TABLE OF CONTENTS CHAPTER ONE... 1 CHAPTER TWO... 3 CHAPTER THREE... REFERENCES... 36

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2 ACKNOWLEDGEMENTS Many people gave generously of their time and expertise to help make sure this work came to fruition. I thank them all special mention for his unstinting willingness to help is due to my supervisor, Dr. Mwema. ii

3 ABSTRACT The electronic temperature sensor described herein provides a means for the user to monitor his temperature and alerts him when it goes above a limit indicative of fever, and below a limit indicative of hypothermia. Use is made of the temperature dependence of the base emitter voltage of a transistor to design a sensor that turns on an LED when the temperature of the user goes above 39 0 c and also when it goes below 35 0 c. The sensor was successfully designed and simulations of the circuit carried out to observe the variation of the output voltage with temperature. iii

4 TABLE OF CONTENTS Acknowledgements... ii Abstract... iii Table of Contents... iv List of Figures... v CHAPTER ONE Introduction... 1 CHAPTER TWO Literature Review Body Temperature General Principles of Temperature Measurement Types of temperature sensors Thermocouple Principle of Operation Thermocouple types Thermocouple Characteristics Resistance Temperature Detector Principle of Operation RTD Characteristics Thermistors Thermistor Characteristics Thermistor Applications Active Semiconductor Sensors Diode Based sensors Transistor based sensors Sensor selection CHAPTER THREE... Error! Bookmark not defined. 3.0 System design and Simulations Design based on the variation of V BE with temperature Comparator Design Window Comparator Operation Discussion REFERENCES iv

5 LIST OF FIGURES Figure 1: Seebeck Effect... 6 Figure 2: The Spectrum of Voltage-Temperature... 9 Figure 3: The Valence band and the Conduction band Overlap Figure 4 : A Simple Diode Sensor Figure 5: Double Diode Sensors Figure 6: Variation of V BE with temperature.. 19 Figure 7: Basic Relationships for one Transistor.. 19 Figure 8: The band gap (V BE ) Voltage reference 20 Figure 9: Basic Relationships for one Transistor. 23 Figure 10: A Circuit with a Voltage Outputs that varies with Temp Figure 11: The Variation of Voltage with Temperature.. 29 Figure 12: Temperature Sensor Circuit 33 LIST OF TABLES Table 1: Thermocouple Types.. 8 Table 2: Main difference among RTDs thermistors, thermocouple and IC Sensors. 24 Table 3: Strength and Weaknesses of the Various Sensor Types 25 Table 4: Variation of Voltage with Temp at the Temp Tap.. 29 v

6 CHAPTER ONE 1.0 INTRODUCTION The aim of this project was to design and implement a temperature sensor in miniature form that will alert the user to the possibility that he may have a fever or hypothermia. The sensor to be implemented will be coupled to an LED that will turn on when the temperature exceeds a certain threshold. There are number of different technologies that could be used to implement a temperature sensor. Thermocouples make use of the Seebeck effect. A net thermoelectric voltage is generated between a pair of junctions formed from two dissimilar metals. The generated voltage is a function of the difference in voltage between the reference junction and the measuring junction. If the reference junction temperature is known, the other junction s temperature can be calculated from the EMF generated in the circuit. Resistance temperature detectors are sensor, typically metals such as platinum, whose resistance changes with temperature. They make use of the fact that the resistance of metals increases with temperature. As the temperature of a metal increases, there is increased vibration of its atoms which in turn impedes the flow of electrons through it. They offer the advantage of being very accurate and stable at moderate temperature ranges. Thermistors also rely on changes in material resistance to measure temperature. They are constructed of solid semiconductor materials that exhibit a negative temperature coefficient. When the temperature of a semiconductor is increased, its atoms vibrate more. This increased vibration gives additional energy to its valence electrons enabling them to cross the energy gap to the conduction band. Thermistors have the highest sensitivity of the sensors discussed here although 1

7 their resistance versus temperature characteristic is highly non-linear thus requiring additional circuitry for linearization. The operating principle of active semiconductor sensors relies on the potential barrier between the conducting layer and the valence layer in semiconductors. Diode based sensors make use of the fact that the forward voltage is dependent on the reverse saturation current which is in turn strongly dependent on the temperature. The reverse saturation current doubles for every temperature increase of 10K. Transistor based sensors make use of the relationship between a Bipolar Junction Transistor s base-emitter voltage and its collector current. The baseemitter voltage exhibits a negative temperature coefficient of approximately -2mV/ 0 C. Semiconductor sensors cost the least to fabricate among the sensors discussed here. The major disadvantage is their limited temperature range. On consideration of the pros and cons of the different sensor types, a transistor based sensor was chosen as the most practical to implement. A sensor with a higher sensitivity (such as a thermistor) would have been better suited for measuring the small temperature increases that signal fever or hypothermia but these were found to be expensive and not readily available locally. 2

8 CHAPTER TWO 2.0 LITERATURE REVIEW 2.1 Body Temperature Normal body temperature is considered to be 37 0 c ( F). However a wide variation is seen among normal individuals. Mean daily temperature can differ by c (0.9 0 F). If the body temperature is too high, the functions of the cells may become impaired or damaged, if too low, then rate at which food is metabolized decreases. A fine balance between heat production and loss is maintained imperceptibly in the normal individual. Production of body heat is primarily the result of conversion of chemical energy in foods to heat by metabolic and mechanical mechanism. Cellular oxidative metabolism produces a constant stable source of heat. Mechanical muscular contraction results in bursts of heat when needed. Heat produced is conserved by vasoconstriction and diversion of blood flow away from the skin. Heat is lost at the skin surfaces by the mechanisms of convection, radiation and evaporation. Dissipation by convection is more efficient when ambient wind current is increased; evaporation is the primary mechanism in high ambient temperatures, unless the atmosphere is saturated with water vapor. Some heat is dissipated by breathing (panting). Heat loss either by conduction through the gastrointestinal (G1) tract via ingestion of cold food and drink or by immersion in cold water is not normally an important mechanism. Variations from the ideal body temperature could be due to: 3

9 a) Physical Activity Active muscles metabolize food faster than muscles at rest, giving off more heat in the process. b) Fever Fever is the state when temperature exceeds c (106 0 F). Patients with fever usually exhibit tachycardia, involuntary muscular contractions or rigors and sweating or night sweats. c) Hypothermia Hypothermia is defined by a rectal temperature of 35 0 c (95 0 c) or less. The early stage of hypothermia (35 0 c to c; 95 0 to 91 0 F) is marked by an attempt to react against chilling including shivering, increased blood pressure and pulse, vasoconstriction and diuresis. An immediate stage ( to 24 0 c; 90 to 75 0 F) is characterized by decrease in metabolism; drop in pulse, blood pressure and respiration; muscular rigidity; a fine tremor and respiratory and metabolic acidosis. At a third stage, when all attempts at compensation by the temperature regulatory center fail, the body loses heat like an inanimate object. Temperature may also vary depending on: The part of the body where the temperature is taken (rectal temperature is nearly a degree higher than oral temperature) The time of day (temperature is generally lowest at around 4.00am and at its peak at about 6.00pm) Sex (Women often have higher normal temperatures than men) 4

10 2.2 General Principles of Temperature Measurement The measurement of temperature differs from the measurement of other fundamental quantities such as mass or voltage not only because of the lack of a physical zero point of temperature, but because of the inconvenience in direct comparison of the thermal state of the system of unknown temperature with the thermal state of the standard. To measure temperature it is necessary to find an extrinsic property that varies in a predictable way with temperature and use these variations to construct the sensor. The sensor interfaces with the system whose temperatures is to be measured by insertion, point contact or by visual contact and converts the thermal state of the system to a determined state of another quantity, such as voltage. This output signal is processed by a transducer and finally presented as a temperature reading. 2.3 Types of temperature sensors Thermocouple A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, jointed together at one end. One of the junctions is held at a reference temperature (usually 0 0 c) and the other junction is held at the temperature to be measured. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. If the reference junction temperature is known, the other junctions can be calculated from the voltage generated in the circuit. 5

11 Principle of Operation. When a temperature differential is maintained across a metal, the vibration of atoms and motion in electrons is affected so that a difference in potential exists across the material. This potential difference is related to the fact that the electrons in the hotter end of the material have more thermal energy than those in the cooler end, and thus tend to drift towards the cooler end. This dislocation of electrons produces a voltage difference, which forces electrons to flow in the opposite direction. In a state of dynamic equilibrium, both processes are in balance. If the circuit is closed by connecting the two ends through another conductor, a current is found to flow in the closed loop. Fig 1 Seebeck Effect T V T 2 The voltage generated in given by: V T = 2 T 1 ( S ( T ) S ( T ))dt B A Where S A and S B are the Seebeck coefficients of the metals A and B T 1 and T 2 are the temperatures of the two junctions 6

12 Thermocouple types A thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. there are high temperature calibrations R, S, C and B. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple, that is, a very thin thermocouple may not reach the full temperature range. 7

13 Table 1: Thermocouple Types Thermocouple Type Type B (Plantinum/Rh odium) Type E (Chromel/Cons tantan) Type J (Iron / Constantan) Type K (Chromel / Alumel_ Type R (platinum / Rhodium) Type S (plantinum / Rhodium) Type T (Copper / Constantan) Overall Range Typical Accuracy Comments 100 to c (at 100 Suited for high temperature measurements. Unusually, type B thermocouples give the same output at 0 0 c and 42 0 c. This makes them useless below 50 0 c TO c Type E has a high output (68μV/ 0 C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is nomagnetic to 760) c Limited range makes type J less popular than type K, J, types should not be used above C as an abrupt magnetic transformation will cause permanent decalibration to c Type K is the general purpose thermocouple. It is low cost and popular. Sensitivity is approx 41 μv/ 0 c. use type K unless you have a good reason not to. -50 to c Suited for high temperature measurements up to c. Low sensitivity (10V/ 0 c) and high cost makes them unsuitable for general purpose use. -50 to c Suited for high temperature measurements up to c. Low sensitivity (10μV/ 0 c) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold ( c) to c Best accuracy of common thermocouples, often used for food monitoring and environmental applications. 8

14 Figure 2: illustrate the spectrum of voltage- temperature characteristics of the most popular standard thermocouples B S R N K Temperature (Centigrade Degrees) T J E Thermocouple Voltage (mv) Voltage Temperature Characteristics of G, E, J, K, N, R, S, and T type thermocouples 9

15 Thermocouple Characteristics a) Sensitivity The sensitivity of thermocouples depends on the type of signal conditioning used and on the type of thermocouple employed. It ranges from 0.05mv/ 0 c for the J thermocouples to 0.006mv/ 0 c for type S thermocouples. b) Range Thermocouples can measure temperatures over a very wide range with type S thermocouples capable of measuring temperatures of up to c c) Time Response Time response is related to the size of the thermocouple used and the type of protective material employed. It varies from 20 seconds for industrial thermocouples to 20ms for small gauge wire thermocouples. d) Signal conditioning Thermocouple output signal is very small, typically less than 50mv. Hence, considerable amplification is necessary for the thermocouples to be practical. The small signal levels also make it susceptible to electrical noise. 10

16 e) Reference Compensation The thermocouple output voltage is proportional to the difference between the reference temperature and the temperature to be measured. As such, it is necessary to know the reference temperature and ensure that any variations in the reference temperature are compensated for. f) Noise This is one of the biggest obstacles to the use of thermocouples. In addition to the output voltage being small, a thermocouple constitutes an excellent antenna that picks up electromagnetic radiation in the radio, TV and microwave bands. It is for this reason that thermocouples have to be shielded Resistance Temperature Detector An RTD (Resistance Temperature Detector) is basically a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. The resistive property of the metal is called its resistivity. The resistive property defines length and cross sectional areas required to fabricate an RTD of a given value. The resistance is proportional to length and inversely proportional to the cross sectional area. R = Where r x L A R= Resistance (ohms) r = Resistivity (ohms) L = Length A = Cross Sectional Area. Platinum with a temperature coefficient of to c and practical temperature range of -452 to F (-269 to c). The platinum RTD has 11

17 the best accuracy and stability. Copper, nickel and nickel iron are also commonly used RTD materials Principle of Operation A metal is an assemblage of atoms in the solid state in which the individual atoms are in an equilibrium position with superimposed vibration induced by their thermal energy. Each atom gives up one electron, called the valence electron, which can move freely throughout the metal. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material. The valence band and the conduction band overlap in energy is as shown in fig 3. Figure 3 The valence band and the conduction band overlap Metals Conduction Overlap no energy gap Valence Conduction Large energy gap Valence Insulator Semi conductor Conduction Small energy gap Valence As electrons move through the metal, they collide with the stationary atoms of the material. As the thermal energy of the metal increases, the conduction electrons tend to collide even more with the vibrating atoms. This impedes the movement of the electrons and absorbs some of their energy, that is, the material exhibits an increased resistance to the flow of electric current. Thus, metallic resistance is a function of the vibration of atoms and hence of temperature. RTDs act 12

18 somewhat like an electrical transducer, converting changes in temperatures to voltage signals by the measurement of resistance RTD Characteristics a) Accuracy Platinum RTDs typically are provided in two classes, class A and class B. Class A is considered high accuracy and class B is standard accuracy. The accuracy will decrease with temperature. b) Stability This is a measurement of drift over time. Most manufacturers specify stability at less than c per year. Stability is affected by the sensor design. A well designed, high quality sensor will have less drift. Stability is also affected by the sensor s service environment; high vibration, mechanical abuse and thermal shock will affect stability. c) Response Time Response time is the sensor s ability to react to temperature changes in the process. The ability to track process changes depends on the sensors thermal mass and proximity to the process. d) Self-heating RTDs are constructed of very fine wires or very thin coatings. The very small cross sectional areas in the sensing elements will tend to heat when electrical current is applied. Most RTDs are specified to be operated with a current of 1 milliamp or less. 13

19 2.3.3 Thermistors The word thermistor is a combination of words thermal and resistors. A thermistor is a temperature sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors can be classified into two types: if the resistance increases with increasing temperature, the device is called a positive temperature coefficient (PTC) thermistor, posistor. If the resistance decreases with increasing temperature, the device is called a negative temperature coefficient (NTC) thermistor. In contrast to metals, electrons in semiconductor materials are bound to each molecule with sufficient strength that no conduction electrons are contributed from the valence band to the conduction band. Thus at low temperatures, semiconductors behave like insulators because there are no conduction electrons to carry current through the material. When the temperature of the material is increased the molecules vibrate more. Such vibration provides additional energy to the valence electrons. When such energy equals to or exceeds the energy gap, the electrons become free of the molecules and more to the conduction band where they are free to carry current through the material. Thus, semiconductors become better conductors of current as their temperature is increased. This is the opposite of what happens in metals. 14

20 Thermistor Characteristics a) Sensitivity The typical change in resistance for a thermistor is 10% per 0 c. Thus a thermistor with a nominal resistance of 10k ohm at some temperature will change by 5k ohm for a 1 0 c change in temperature. b) Range The temperature range of thermistors depends on the materials used to construct the sensor. In general, there are three range limiting effects. Melting or deterioration of the semiconductor Deterioration of the encapsulation material Insensitivity at higher temperature Thermistors function between the range of c and 80 0 c. The lower limit arises because the resistance of the thermistor becomes very high at low temperature. d) Response time The response time depends on the size of the thermistor and on the environment. Typical values are between 0.5 seconds and 10 seconds. The features of a thermistor that favour its use as a temperature sensing element are: High sensitivity 15

21 Availability in small sizes which enables measurements at a point and with fast response. Wide range of resistance values Possibility of covering a large temperature range from 120k to 470k or higher Thermistor Applications NTC thermistors are used as resistance thermometers in low temperature measurements of the order of 10k. NTC thermistors can be used also as inrush- current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purpose designed for this application. Thermistors are also commonly used in modern digital thermostats and the monitor the temperature of battering packs while charging Active Semiconductor Sensors. These are sensor whose operating principle relies on the potential barrier between the conducting layer and the valence layer in semiconductors such as transistors and diodes Diode Based sensors According to the Shockley theory, the relationship between current I and voltage V in a forward polarized diode may be expressed as: V = V + a KT nq I Is ( T) ln + Is ( T ) Where V a is the barrier voltage, n is a coefficient, I s (T) is the reverse saturation current and I is the forward current. 16

22 I s (T) is many times smaller than the forward current but very strongly dependent on temperature, every temperature increase by 10k results in its value doubling, and this relationship determines the temperature behaviour of the diode. Figure 4. A simple diode sensor 1 v One disadvantage of using this code configuration as a temperature sensor is that the value of I s depends not only on the temperature but also many other factors that are hard to control during the manufacturing process. Thus, the diode sensor s interchangeability is poor and each diode has to be individually calibrated at one or two points. Even after calibration, the error in measurement is at the level of +2K. In order to improve the properties of the diode sensor, two integrated diodes fed from two different current sources I 1 and I 2 are used. The difference in voltage drop between these diodes is: V = V V = 1 2 kt nq In I I 1 2 When two diodes are used, the output voltage does not depend on the reverse saturation currents because of their similarity due to integration. The output 17

23 voltage bias is also significantly reduced, which leads to simpler measuring circuits. Figure 5. Double diode sensors Transistor based sensors Transistor based temperature sensors make use of the relationship between a bipolar junction transistors base-emitter voltage and its collector current. The base emitter voltage of a transistor is universally proportional to its temperature. It exhibits a negative temperatures coefficient of approximately -2 mv/ 0 c. As the temperature increases, the reserve saturation current increases exponentially. Based on the Eber-Moll s equation, in order to maintain the same value of collector current, the base-emitter voltage decreases. 18

24 Fig 6. Variation of V BE with temperature 1.6 Typical Variation of V BE with temperature Vbe in Volts Temperature in kelvin Fig 6 shows that at a temperature of absolute zero, the value of V BE extrapolates to approximately 1.22 V. this is the band-gap of silicon. The graph also shows that the value of V BE at room temperature (27 0 c/300k) is approximately 0.6V Fig 7. Basic Relationships for one transistor 1 c V BE The base-emitter voltage for this configuration is given by: 19

25 kt V BE = q In Ic Is Where k is Boltzmann s constant, T is the absolute temperature, q is the charge of an electron, and 1s is the saturation current that is related to the geometry and the temperature of the transistors. If we take N identical transistors and allow the total current I c to be shared equally among them, the new base-emitter voltage is given by the equation kt 1c V N = In q N. Is Fig 8. The band-gap (V BE ) Voltage reference R 1 R 2 Ic 1 Ic V ref Q 1 Q 2 V BE R 3 V PTAT R 4 20

26 Biasing currents are set using current mirrors which are transistor circuits so laid out that a current flowing in one pair is proportional to that flowing in another part. If Q1 has the same parameters as Q2 then I c1 = I c2. I c1 can be set by connecting an appropriate resistor in the collector. This involves generating a voltage with a positive temperatures coefficient (tempco) which is the same as the negative V BE tempco. This when added to V BE, the resultant voltage will have a zero tempco. The ratio, r of the emitter current densities of the two transistors is typically 10:1 and using Elbers-moll diode current equations, the collector current I c2 = ri c1 can be shown to have a positive tempco. J E = I O qvbe ( exp 1 kt Ι 0 qv BE exp kt J I E o qv = exp kt BE J In Ι E 0 = Q kt V BE V BE = kt q In J E I 0 21

27 V BE = V BE 2 V BE1 = kt q J In Ι E 2 0 kt x q In J Ι E1 0 = kt q In J J E 2 E1 = kt q In r J E2 is then converted to a voltage using the resistor R2 which thus sets the amount of positive tempco voltage required to get an overall zero tempco reference voltage. This occurs when the voltage is equal to the silicon band gap voltage at 0K which is approximately 1.22V. Analysis V REF = V BE1 +V R4 V R4 = I R4 R 4 = 2 I R3 R 4 V R 4 = 2 V R3 R 3 R 4 V R3 V = V REF BE = V BET R + 2 R 4 3 V BE = V BEI R + 2 R 4 3 kt q In r 22

28 The Voltage VBE appears across R 3. The I E in Q 1 is therefore V BE R 3. The Opamps serves loop and the resistors force the same current to flow through Q2. The Q 1 and Q 2 current are equal and are summed and flow into R 4. The corresponding voltage developed across R 4 and V PTAT V PTAT = 2 R 4 R 3 kt In r q Fig 9. Basic Relationships for one transistor 1 c V BE The base-emitter voltage for this configuration is given by: kt V BE = q In Ic Is Where k is Boltzmann s constant, T is the absolute temperature, q is the charge of an electron, and I s is the saturation that is related to the geometry and the temperature of the transistors. If we take N identical transistors and allow the total current Ic to be shared equally among them, the new base-emitter voltage is given by the equation. kt Ic V N = In q N. Is 23

29 2.4 Sensor selection In order to better make an informed choice on the type of sensor that would be most appropriate for design and fabrication in the lab, the major characteristics of various sensors types were summarized in Table 2 below. The main strengths and weaknesses of the various sensors were also summarized in table 3. Table 2 Main difference among RTDs. Thermistors, thermocouples, and IC sensors Characteristic RTD Thermistor Thermocouple IC sensor Active material Platinum Metal oxide Two dissimilar Silicon ceramic metals Changing parameter Resistance Resistance Voltage Voltage Cost of sensor (relative) Cost of System (relative) Additional circuitry Moderate to Moderate to low Low Low low Moderate to Moderate to low High Low low Lead Linearisation Reference None Compensation junction Interchangeability -0.06% - 0.1%, 10%, 2 0 c typ. 0.5%, 2 0 c 1%, 3 0 c c c Stability Excellent Moderate Poor Moderate Sensitivity 0.39% / 0 c -4% / 0 c 40V/ 0 c 10mV/ 0 c Relative Sensitivity Moderate Highest Low Moderate Linearity Excellent Logarithmic/ Moderate Moderate poor Slope Positive Negative Positive Positive Noise susceptibility Low Low High Low 24

30 Table 3. Strengths and Weaknesses of the various sensor types Sensor Type Strengths Weaknesses RTDs Very accurate Very stable Moderate temperature range IC Sensors Good linearity Low cost O/P Values prop to temp, no additional circuitry required Addressability, data storage and retrievability Durable Available in tiny packages Thermistor Good resolution, resistance vs. temperature characteristics is large High resistance allows for longer lead wires without lead wire compensation Available in tiny packages React fast to changes in temperature Inexpensive Thermocouple Wide temperature range Durable Cost: 4 10 x more expensive than thermocouples Not suitable for high-vibration environments Limited temperature ranges Moderately stable Must be tested under controlled conditions Thermocouple extension wires must be used in hooking up to equipment, hence results in error when ambient temperature changes. 25

31 CHAPTER THREE 3.0 System design and Simulations The choice of what type of temperature sensor to use was based on the following criteria: 1. Temperature range The different types of sensors have different ranges in which they effectively measure temperature 2. Accuracy required Different sensors have different sensitivities which affect their suitability for a particular application. For example, thermocouples have a sensitivity of 40V/ 0 c which makes them unsuitable for measuring small variations in body temperature. 3. Economic Considerations There are a number of factors that can affect the price of a sensor, including The type of material used in their construction. For example, RTDs should ideally be fabricated from platinum which is an expensive metal. Based on all these considerations, a transistor based sensor was picked as the most practical to fabricate. 26

32 3.1 Design based on the variation of V BE with temperature As outlined in the literature review section, the base emitter voltage of a transistor exhibits a negative temperature coefficient of approximately -2 mv/ 0 C. Based ion this variation, the circuit in Fig 3.1 was designed. Fig 10. A circuit with a voltage output that varies with temperature BC108BP 5v 10 kω 3.9 kω Q 3 56KΩ 5.6kΩ 10 kω 1.3 kω + / Op amp 1 Q 1 Q 2 BC108BP BC108BP 1.0kΩ Temperature Tap 220kΩ The circuit is fed from Vcc of 5V. The 3.9kΩ and 1.3kΩ resistors form a voltage divider that feeds a voltage of 1.25 volts to the bases of Q1 and Q2. This is approximately equal to the band-gap voltage of silicon (1.22V). Q1 and Q2 are forced to operate at current ratios of 10:1(56KΩ & 5.6KΩ) by feedback from the 27

33 collector voltages. Op amp 1 acts as an error amplifier that ensures that V c1 and V c2 are equal. As shown in the preceding chapter, the difference in V BE between Q1 and Q2 is given by equation below. VBE = kt q In n = kt q In 10 Thus, the difference in the forward base-emitter voltage is directly proportional to temperature. With accurate forcing of the two current levels, we can calculate temperature from a measured V BE. The V BE reading will not be affected by the initial forward voltage, physical size of the junction, leakage currents or other junction characteristics. As the current flows to the ground through the 1 kω and 220 kω resistors, a voltage is developed across the 220 kω resistor that is directly proportional to the difference in V BE and hence to the temperature. To confirm that the circuit works as it was designed to, the circuit in fig 8 was simulated using Electronic Workbench. A temperature sweep at the node marked Temperature tap gave the readings in Table 4. A plot of the temperature readings obtained against the voltage was also plotted using Matlab and the graph shown in Fig 9 obtained. The graph confirmed that the temperature varies linearly with the voltage with a scope of 2.29mV/ 0 c 28

34 Table 4 Variation of Voltage with temperature at the temperature tap TEMPERATURE VOLTAGE (v) $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m $1, Temperature = m Fig 11 Graph showing the variation of voltage with temperature Voltage in mv Temperature in degrees Centigrade 29

35 3.2Comparator Design A comparator is a circuit that switches output states when its input exceeds a certain level called the trip or set point or reference voltage. In Out The idea is that as long as V in is less than the trip level V t the output is low. When V in exceeds V t, the output instantly switches high mathematically, it can be written as; V 0 = V V max min for for V V in in > V < V t t ` V 0 V max V minx V t 30

36 3.2.1 Window Comparator A window comparator is used to indicate whether or not a voltage is within a range of values that is determined by two reference voltages. +V cc R 1 V L D 1 R 2 VI nnn +V cc D 2 V 0 R 3 R 4 R L Divider networks R 1 -R 2 and R 3 -R 4 are used to establish the upper and lower limits of the window. That is, V = V u cc R 3 R4 + R 4 V L = V cc R2 R1 + R 2 We must have V u > V L 31

37 3.2.2 Operation Initially, V in = OV, which is less than V 1 or V 2, which causes the output of A1 to drive high while A2 drives low. This causes D1 to be forward biased while D2 is reverse biased, thus, the output voltage will be high. When the input voltage enters the lower limit of the window, A1 drives low along with A2, which causes the output to be pulled low via R L. When the input voltage exceed the upper limit of the window, A2 drives high while A1 remains low. Thus, we see that as long as the input voltage is within the window, the output of the comparator is low Discussion The supply voltage that is used to drive the voltage dividers must be relatively stable for good performance. In some cases a very stable and accurate reference voltage, separate from the normal supply voltage, will be employed to drive the trigger voltage levels and for transducer excitation. Resistor R 1 and R 3 would most likely be implemented with multiple turn potentiometers, which would be adjusted to set the upper and lower trip levels to the desired levels. The aim of having a comparator in this case was that we can have a voltage that toggles between 5v and Ov. Since the voltage at 39 0 c is mv, this was picked as the upper trigger point and the voltage at 35 0 c is mv, this was picked as the lower trigger point. The design is to be such that that the comparator output is Ov at voltages below the upper and lower trigger levels. Voltage dividers comprising of 12.8kΩ and 2.2kΩ, 16kΩ and 2.7kΩwere used to scale down the 5v source to obtain the upper and lower trip levels. 32

38 33 v x V v x V L u = + = = + = Fig 12. Temperature Sensor Circuit VCC 5V R1 3.9kΩ 5% R2 1.3kΩ 5% R3 10kΩ 5% R4 10kΩ 5% Q1 BC108BP 3 R5 5.6kΩ 5% R6 56kΩ 5% Q2 BC108BP Q3 BC108BP R7 1kΩ 5% 9 7 R8 220kΩ 5% U2A LM339N U3A LM339N D1 1N D2 1N R9 1kΩ 5% 0 0 VCC 5V VCC VCC 5V VCC 0 VCC 5V VCC 5V U1 LM741H LED1 R12 2.7kΩ 5% R11 2.2kΩ 5% 0 1 VCC 0 R10 16kΩ 5% R kΩ 5% VCC VCC

39 Having simulated the circuit and found it to work as designed, the next step was the design of the actual circuit in a form that could be implemented. This was done using a bread board. Results The circuit was successfully implemented on a bread board. It was, however, not possible to test it and get accurate results due to the difficulty in getting a temperature of 39 0 c or 35 0 c given the equipment in the lab. Based on the results from the simulations, the author is fairly confident that the circuit will work as designed. Discussion and recommendations The circuit that was designed and implemented but had a number of shortcomings including: In order to the circuit to have been capable of detecting occurrence of fever or hypothermia, it should have had a higher sensitivity of the order of c. However, it was not possible to design a transistor based sensor with that sensitivity. The sensor designed is only capable of detecting temperatures associated with fever and hypothermia occurrence. Ideally, the sensor designed should have had a data logger and a display such that the user would be able to tell their exact body temperature. Due to time constraints, however the sensor designed has as an LED at its output that turns on when the temperature exceeds 39 0 c or goes below 35 0 c. The sensor should have been designed in a miniature form that the user could either carry around with them or wear on their person as a bracelet or maybe a locket. Design of the sensor in miniature form was not possible 34

40 given the type and size of components and the design resources available to the student. Recommendation for future work Based on the shortcomings of the designed sensor, the author would like to suggest the following improvements to the sensor: Given a bit more time and resources, it would be easy to couple the sensor to a data logger and a display unit. This would be a lot more convenient to users since they would be able to tell their temperatures at all times and not just when it exceeds the threshold. Although this would mean the use of expensive components, the sensor could be redesigned (using a thermistor, for example) so that it can detect smaller variations in body temperature. Conclusion The main objective of designing and implementing a temperature sensor in miniature form was successfully met. The design was based on the temperature dependence of the base-emitter voltage of a transistor. A number of compromises such as in the sensitivity of the sensor had to be made to keep its cost low and to make sure the sensor was implemented in time. It was not possible to test the implemented sensor but simulations of the circuit were successfully done using the temperature sweep function in Electronic Workbench. 35

41 REFERENCES D.D Pollock, (1991), Thermocouples-Theory and Properties, Boca Raton, FL: CRC Press. Denton J. Dailey. Electronic Devices and Circuits, New Jersey: Prentice Hall. Hensel H., (1981), Thermoreceptors and Temperature Regulators, New York: Academic Press. to=app notes. http //materials.usak.ca Lomax P. Schonbaum (1979), Body Temperature, New York : Marcel Dekker. R.D. Barnard, (1972), Thermoelectricity in Metals and Alloys, London: Taylor and Francis. S. Middle Hoek (1989), Silicon Sensors London: Academic Press, 36