source is used to replace the resistor R1
used in the Figure 1 circuit. The capacitor charging period is inversely proportional to
the strength of the current source. The discharge period of the capacitor is constant. The
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derived frequency of oscillation is shown in Equation 1 below. If the term t Discharg e is
kept proportionally small to the charge period, the circuit exhibits a linear response to the
current source ISRC.
1
f 
t Discharg e 
(1)
C *VCC
3 * I SRC
Now what is needed is a current source with a linear sensitivity to temperature. Such a
device has been around for over 25 years. The device is the LM234 with use as a current
source and a temperature sensor. It produces a current that is related to the value of a set
resistor (RSET) and with sensitivity to the absolute temperature. The LM234 is shown
incorporated in to the timer circuit in Figure 3. The resulting theoretical frequency
response is shown if Figure 4.
Figure 3: Proposed 555-timer oscillator circuit
incorporating the LM234 temperature sensor.
Figure 4: Theoretical linear frequency response to
temperature.
SCHEDULE AND PLAN
Item
#
1
2
Task
Thorough evaluation of the present-version CricketSat
 Issues
 Nonlinear response to temperature
 Undesired sensitivity to supply voltage
 Undesired sensitivity to temperature
 Lab testing
 Results and documentation
ASEE conference paper
 Compose and submit
 PPT presentation
 Attend conference
8
Weeks
Activity Dates (2005)
2
Feb 28 - March 14
2
1
Feb 28 - March 18
March 28 - April 4
April 8, 9
3
4
5
6
7
8
9
10
11
12
13
14
Minimization of CricketSat undesired sensitivities
 Theory and methods
 Voltage regulation
 Component tolerance
 Simulation
 Lab testing
 Results and documentation
Improving CricketSat nonlinear temperature response
 Theory and methods
 Thermistor compensation
 Linear-response circuits
 Simulation
 Lab testing
 Results and documentation
Minimal calibration effects on accuracy
 Theory and methods
 Simulation
 Lab testing
 Results and documentation
Linear-response adaptations for other sensors
 Theory and methods
 Air pressure
 Humidity
 Light intensity
 Simulation
 Lab testing
 Results and documentation
In-flight tests
 Data collaboration
 NOAA/NWS balloon sounding flights
 BalloonSat on-board instruments
 Results and documentation
Reliability and accuracy issues
 Theoretical
 Testing
 Results and documentation
Summary of results and conclusions
Thesis format check with Graduate College
Completion of the thesis
Submit defendable thesis copies to committee members
Defend thesis
Submit final thesis copies to Graduate College
FACILITIES NEEDED AND AVAILABILITY
KEY
 Item is available
 Item is needed
Facilities
 Office space and lab space
Equipment
9
2
April 11 - 25
4
April 25 - May 23
1
May 23 - May 30
4
May 30 - June 27
1
TBD
2
June 27 - July 11
1
July 11 - July 18
2
July 18 - Aug 5








Laptop computer
UHF radio receiver
Wide-range thermometer
UHF Yagi antenna
RF spectrum analyzer
Oscilloscope
Soldering equipment
Freezers functional down to -50 C for calibration and testing
 Dry ice (-78.5 C) for colder calibration and testing
Required Software
 Audio frequency measurement: Spectrogram
 Mathematical: Excel, Matlab
 Circuit design and analysis: PSpice
 Documentation: MS Word
 Website: HTML, MS Word
Materials to be purchased
 Electronic components
 Printed circuit boards
 Balloons
 Parachutes
 Helium
Funding
 $500 - Vermont Space Grant
 $500 - HELiX / EPSCoR
DELIVERABLES
1. ASEE conference paper describing CricketSat educational activities and outreach
at UVM
2. A written thesis
3. A technical conference paper
4. Various CricketSat type sensors resulting from the work
5. A UVM CricketSat website
6. Additional design documentation not included in the thesis
REFERENCES
1
Infoplease, Highest Recorded Temperatures, online: http://www.infoplease.com/ipa/A0001375.html
Infoplease, Lowest Recorded Temperatures, online: http://www.infoplease.com/ipa/A0001377.html
3
NASA, National Space Grant and Fellowship Program, online: http://www.hq.nasa.gov/spacegrant/
4
NASA, Learning to Fly on Mars, online:
http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Learning_to_Fly_on_Mars.html
5
NASA Space Grant Consortium, StudentSat Workshop, online: http://spacegrant.colorado.edu/studentsat/
6
Stanford University, Space Systems Development Laboratory, online: http://ssdl.stanford.edu/
7
555-Timer IC History, The 555 Timer IC, an Interview with Hans Camenzind - The Designer of the Most
Successful Integrated Circuit Ever Developed, online:
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Index.htm
8
Doctronics Educational Publishing, 555-Timer IC - Astable Circuits, online:
http://www.doctronics.co.uk/555.htm
2
10
9
Kirschman, Randall K., Extreme-Temperature Electronics, online:
http://www.extremetemperatureelectronics.com
10
NASA Electronics Parts and Packaging Program, Extreme Environment Electronics and Packaging,
online: http://nepp.nasa.gov
11
Mims, Forrest M., 1990, Engineer’s Mini-Notebook Science Projects, Electronic Cricket, page 45
12
Velleman Components NV, Electronic Cricket, online: http://www.velleman.be
13
Ramsey Electronics, Electronic Cricket Sensor Kit, online: http://www.ramseyelectronics.com/
BIBLIOGRAPHY
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2.
3.
4.
5.
Wallace, J.M., Peter V. Hobbs. 1977. Atmospheric Science, An Introductory Survey, San Diego, CA:
An Elsevier Science Imprint, Academic Press.
Brock, F.V, Scott J. Richardson. 2001. Meteorological Measurement Systems, New York, NY: Oxford
University Press.
NWS Radiosonde Observations - Factsheet: U.S. National Weather Service Upper-air Observations
Program, online: http://www.ua.nws.noaa.gov/factsheet.htm
Radiosonde Database Access: NOAA/FSL Radiosonde Database, online: http://raob.fsl.noaa.gov
Federal Meteorological Handbook No. 3, Rawinsonde and Pibal Observations: Office of the Federal
Coordinator of Meteorology, online: http://www.ofcm.gov/fmh3/text/
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