University of Vermont
CricketSat
What’s a CricketSat?
• An electronic cricket
• Produces chirps, clicks or tones
relating to temperature
• Provides wireless balloon-borne
or ground-based measurements
• Entry level of NASA Student
Satellite Program
CricketSat Benefits
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•
•
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Low cost (<$15)
Easy to assemble
Simple electronic operation
Provides live telemetry
Easy data collection methods
Adaptable for custom
applications
• Middle school through
college use
CricketSat Sensor Circuit
• Oscillator frequency
determined by temperature
• Oscillator output signal
modulates RF carrier
frequency
Receiving Station
• Receiver extracts oscillator
frequency from radio signal
• Oscillator frequency
measured by instruments
or software
• Calibration charts used to
determine temperature
CricketSat Circuit Board
Power Supply
Oscillator
Test Custom Circuit
Points Prototype Area
Radio
Transmitter
Output
CricketSat Schematic Diagram
Power Supply
Oscillator
Output
Transmitter
Power Supply Operation
Power Supply
Power Supply Operation
• 9 Volts unregulated supply
– Max power to RF transmitter for maximum distance.
• 5.0 Volts regulated supply
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–
–
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5-Volt regulator (U2), 5.5-20 Volts input, 5.0 Volts output
Provides constant output as battery discharges (dies).
Required by oscillator circuit for consistent operation.
May be required for student-added circuitry.
• Short-circuit protection
– Prevents damage upon reverse battery connection.
– 5-Volt regulator has built-in protection.
– Diode D2 added to protect RF transmitter module.
Oscillator Operation
Oscillator
Output
Sensing the Temperature
Thermistor Device
• Resistance changes with
temperature as shown in
the graph
• Exponential resistance
relationship
• Use with an oscillator
circuit provides a simple
and low cost solution
Thermistor
Temperature Sensitive Oscillator
Thermistor
• Produces an oscillation that
changes with temperature
• Circuit based on the popular 555Timer IC
• Oscillator frequency determined
by two resistors and a capacitor
• Resistive and capacitive type
sensors may be substituted
• In our case, the upper resistor is
replaced with the thermistor
• Changes in temperature affect the
oscillator frequency as shown in
the chart
Oscillator Demo
Thermistor
R1
555 Timer IC
Vcc
Not Used
Voltage on capacitor C1
R2
Digital
Output
Signal
C1
LED
Capacitor
Charge &
Discharge
Waveform
Time
Simulation courtesy of Williamson Labs: http://www.williamson-labs.com
Frequency vs Temperature
• Oscillator Frequency
– Increases with warmer temperatures
– Decreases with colder temperatures
• Finding the Temperature
– A calibration graph, similar to the
one shown right, allows the
temperature to be determined
Oscillator Circuit Operation
• Based on the popular 555 timer IC design.
• Timing components
– Capacitor C1 is the electrical charge storage vessel.
– Resistors R1 and R2 behave as electrical conduits for the
charge to flow into and out of the C1 capacitor.
– R1 is a thermistor whose resistance (conductivity) varies
with temperature.
– The timer, U1, monitors the operation and the discharging
of the C1.
– Timing is controlled by R1, R2 and C1 represented by the
formula:
f ( Hz )   1.44  R1 2 R 2 C 1 
Oscillator Circuit Operation
• Oscillator operation
– Voltage level on C1 oscillates between 1/3 and 2/3 of the
supply voltage (5 Volts).
– Charging interval
• Voltage increases on the capacitor with charge entering from the
series combination of R1 and R2.
• The timer IC monitors the voltage on the capacitor waiting for it
to rise to 3.33 Volts.
• Once it does, it begins to discharge it through R2 alone.
– Discharging interval
• The timer now monitors the voltage on the capacitor until it drops
to 1.67 volts.
• At this point, it ceases the discharge and allows the charging cycle
to repeat.
Oscillator Circuit Operation
• Timer Output
– The timer also provides a digital output relating to
capacitor charging and discharging .
– The output pin is high during the charging interval and
low during the discharge interval.
– The output drives an LED for visual cue, a speaker for
audio and the RF transmitter for remote monitoring.
• Temperature Relationship
– The resistance of R1 increases with colder temperatures
causing the charging interval to increase, and thereby
reducing the oscillator frequency.
– The opposite effect occurs for warmer temperatures.
Transmitter Operation
Transmitter
RF Transmitter Operation
• Purpose
– Modulate (mix) 434 MHz “carrier”
signal and 555-Timer output signal
– Amplify and transmit signal through
antenna sized for 434 MHz
• Common Types of Modulation
– FM: Frequency Modulation
– PM: Phase Modulation
– AM: Amplitude Modulation
• Analog
– Ex: Audio
• Digital (CricketSat)
– Amplitude Shift Keying (ASK)
– Also known as….
– On-Off Keying (OOK)
AM and FM Waveforms: Washington State University,
http://cbdd.wsu.edu/kewlcontent/cdoutput/TR502/page21.htm
CricketSat System Capabilities
CricketSat Sensor Options
Resistive Sensors
Capacitive
Sensors
Other Sensor Types **
Thermistor (temperature)
Humidity
Switch (activation, proximity)
Photocell (light)
Pressure *
Reed Relay (activation, proximity)
Potentiometer (position)
Hall Effect (proximity)
Strain gage* (force, flex)
Infra Red (proximity)
DC Motor (rotation)
* May require signal conditioning circuitry
** Enable CricketSat or trigger it
Compatible Radio Receivers
• Kenwood TH-D7A or Equivalent UHF
• Amateur radio transceiver
• Use with Yagi antenna for longer balloon flights
• CricketSat Low-Cost Receiver Kit
• Simple to assemble
• Range: 600 feet for ground operation, >1 mile for balloons
• Volume control with speaker
• Audio jack allows connection to equipment for data
collection
• Kit or assembled
Data Collection
•
Audio Source: Audio output from receiver
•
Spectrogram
• Audio spectrum analyzer software
• Allows measurement of tone data present
on computer’s audio input jack
• Will data-log to file but results may be
intensive and difficult to decipher
•
Radio Shack Data-logging Multimeter
• Measures dominate frequency of signal
• Works well with strong, clean signal
• Connects to computer using RS232 interface
• Software will data-log at rates from once-asecond to hours
•
CricketScan
• BASIC Stamp - StampPlot
Tone Decoding and Control
•
Two Circuit Options
• Tone decoder
• Frequency to voltage converter
•
Tone Decoder IC
• IC digital output activated if received tone is within
bandwidth of decoder center frequency
• eg: 1000 Hz +/- 50 Hz will activate
•
Frequency to Voltage Converter IC
• IC can provide a digital output if tone is >= to the
tuned frequency
• Good for heating or cooling projects
•
Control
• Decoded output signals can be used to activate
power transistors or relays to activate motors, fans
etc.
Sample Designs (1)
Wireless sound level measurement system
Mountain Skateboard Speedometer
Golfing Wind Caddy
Wind Over Water Monitor
Sample Designs (2)
Intruder Alarm
Bike Stress
Wind and Temperature Probe
Snow Load Alarm
HELiX 2003: Wireless Weather Station
SpaceShipOne
Atmospheric Profile
Measurements
BalloonSat
CricketSat
Commercial Airliners Fly Here
CricketSat Sensor Array System
Audible
External
Alarm
Temperature
Internal
Temperature
Accelerometer
External
Batteries
Strobe Light
Temperature
Switch Panel
Altimeter
Power
Sequencing
Controller
Camera
MHS-3 Flight Results:
Temperature vs Altitude Correlation
More Info
• CricketSat Web Site
– http://www.uvm.edu/~cricksat
• This presentation
– http://www.uvm.edu/~cricksat/documents/ME1Csat2007.ppt
• CricketSat Assembly Manual
– http://www.uvm.edu/~cricksat/RevG/UVM_CSat_G_Assembly.ppt
• Mike Fortney email
– mfortney@uvm
– mfortney@cems.uvm
Questions?