A LOW-COST LINEAR-RESPONSE WIRELESS TEMPERATURE SENSOR FOR
EXTREME ENVIRONMENTS
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.
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, balloonborne 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.
1

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 high
1
as +58 C and as low
2
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 circuit output 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
CricketSat sensors developed at the university, since they suffer from similar nonlinearity and sensitivity issues.
NASA Space Grant Program
The CricketSat wireless temperature sensor was developed to support the
NASA National Space Grant Fellowship
3
"Crawl, Walk, Run, Fly" Student Satellite
Program
4
. This program exists to teach students the fundamentals of space hardware
2

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.
3

Figure 1: Students launching a BalloonSat payload at the Milton High
School, Vermont.
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 hands-on 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 Consortium
5
conducts "Starting Student
Space Hardware Programs" workshops at the University of Colorado Boulder
4

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 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 Laboratory
6
, 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.
5

Timer Frequency vs Temperature
1800
1600
1400
1200
1000
800
600
400
200
0
-90 -80 -70 -60 -50 -40 -30 -20 -10 0
Temperature (C)
10 20 30 40 50 60
Figure 2: CricketSat frequency response to temperature.
The CricketSat Wireless System
The CricketSat wireless system is composed of one or more CricketSat sensors, a UHF receiver and a instrument for measurement as shown in Figure 5. The method for determining the remote temperature is straightforward. The CricketSat timer circuit produces an audio frequency tone that varies with temperature. An onboard radio module transmits this tone over 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 prepared before the launches are used to determine the temperature from the measured frequency.
6

Figure 3: The CricketSat wireless system consists of the remote sensor and a receiving station. A commercial amateur (ham) UHF radio can be used for long-range reception during balloon flights.
CricketSat Circuit Operation
The CricketSat contains simple circuitry to produce a temperature sensitive tone. The heart of the circuit is the popular 555-Timer
7
IC, configured as an oscillator, shown in Figure 2 below.
Figure 4: The original Stanford CricketSat design. Changes in temperature affect the resistance of the thermistor, R
1
, and thereby the oscillation frequency of the 555-timer circuit. The oscillation is presented to an LED for visual cue and RF transmitter for wireless use.
The frequency of oscillation is determined by resistive and capacitive timing components R
1
, R
2
and C
1
. All component values are fixed except for the thermistor,
7

R1, whose resistance decreases with temperature. The capacitor charges from the power supply through the series combination of R1 and R2. Thus, as the temperature increases, the thermistor resistance decreases, thereby shortening the charging interval. The opposite is true of colder temperatures, as illustrated in Figure 7 below.
The discharge interval remains constant since it occurs only through the fixed value resistor, R2.
The CricketSat can be used in two different modes: pulse and tone. The only difference between these modes is the magnitude of the frequency. The pulse mode is useful when radio receiver and frequency measurement equipment is not available.
This situation is likely to occur when CricketSat sensors are assembled for student use at home. In this low frequency mode, LED flashes, which are produced once every ½ to 10 seconds, are counted over a timed interval to determine the temperature. This is a simple procedure, but prone to high error when the events are few. A more accurate method is to measure the time interval of ten or more LED flashes with a stopwatch.
The tone mode of operation will typically produce an oscillation in the range of 500-2400 Hz. This mode is preferred for wireless measurements, since the frequency is easier to measure than the slower method, providing higher temperature resolution and accuracy. Modes are changed with the selection of C1. A 47 uF capacitor is used for the pulse mode and a 0.1 uF capacitor is used for the tone mode.
8

Figure 5: The effect of temperature on frequency produced by the CricketSat circuit.
Changing temperature affects only the charge interval.
The timer works to monitor and control the charging and discharging of timing capacitor C1. Internally, it has a pair of voltage comparators with thresholds set for 1/3 and 2/3 of the supply voltage. These values determine the voltage limits of the signal on the timing capacitor and state transition. The 555 timer contains a latch, which is used to determine the charging state and to control an internal transistor for discharging the timing capacitor.
Figure 6: PSpice astable timing waveforms of the 555-timer circuit. The top trace shows the voltage on the timing capacitor. The levels alternate between 1/3 and 2/3 of the 6-Volt supply voltage. The bottom trace represents the digital output of the timer.
9

The timing capacitor charges and discharges in an exponential fashion shown in the upper trace of the figure above. Note that for a 6-Volt power supply, the voltage on the capacitor cycles between 1/3 and 2/3 the supply voltage, as expected.
The latch output is low during the discharge interval and high during the charging interval, as shown on the lower waveform. It is also the timer’s output signal, used to drive the LED and RF transmitter devices.
The output of the timer toggles the power and data lines of the RF transmitting module, producing AM modulation of the 434 MHz carrier signal, shown in Figure 8.
The radio signal is output through a simple dipole antenna, providing a near uniform horizontal beam pattern. The 10 mW signal can be received for tens of kilometers using a standard UHF amateur radio receiver, fitted with a moderate-gain yagi antenna.
Figure 7: Modulation of the RF signal produced by the CricketSat sensor. Note that this is AM modulation, with the carrier signal toggled on/off at the slower rate of the 555-timer oscillation.
A printed circuit board, shown in Figure 3, is also part of the Stanford design, to house the components and facilitate assembly of the circuit. The dipole antenna is attached to one end of the circuit board and a 9-Volt battery to the other end. The board also includes a prototype area to support additional circuitry.
10

Figure 8: The Stanford CricketSat circuit board. The prototype area allows for circuit modifications.
Artwork Similar to the CricketSat
A similar 555-Timer based, temperature-sensing circuit
8
, 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 NV
9
. 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
Electronics 10 . This kit uses three 555-Timer ICs to mimic a cricket sound.
11

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 only the temperature sensor will be exposed to the environment in which to be measured. The CricketSat circuit board will be protected, operating within the allowed temperature range of its’ components. 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
12

the performance and cost criteria. If linearization is possible, CricketSat adaptations for other sensor types will be explored.
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
2.
Minimization of undesired circuit sensitivities :

Select cost-effective components to minimize undesired sensitivities.

Apply circuit design changes as necessary.

Perform sensitivity analysis based on component tolerances.

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.
13


Pspice will be used to verify the final results and to generate schematic diagrams
4.
Effect of minimal calibration on measurement accuracy :

Test effects of no, single and double-point calibration methods on the accuracy of a linear-response CricketSat
5.
Application of a linear-response circuit for other types of sensors:

Design and test implementations for pressure, humidity and light intensity.
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.
14

Figure 9: The UVM CricketSat RevF circuit board. Many board modifications are apparent including silkscreen, solder mask, enlarged prototype area, test points and strain relief holes for the wires.
15

Figure 10: UVM CricketSat RevF schematic diagram. Primary electrical improvements are seen over the original design with the addition of the voltage regulator circuit, power switch and D2 protection diode.

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.
16


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 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
17

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.
2500
2000
1500
1000
500
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Elapsed Time (Minutes)
ExtTemp1 ExtTemp2
Figure 11: Raw frequency data collected during the 07/30/2004 BalloonSat flight. Note that both sensors experienced very low frequencies at the coldest temperatures. Peak altitude occurs at 87 minutes.
18

100
80
60
40
20
0
-20
-40
-60
-80
-100
0 10000 20000 30000 40000 50000 60000 70000 80000 90000
Altitude (Feet)
NWS T1 T2
Figure 12: CricketSat temperature measurement (T1 and T2) correlation with the Albany, NY
National Weather Service (NWS) sounding balloon data. CricketSat data tracks well at lower altitudes. High altitude error likely to radiation heating of the black thermistor sensor in the thin atmosphere.
The operation of the CricketSat at these low temperatures is intriguing. The commercial-grade 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
19

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 set the groundwork for the development of a low-cost CricketSat-based radiosonde (CricketSonde) developed by the author. This educational instrument would aid classroom instruction and compliment low-spatial resolution of NWS atmospheric sounding data. The CricketSonde is used as the test platform for evaluating the CricketSat sensors.
Classic 555 Timer Astable Equation Development
The equation relating to the frequency of the 555 timer astable oscillator is given as: f


R
1

1 .
44
2 R
2

C
1
20

Since this equation already exists, it may seem futile to develop this result from basic principles. It does however serve a useful purpose in establishing the groundwork for further equation development. Developing the equation also allows one to become more familiar with the 555 timer timing mechanisms. This understanding allows one to consider and evaluate other methods of frequency control.
To determine the oscillator frequency, the timing cycle period must first be determined. f

1
Period
 t
CHG
1
 t
DIS
This in turn must be evaluated for the charge and discharge intervals of the cycle period. These intervals occur between the 1/3 and 2/3 VCC voltage levels.
CHARGING INTERVAL:
For the charging interval, the capacitor voltage (Vc) has the following relationship.
V
C
( t )

V
0

A ( 1
 e
 t /

) where
V
C
( t )
 the final voltage on the capacitor after time t
CHG
,
2
3
V
CC
.
V
0

the initial voltage on the capacitor,
1
3
V
CC
.
A
 the applied voltage potential, V
CC

1
3
V
CC
.
  the time constant,

R
1

R
2

C
1
.
Substituting these values into the previous equation yields:
21

2
3
V
CC

1
3
V
CC

V
CC

1
3
V
CC

1
 e
 t
CHG
/

1

Rearranging and collecting terms: e
 t
CHG
/

1

0 .
5
Taking the natural log of both sides and solving for t : t
CHG

0 .
6931

R
1

R
2

C
1
This equation determines the charge-timing interval. Now the discharge-timing interval will be evaluated.
DISCHARGE INTERVAL:
The voltage on the capacitor during the discharge interval has the following relationship:
V
C
( t )

Ae
 t /
 where
V
C
( t )
 the minimum voltage on the capacitor after time t
DIS
,
1
3
V
CC
.
V
0

the initial voltage on the capacitor,
2
3
V
CC
.
A
 the applied voltage potential,
2
3
V
CC

0

2
3
V
CC
.
  the time constant, R
2
C
1
.
Substituting these values into the previous equation yields:
1
3
V
CC

2
3
V
CC
 e
 t
DIS
/

2

Rearranging and collecting terms:
22

e
 t
DIS
/

2

0 .
5
Taking the log of both sides and solving for t : t
DIS

0 .
6931 R
2
C
1
FREQUENCY:
Now the equation for the oscillator frequency can be determined: f
 t
CHG
1
 t
DIS

0 .
6931

R
1

R
2

1
C
1

0 .
6931 R
2
C
1
Simplifying reveals: f


R
1

1 .
44
2 R
2

* C
1
This is the same result as the original equation.
Inspection of the equation leads to a couple of observations for this configuration.

Oscillator frequency has no dependency on the supply voltage, V
CC
.

Linear variances in R
1
, R
2
or C
1
produce non-linear effects in frequency as seen in Figure 13.
23

2500
2000
1500
1000
500
0
0 50 100
Resistance (k Ohms)
150 200
Figure 13: Timer frequency vs. resistance. A linear change in the resistance of R1 produces a very non-linear frequency response over a useful frequency range.
Evaluation of Thermistor-based Application
Intro.
24

Figure 14: PSpice simulation circuit used to evaluate CricketSat thermistor-based operation. R1 represents the thermistor with a nominal 10K value at room temperature. Waveforms are monitored on timing capacitor C1 and the output of the 555 timer IC. R3 is used to provide a light load to please the simulator

Discuss results.

Insert graph of thermistor resistance vs temperature.

Update following graph with PSpice results to compliment shown theoretical.
Timer Frequency vs Temperature
1800
1600
1400
1200
1000
800
600
400
200
0
-90 -80 -70 -60 -50 -40 -30 -20 -10 0
Temperature (C)
10 20 30 40 50 60
Figure 15: CricketSat non-linear frequency response to temperature. Performance is nearly linear down to 0 degrees C, but suffers at much colder temperatures experienced in the upper atmosphere.
25

Frequency Sensitivity to Timing Component Tolerances

Perform PSpice sensitivity analysis based on resistor and capacitor tolerances.
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.
Three methods of frequency control are investigated:

Threshold Control Voltage Method

Ladder Voltage Control Method

Current Source Control Method
THRESHOLD CONTROL VOLTAGE METHOD
This method takes advantage of the control voltage (pin 5) on the 555 timer, by way of forcing an external voltage, seen as source V2 in Figure 16. Normally one would measure 2/3 V
CC
on this pin, representing the upper compare threshold value.
Internally this voltage is produced through the use of a resistor ladder composed of three 5K resistors. This pin is connected at the junction of the upper two resistors.
The lower threshold is one-half that value, produced at the junction of the lower two resistors.
26

Figure 16: PSpice simulation circuit for using a voltage source (V2) to override the timer's internal threshold compare voltages. Stanford CricketSat timing component values are used, with a 10K resistor replacing the thermistor. The 5-Volt source is used to represent the regulated voltage, which is now necessary for this mode of operation.
Through the use of the external forcing voltage, the upper and lower threshold voltages can be overridden and used for frequency control. Since the lower threshold is always one-half of the upper threshold voltage, the size of the comparator window varies with the changing voltage. A larger forced voltage produces a larger window.
Since these compare values have no effect on the timing capacitor rate of charge and discharge, a larger compare window results in longer timing intervals for the capacitor voltage to reach the comparator thresholds.
To determine the oscillator frequency, the timing cycle period must first be determined as before. f
 t
CHG
1
 t
DIS
27

What is different in this configuration is the value of the upper and lower threshold limits that determine charge and discharge states inside the timer. The externally forced level provided by voltage source V2 in the schematic defines the upper threshold limit and one-half of that voltage defines the lower limit.
Figure 17: Simulation waveforms showing the effect of control voltage on the charging cycle of the timing capacitor. The top and bottom waveform shows the capacitor voltage using control voltage of
4.0 Volts and 2.0 Volts respectively. Note that the lower limit is always one-half the control threshold value. Enlarging the amplitude of timing window lowers the frequency and reducing the amplitude increases the frequency.
Charging Interval
For the charging interval, the capacitor voltage (Vc) has the following relationship.
V
C
( t )

V
0

A ( 1
 e
 t
CHG
/

1 ) where
V
C
( t )
 the final voltage on the capacitor after time t
CHG
, V
THR
.
V
0

the initial voltage on the capacitor,
1
2
V
THR
.
28

A
 the applied voltage potential, V
CC

V
THR
2
.
  the charging time constant,

R
1

R
2

C
1
.
Substituting these values into the general equation yields:
V
THR

V
THR
2

V
CC

V
THR
2
( 1
 e
 t
CHG
/

1 )
Rearranging to separate the exponential component: e
 t
CHG
/

1

1

1
2 V
CC
V
THR

1
Taking the natural log of both sides and solving for t : t
CHG
  ln




1

1
2 V
CC
V
THR

1





R
1

R
2

C
1
Note the dependency on the supply voltage (V
CC
).
Discharge Interval
The discharge interval follows the familiar form:
V
C
( t )

Ae
 t /
 where
V
C
( t )
 the final voltage on the capacitor after time t
DIS
,
V
THR
2
.
V
0

the initial voltage on the capacitor, V .
THR
A
 the applied voltage potential,

V
THR

0


V
THR
.
29

  the discharge time constant, R
2
C
1
.
Substituting these values into the general equation yields:
V
THR
2

V
THR
* e
 t
DIS
/

2
Rearranging to separate the exponential component: e
 t
DIS
/

2

0 .
5
Taking the natural log of both sides and solving for t : t
DIS

0 .
6931 R
2
C
1
This is the same result as equation xx. Note that the discharge interval has no dependency on the supply voltage.
Frequency
Now the frequency can be determined as: f
 t
CHG
1
 t
DIS

 ln




1

1
2 V
CC
V
THR

1





R
1
1

R
2

C
1

0 .
6931 R
2
C
1
This is a rather unpleasant looking result. To test the derivation, substituting
V
CC
3
for
V leads to the classic equation xx.
THR
SIMULATION RESULTS
Results of the PSpice simulation are shown in Figure 16 below. Over a limited voltage range, the frequency response is somewhat linear. A frequency sensitivity of
515 Hz/Volt is demonstrated with a linear correlation factor of 0.991.
30

Timer Frequency vs Threshold Control Voltage
1800
1600
1400
1200
1000
800
600
400
200 y = -515.68x + 2616.5
R
2
= 0.991
0
2 2.5
3 3.5
Threshold Control Voltage (Volts)
4
Figure 18: Simulation results demonstrating 555-timer frequency response to vary threshold control voltage. Note that over a limited range, the effect is somewhat linear. Error bars represent the error introduced by the voltage regulator.
Supply voltage sensitivity must be investigated as well. This is determined by the tolerance of the TL750L05 voltage regulator. The specifications claim an output voltage of 5.00 Volts, with a tolerance of +/- 0.25 Volts (+/- 5%). PSpice simulation runs were performed over this span with the results in Table 1 below.
Table 1: Threshold control method sensitivity to regulated supply voltage.
V
CC
= 4.75V V
CC
= 5.0V V
CC
= 5.25V
Variance % Variance
V
THR
= 2V
V
THR
= 3V
1555 Hz
950 Hz
1630 Hz
1025 Hz
1700 Hz
1100 Hz
V
THR
= 4V 515 Hz 595 Hz

Discuss error.
 Discuss advantages and disadvantages.
670 Hz
+70/-75 Hz
+75/-75 Hz
+75/-80 Hz
+4.1/-4.4
+7.3/-7.3
+13/-13
31

LADDER VOLTAGE CONTROL METHOD

Discuss Method.
Figure 19: Simulation circuit used to investigate the effect of RC ladder voltage (V2) on timer frequency. The value of R2 has been reduced to increase duty cycle and minimize the nonlinearity.

Discuss equation development. f
 t
CHG
1
 t
DIS
Charging Interval
2 V
CC
3

V
CC
3

V
LAD

V
CC
3
( 1
 e
 t
CHG
/

1 ) t
CHG
  ln


1

3 V
V
CC
LAD

V
CC



R
1

R
2

C
1
32

Discharge Interval t
DIS

0 .
6931 R
2
C
1
Frequency f
 t
CHG
1
 t
DIS

 ln

 1

3 V
V
CC
LAD

V
CC
1



R
1

R
2

C
1

0 .
6931 R
2
C
1
Timer Frequency vs RC Ladder Voltage
2000
1500
1000
500 y = 549x - 1527
R
2
= 0.9963
0
3 4 5
RC Ladder Voltage (V)
6
Figure 20: Simulated response of timer frequency to control of RC ladder voltage. Note that the result is nearly linear over a narrow voltage range (3.5V - 5.0V). Also demonstrated is that the control range can exceed the supply voltage (5V) to the timer IC, increasing the range of control.

Discuss results.
Table 2: Ladder voltage control method sensitivity to regulated supply voltage.
V
CC
= 4.75V V
CC
= 5.0V V
CC
= 5.25V
Variance % Variance
+135/--- Hz +36/--- V
LAD
=
3.5V
V
LAD
=
4.0V
510 Hz
810 Hz
375 Hz
690 Hz
--- Hz
585 Hz
+120/-105 Hz +17/-15
V
LAD
=
1075 Hz 960 Hz 850 Hz +115/-110 Hz +12/-13
33

4.5V
V
LAD
=
5.0V
1320 Hz 1200 Hz

Discuss error.

Discuss advantages and disadvantages.
1090 Hz
+120/-110 Hz +11/-10
CURRENT SOURCE METHOD
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 derived frequency of oscillation is shown in Equation 1 below. If the term t
Disch arg e
is kept proportionally small to the charge period, the circuit exhibits a linear response to the current source I
SRC
.
(1)
34

Figure 21: PSpice simulation circuit used to investigate the use of a current source method for 555timer frequency control.

Discuss equation development.
Charging Interval f
 t
CHG
1
 t
DIS
Q

CV

Q
 t

I

C

V
 t
 t

C

V
I t
CHG

C
1
V
CC
3 I
SRC
Discharge Interval
V ( t )

2
3
V
CC

2
3
V
CC

V min

 e
 t
DIS
/

2
35

t
DIS
  ln




1
3
V
CC

V
MIN
2
3
V
CC

V
MIN




R
1
C
1
Frequency f
 t
CHG
1
 t
DIS
If t
DIS
is kept proportionally small, then linear f

3 I
SRC
C
1
V
CC
Figure 22: PSpice simulation waveforms shown for the current source method of 555-timer frequency control. In the upper waveform, note the linear voltage increase on the timing capacitor due to constant current source. The lower waveform shows the digital output signal from the 555 timer.
 Discuss results.
Table 3: PSpice frequency measurements as a function of amount of current supplied by the current source. Note the slight effect on the discharge period as well.
Current Source
(uA)
Charge Time
(us)
Discharge Time
(us)
Period
(us)
Frequency
(Hz)
175
200
225
953.948
834.704
741.959
76.608
77.539
78.493
1030.556
912.243
820.452
970.3
1096
1219
36

250
275
300
325
350
375
667.763
607.057
556.468
513.663
476.973
445.174
79.473
80.444
81.470
82.533
83.659
84.779
747.236
687.501
637.938
596.196
560.632
529.953
1338
1455
1568
1677
1784
1887
555 Timer Frequency vs Current Source
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
175
R
2
= 0.9989
200 225 250 275 300 325
Current Source (uA)
350 375 400
Theoretical PSpice
Figure 23: Graph demonstrating the linear frequency response of the current source control method. Note the agreement between the theoretical, equation based results, and the PSpice simulation results. Also note the linearity over this wide current range.
Table 4: Current source method sensitivity to regulated supply voltage.
V
CC
= 4.75V V
CC
= 5.0V V
CC
= 5.25V
Variance
% Variance
+44/-44 Hz
+4.5/-4.5
I
SRC
=
175mA
I
SRC
=
275mA
1016 Hz
1520 Hz
972 Hz
1455 Hz
928 Hz
1395 Hz
I
SRC
=
375mA
1965 Hz 1885 Hz

Discuss error.

Discuss advantages and disadvantages.
1815 Hz
+65/-60 Hz
+80/-70 Hz
+4.5/-4.1
+4.2/-3.7
37

Implementation of a Linear Temperature Current Source
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 24: Schematic showing implementation of the LM234 current source device as a temperature sensor. The value of R1 is selected to provide a suitable sensitivity and frequency range.

Discuss results and error.

Update following graph with PSpice results to compliment shown theoretical.
38

Timer Frequency vs Temperature (LM234 Sensor)
2000
1900
1800
1700
1600
1500
1400
1300 y = 5.222x + 1647.8
R
2
= 0.9993
1200
1100
1000
-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
Temperature (C)
Figure 25: Theoretical linear frequency response using the LM324 current source as a temperature sensor. Note the sensitivity at 5.22
Hz

C
and the linearity. Error bars represent sensitivity to voltage regulator tolerance.
CONTROL METHODS SUMMARY

 Discuss advantages and disadvantages of frequency control methods leading to the development and testing of one.
Use a table/decision tree to compare things like accuracy, cost, complexity.
TESTING

Discuss testing methods
 Perform testing
CricketSat Designs to test

Thermistor-based UVM CricketSat (qty 3)

Current source UVM CricketSat (qty 3)
Temperature ranges and methods

Cold


-80C Freezer on 1 st floor of Votey bldg
Dry Ice (-78C)

What is temperature variance vs atmospheric pressure and humidity?

Warm

Room temperature

Hot

Oven
39


Concern: IR absorption causing self heating

Boiling Water (100C)

What is temperature variance vs atmospheric pressure?
Equipment

Electrical test platform

CriketSonde

Measurement equipment

Temperature

Radio Shack meter with thermocouple probe

What is accuracy below specifications?

Low temperature thermometer, where?

Frequency

Radio Shack meter

HP digital scope
TEST RESULTS
 Obtain and discuss test results
 Compare simulated vs actual results.
 Discuss error.
Timer Frequency vs Temperature (LM 234 Sensor)
2000
1900
1800
1700
1600
1500
1400
1300
1200
Theory
Actual
Therm
1100
1000
-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
Temperature (C)
Figure 26: Theoretical vs preliminary measurements for the LM234 CricketSat temperature sensor. Error bars represent dominant voltage regulator tolerance.
40

EXTENSION: A VOLTAGE CONTROLLED CURRENT SOURCE
Figure 27: Simulation circuit used to demonstrate a voltage controlled current source, allowing the use of differential output type sensors to produce a linear frequency response CricketSat sensor.
Interfacing with the CricketSat, the collector of Q1 here would connect to CricketSat timing capacitor C1, instead of R6 in this schematic.
Figure 28: Simulated output of voltage controlled current source, demonstrating linearity and range of operation.
1 Infoplease, Highest Recorded Temperatures, online: http://www.infoplease.com/ipa/A0001375.html
2 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/
41

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 Mims, Forrest M., 1990, Engineer’s Mini-Notebook Science Projects, Electronic Cricket, page 45
9 Velleman Components NV, Electronic Cricket , online: http://www.velleman.be
10 Ramsey Electronics, Electronic Cricket Sensor Kit , online: http://www.ramseyelectronics.com/
1.
Wallace, J.M., Peter V. Hobbs. 1977. Atmospheric Science, An Introductory Survey , San Diego,
CA: An Elsevier Science Imprint, Academic Press.
2.
Brock, F.V, Scott J. Richardson. 2001. Meteorological Measurement Systems, New York, NY:
Oxford University Press.
3.
NWS Radiosonde Observations - Factsheet : U.S. National Weather Service Upper-air
Observations Program, online: http://www.ua.nws.noaa.gov/factsheet.htm
4.
Radiosonde Database Access : NOAA/FSL Radiosonde Database, online: http://raob.fsl.noaa.gov
5.
Federal Meteorological Handbook No. 3, Rawinsonde and Pibal Observations : Office of the
Federal Coordinator of Meteorology, online: http://www.ofcm.gov/fmh3/text/
42