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DESIGN OF A WIRELESS STETHOSCOPE A Project Presented to the faculty of the Department of Electrical and Electronic Engineering California State University, Sacramento Submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in Electrical and Electronic Engineering by Gokul Krishnappa SUMMER 2012 DESIGN OF A WIRELESS STETHOSCOPE A Project by Gokul Krishnappa Approved by: ______________________, Committee Chair Dr. Warren D. Smith ______________________, Second Reader Dr. Preetham Kumar _____________________ Date ii Student: Gokul Krishnappa I certify that this student has met the requirements for format contained in the University format manual, and that this Project is suitable for shelving in the Library and credit is to be awarded for the Project. _____________________, Graduate Coordinator Dr. Preetham Kumar Department of Electrical and Electronic Engineering iii _______________ Date Abstract of DESIGN OF A WIRELESS STETHOSCOPE by Gokul Krishnappa A stethoscope is a primary device used by medical doctors. It is used to listen to a patient’s heart and lung sounds. Acoustic, electronic, and wireless stethoscopes are commercially available. A problem with the acoustic and electronic stethoscopes is that harmful germs can be spread to other patients as the doctor uses the same stethoscope from patient to patient. Also, the doctors may get infected with the germs as they sit close to the patients they are examining. A wireless stethoscope, consisting of a chest piece and a separate head set, can be used to reduce the spread of germs, because the doctors do not have to sit as close to the patients as when using the acoustic or electronic stethoscopes. But due to their cost, they are rarely used, and patients are still in danger of getting infected as the doctors use the same chest piece (transmitter module) from patient to patient. A lower-cost wireless stethoscope that is more affordable, allows providing a separate transmitter module to each patient, and perhaps allows the chest piece to be disposable, is proposed. The design requirements are that the device has to provide clear sound, have a range of wireless transmission of 10-15 feet, and have a cost lower than that of the iv wireless stethoscopes that are currently available in the market. The design uses a microphone, low voltage amplifiers, analog RF transmitter, analog RF receiver, and earphones. The microphone is a transducer that converts heart/lung sounds to an electrical signal. This converted electrical signal then is amplified and transmitted. The receiver receives the transmitted signal and then amplifies the signal. The amplified signal then is heard through earphones. The signal-to-noise ratio, frequency response, and cost of the design are compared to the existing acoustic, electronic and wireless stethoscopes. The device developed does not provide sound as clear as that of the electronic stethoscope, and the noise increases as the distance increases between the chest piece and the head set. The frequency response of the device is not satisfactory, as it is not flat for heart and lung sounds (30 – 1,000 Hz). Without considering manufacturing costs, the cost of the device is less than the cost of the wireless stethoscope and less than the average cost of the electronic stethoscope but more than the average cost of the acoustic stethoscope. ______________________, Committee Chair Dr. Warren D. Smith _____________________ Date v ACKNOWLEDGEMENTS I would like to acknowledge Dr. Warren D. Smith for his guidance and support in completion of this project. I would also like to thank Dr. Bruce Jobe for suggesting this topic and providing the goals for this project. vi TABLE OF CONTENTS Page Acknowledgements……………………………………………………………………….vi List of Figures………………..………………………………………………………….viii Chapter 1. INTRODUCTION…………………….………………………………………………1 2. BACKGROUND…………………………………...…………………………………5 2.1. Microphone……………………….……………………………………………..5 2.2. Audio Amplifier……………………….………………………………………...6 2.3. Transmitter…………….…………………….…………………………………..7 2.4. Receiver……………….………………………….……………………………..8 2.5. Earphones……………….……………………………..………………………...9 3. METHODOLOGY…………………………………………………………………..10 3.1. Conditioning Circuit…………………………...…………...……….................11 3.2. RF Transmitter and Receiver Evaluation Boards……………..……..….……..12 3.3. Testing……………………………………………………………..………….12 4. RESULTS…………………………………………………………………………....19 5. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS………….……….25 Appendix. Prices of the Components Used……………………………………………27 References…….……………..…………………………………………………………...28 vii LIST OF FIGURES Figures Page 1. Acoustic and Electronic Stethoscopes…………….………………….……………….3 2. An Electret Condenser Microphone…………………………………………………...5 3. Pin Layout of LM386N-3-ND Audio Amplifier….…………………………………..6 4. Pin Layout of TXM-916-ES RF Transmitter….………………………..……………..8 5. Pin Layout of RXM-916-ES RF Receiver……...………………………...…………...8 6. Evaldi Earphones…………………………….....…………………………………...9 7. Block Diagram of the Device……………………………………………………......10 8. Circuit Diagram of the Conditioning Circuit…...……………………………....……11 9. Transmitter Evaluation Board…………………..……………………………………12 10. Receiver Evaluation Board…………….………………………………………..…...13 11. FG - 8002 Function Generator……..…………...…………...……………………….14 12. Tektronix 2201 Digital Storage Oscilloscope...………………………...……………14 13. Circuit Diagram of the Transmitter Module………………………………………....16 14. Circuit Diagram of the Receiver Module…...………………………………………..17 15. Frequency Response of the Audio Amplifier…………...….………………………..19 16. Frequency Response of the Wireless Link……………………………………….…..20 17. Signal-to-Noise Ratio of the Wireless Link at 4 Feet…………………………….….21 18. Frequency Response of the Device with the Wireless Link.……………………...…21 19. Frequency Response of the Device without the Wireless Link……………………22 viii 20. Frequency Response of the Wireless Link for 1, 4, and 6 Feet…………………...…23 21. Signal-to-Noise Ratio of the Wireless Link for 1, 4, and 6 Feet……….……………24 ix 1 CHAPTER 1 INTRODUCTION A stethoscope is a basic, simple, and practical tool used in the field of medicine in diagnosing health problems [1][2]. It was invented in 1816 by Dr. Rene Theophile Hyacinthe Laennec in France [3][4]. The name stethoscope derives from Greek words, ‘stethos – chest and skope – examination’. It is “an acoustic medical device for auscultation or listening to the internal sounds of a human or animal body” [4, introduction]. The stethoscope often is considered a symbol of the doctor’s profession. Types of stethoscopes that are available presently are 1. Acoustic stethoscope 2. Electronic stethoscope 3. Wireless stethoscope. The acoustic stethoscopes are the most popular and familiar [1]. They operate by the transmission of sound through a chest piece via air-filled hollow tubes to the doctor or listener’s ears. The chest piece of the acoustic stethoscope usually consists of two sides, a diaphragm (which is like a plastic disc) and bell (which is like a hollow cup). The chest piece is placed on the patient’s chest for listening to the sounds of the heart/lungs. “The bell transmits low frequency sounds, while the diaphragm transmits higher frequency sounds” [4, acoustic]. The disadvantages of the acoustic stethoscope are that its frequency response shows maxima and minima at specific frequencies due to tubular resonance 2 effects, its sound level is low, it attenuates sound transmission proportional to frequency, and its transmission properties differ from model to model [2]. Electronic stethoscopes are designed to overcome the disadvantages of the acoustic stethoscopes. They are designed to have a uniform frequency response and to amplify the sound level [2]. In general, an electronic stethoscope is comprised of a chest piece, sound transducer, adjustable gain amplifier, frequency filters, mini-speaker/head phones, and a dry cell or battery. The chest piece consists of a sound transducer (microphone) that converts the sound to an electrical signal, and this converted electrical signal is transmitted to the conditioning circuit, which may consist of an amplifier and a frequency filter. This conditioned signal then is transmitted through an electrical cable to a headset. An acoustic stethoscope and an electronic stethoscope are shown Figure 1. However, the advantages of an electronic stethoscope over an acoustic stethoscope are diminished by the sensitivity of the electronic stethoscope to manipulation artifact and cost. The cost of the acoustic stethoscope ranges from $3 to $281, and the cost of the electronic stethoscope ranges from $200 to $575 [5]. Hence, acoustic stethoscopes are mostly used still today [2]. Both acoustic and electronic stethoscopes have a common disadvantage, i.e., they can spread harmful germs to other patients as the doctors use the same stethoscope from patient to patient, and even the doctors are in danger of getting infected with the germs as they sit close to the patients. Wireless stethoscopes consist of a chest piece (used to transmit the heart/lung sounds from the patient) and a head set (used to receive the transmitted sound from the patient). Wireless stethoscopes can be used to reduce the spread of germs, because the 3 doctors do not have to sit as close to the patients as when using the acoustic or electronic stethoscopes. But, the wireless stethoscopes are rarely used due to their cost. The cost of the wireless stethoscope ranges from $625 to $5600 [6][7]. And still there is a chance of germs being spread to other patients if the doctor uses the same chest piece from patient to patient. Figure 1. Acoustic and Electronic Stethoscopes [2, p. 653, 656] Left: An acoustic stethoscope. Right: An Electronic stethoscope. Dr. Bruce Jobe, of Kaiser Permanente, Roseville, California, proposes that a low-cost wireless stethoscope be developed that is more affordable, allows providing a separate transmitter module to each patient, and perhaps allows the transmitter module to be disposable. This project deals with the development of a transmitter module and a 4 receiver module for a prototype of such a device. The design requirements given by Dr. Jobe are that the device has to provide clear sound, have a range of wireless transmission of around 10-15 feet, and have a cost that is lower than that of the wireless stethoscopes that are available in the market. For design, cost, and development purposes, this project uses the RF (radio frequency) analog wireless transmission technique, audio power amplifiers (in the transmitter and receiver modules), an electret condenser microphone, and headphones. The assembly is laid out on breadboards and connected to RF transmitter and receiver kits. The details are discussed in the remainder of this report. Chapter 2 provides a brief background about the hardware components that are used in this design. Chapter 3 presents the methodology for the design and testing. Chapter 4 shows the results. In Chapter 5, the summary and conclusions of the report are presented and also recommendations for future work. 5 CHAPTER 2 BACKGROUND 2.1. Microphone A microphone is a sound transducer that converts sound to an electrical signal. It is used as a chest piece to sense heart and lung sounds in the design of a wireless stethoscope. There are different varieties of microphones having different transducer principles and directional characteristics [8]. The microphone used in the project is the WM-61A omnidirectional back electret condenser microphone (Panasonic Corporation, Secaucus, New Jersey) because of its cost, its supply voltage, and its frequency range. The images of the front side (with black top) and back side (for soldering) of the microphone are shown in Figure 2. It is of circular shape, 6 mm in diameter and Figure 2. An Electret Condenser Microphone [9, p. 1] Left: back side for soldering. Right: front side 6 3.4 mm in thickness, and has solder pads to connect to other parts. It has an operating voltage range of 2 – 10 V and consumes a maximum of 0.5 mA. Its frequency range is 20 – 16,000 Hz [9]. It has a low cost of $1.92 (Appendix).The supply voltage is in the range of the supply voltages of the other components. This frequency range makes it compatible for this design, as the heart and lung sounds are in the range of 30 – 1,000 Hz [1]. 2.2. Audio Amplifier The audio amplifier used in this project is the LM386N-3-ND low voltage audio amplifier (National Semiconductor Corporation, Santa Clara, California). The factors that were considered in the choice of the low voltage audio amplifier are cost, operating voltages, and operating frequencies. Figure 3 shows the pin layout of the LM386N-3-ND. Figure 3. Pin Layout of LM386N-3-ND Audio Amplifier [10, p. 1] 7 It is packaged in an 8-pin Dual-In-Line Package (DIP), and it can operate at a voltage in the range of 4 – 12 V. Its cost is low ($0.95) (Appendix). The supply voltage is in the range of the supply voltages of other components used in this design. It operates in the audio range. The voltage gain of the amplifier is internally set to 20 (26 dB) and can be increased to 200 (46 dB) with the addition of an external capacitor and resistor between pins 1 and 8 of the chip [10]. 2.3. Transmitter The TXM-916-ES RF transmitter (LINX Technologies, Merlin, Oregon) is used in this project. This transmitter is chosen for the design for its cost, its supply voltage, and its frequency range, and because it does not require any external RF components except for an antenna. It is packaged in a 10-pin SMD (surface mount device) package. It uses the FM modulation technique [11]. The pin layout of the TXM-916-ES RF transmitter is shown in Figure 4. The TXM-916-ES RF transmitter can transmit an analog/audio signal with a bandwidth of 20 – 28,000 Hz and an amplitude of 0 – 5 V and operates at a voltage in the range of 2.1 – 4 V. The transmit frequency (Fc) is 916.48 MHz, which in North America allows an unlimited variety of applications [12]. The cost is low ($13.84) (Appendix), the supply voltage is in the range of the supply voltages of the other components used, and its frequency range covers the range of heart/lung sounds. 8 Figure 4. Pin Layout of TXM-916-ES RF Transmitter [11, p. 3] 2.4. Receiver The RXM-916-ES RF receiver (LINX Technologies, Merlin, Oregon) is used in this project. It is packaged in a 10-pin SMD package. It uses FM demodulation to work with the TXM-916-ES RF transmitter [9]. Figure 5 shows the pin layout of the RXM-916-ES RF receiver. Figure 5. Pin Layout of RXM-916-ES RF Receiver [13, p. 3] 9 It recovers the audio signal of 20 – 28,000 Hz in bandwidth and has an output level of 360 mVp-p for an input to the transmitter of 5 Vp-p. It operates at a voltage in the range of 4.5 – 5.5 V. The receiver can receive a signal as low as -97 dBm [13]. Its cost is low ($17.17) (Appendix), its operating voltage is in the range of other components used, and its frequency range covers the range of heart/lung sounds. 2.5. Earphones The Evaldi earphones set (MOLEX, Wilmington, Delaware) is used for this project. It is chosen for its frequency range, i.e., 20 – 20 kHz [14], which is compatible with the heart/lung sounds. Figure 6 shows a picture of the Evaldi earphones. The cost is $19.99 (Appendix). Figure 6. Evaldi Earphones [14, p. 2] 10 CHAPTER 3 METHODOLOGY Figure 7 shows a block diagram of the device. The device is comprised of transmitter and receiver modules. The transmitter module (used at the patient to transmit heart/lung sounds) consists of a microphone, a conditioning circuit, and a transmitter. The receiver module (used by the doctor to receive the transmitted signal from the patient) consists of a receiver, a conditioning circuit, and an earphones set. The signal from the heart/lungs is received by the microphone, which converts the sound signal to an electrical signal. The converted electrical signal then is sent through the transmitter Antenna Antenna Receiver Transmitter Wireless link Conditioning circuit Conditioning circuit Microphone Earphones Transmitter Module Receiver Module Figure 7. Block Diagram of the Device 11 module conditioning circuit where it is filtered and amplified before being transmitted. The receiver receives the transmitted signal via its antenna, and then this received signal is sent through the receiver module conditioning circuit. The conditioning circuit filters and amplifies the received signal which then can be heard through the earphones. 3.1. Conditioning Circuit The conditioning circuit consists of a variable resistor (Panasonic Electronic Components, Secaucus, New Jersey) [15] and LM 386N-3-ND low voltage audio amplifier. The circuit diagram is shown in Figure 8. The variable resistor makes it possible to vary the gain. Circuits of this design are used both in the transmitter and receiver modules. Figure 8. Circuit Diagram of the Conditioning Circuit [16] 12 3.2. RF Transmitter and Receiver Evaluation Boards For convenience, evaluation boards, EVAL-916-ES (LINX Technologies, Merlin, Oregon), were used in both the transmitter and receiver modules. The evaluation boards were useful in the connection of the antennas to the transmitter and receiver and for supplying constant power to the transmitter and receiver. The transmitter and its antenna on the transmitter evaluation board are shown in Figure 9. The receiver and its antenna on the receiver evaluation board are shown in Figure 10. Transmitter Antenna Figure 9. Transmitter Evaluation Board 3.3. Testing Testing is carried out to find if the device meets Dr. Jobe’s requirements. An FG - 8002 function generator (EZ Digital Co. Ltd., Melrose, Massachusetts), shown in 13 Receiver Antenna Figure 10. Receiver Evaluation Board in Figure 11, is used as the source in place of the microphone, and two Tektronix 2201 digital storage oscilloscopes (Tektronix, Inc., Beaverton, Oregon), Figure 12, are used to observe the signals at both the transmitter and receiver modules. The frequency responses for the LM386N-3-ND audio amplifier and the wireless link are individually observed. The average output of the microphone observed on the digital storage oscilloscope while talking into the microphone is approximately 110 mVp-p. So a sinusoidal signal of 110 mVp-p and frequency in the audio range from the function generator is used as an input for the LM386N-3-ND audio amplifier. For measuring the frequency response of the audio amplifier, the input from the function generator is fed into the Vin Pin in Figure 8, and the output is measured at the Vo Pin in Figure 8. The maximum output of the audio amplifier, without being saturated, is 14 Figure 11. FG - 8002 Function Generator Figure 12. Tektronix 2201 Digital Storage Oscilloscope measured to be 3.6 Vp-p. So, for measuring the frequency response of the wireless link, an input of 3.6 Vp-p (0 to 3.6 V) is fed into Pin 5 in Figure 4, and the output is measured at Pin 11 in Figure 5. Now the frequency response for the device is observed. The transmitter module 15 and receiver module circuits are set up on the individual breadboards. Figures 13 and 14 are the circuit diagrams of the transmitter module and the receiver module, respectively. For measurements of the device, the input from the function generator is fed into A in Figure 13 (with the microphone disconnected), and the output is measured at E in Figure 14. A BNC cable (has red and black wire terminations) is used to connect the signal generator to the circuit. The red wire of the BNC cable is connected to A in Figure 13, and the black wire is connected to ground. Pin 1, Pin 2, and Pin 3 of the variable resistor are connected to B, to Pin 3 of the audio amplifier, and to ground, respectively. The audio output at C in the transmitter module (Figure 13) is connected to Pin 5 (DATA pin) of the TXM-916-ES transmitter (Figure 4). In the receiver module, Pin 11 (AUDIO pin) of the RXM-916-ES receiver (Figure 5) is connected to D, and the earphones are connected to E (Figure 14). Channel 1 of one digital storage oscilloscope is connected to A, and channel 2 of the same digital storage oscilloscope is connected to C in Figure 13. Channel 1 of a second digital storage oscilloscope is connected to D, and channel 2 of the same digital storage oscilloscope is connected to E in Figure 14. The supply voltage to the microphone is 5 VDC, to the audio amplifier is 5 VDC, to the transmitter is 3 VDC, and to the receiver 5 VDC. The variable resistor in the transmitter module (Figure 13) is varied such that the audio amplifier attains maximum gain without the output signal being saturated or clipped off and also to ensure that the input signal to the transmitter is in the required range of 0 – 5 V. After the variable resistor is set to satisfy these conditions, the input frequency is varied from 20 Hz to 20,000 Hz with amplitude 16 17 18 held constant. The transmitter module and receiver module are placed 4 feet apart. The voltage values are observed and measured at A (Figure 13), at C (Figure 13), at D (Figure 14), and at E (Figure 14). The noise of the wireless link first is measured by observing the output signal at the receiver (Pin 11 in Figure 5) in the receiver module with the input of the transmitter (Pin 5 in Figure 4) connected to ground. When the input signal of 3.6 Vp-p (0 to 3.6 V) is fed into the input of the transmitter (Pin 5 in Figure 4), the noise observed is greater than the noise as measured above and varies with frequency. So, the noise in the wireless link is measured versus frequency within the audio range, with amplitude held constant at 3.6 V, and signal-to-noise ratios are calculated. The total noise of the device is measured by observing the output signal at E in the receiver module (Figure 14). The measured gain values are plotted versus frequency. The measured signal-to-noise ratio values are plotted versus frequency. Similarly, the frequency response and noise on the signal are measured when the transmitter module and receiver module are 1 foot and 6 feet apart. The gain values (dB) of the device without the wireless link are calculated by subtracting the gain (dB) of the wireless link from the gain (dB) of the overall device with respect to frequency. The frequency response of the device without the wireless link then is plotted. The performance of the device is compared with the performances of acoustic, electronic, and wireless stethoscopes. The cost of the design (Appendix), without considering the manufacturing cost, is compared with the cost of the acoustic, electronic, and wireless stethoscopes available in the market. 19 CHAPTER 4 RESULTS The frequency response of the LM386N-3-ND audio amplifier is shown in Figure 15. The maximum output of the audio amplifier observed is 3.6 Vp-p when a sinusoidal signal of 110 mVp-p and 100 Hz is given as input. The maximum gain measured for the audio amplifier is 30.2 dB, which is less than the manufacturer’s datasheet value of 46 dB. 35 30 Gain (dB) 25 20 15 10 5 0 1 10 100 1000 10000 100000 Frequency (Hz) Figure 15. Frequency Response of the Audio Amplifier 20 The output of the receiver is about 360 mVp-p when a sinusoidal signal of 3.6 V and frequency in the audio range is applied as input to the transmitter. Figure 16 shows the frequency response of the wireless link. 0 -5 -10 Gain (dB) -15 -20 -25 -30 -35 -40 -45 1 10 100 1000 10000 Frequency (Hz) Figure 16. Frequency Response of the Wireless Link 100000 The noise of the wireless link when the transmitter input is grounded is 4 mV. When the input to the transmitter is a 3.6 Vp-p sinusoid, the signal-to-noise ratio versus frequency of the wireless link is shown in Figure 17. The frequency response of the device is shown in Figure 18. The frequency response of the device without the wireless link is shown in Figure 19. The frequency response of the wireless link for 1, 4, and 6 feet are shown in Figure 20. It is approximately the same when the transmitter module and receiver 21 35 Signal-to-Noise Ratio (dB) 30 25 20 15 10 5 0 10 100 1000 10000 100000 Frequency (Hz) Figure 17. Signal-to-Noise Ratio of the Wireless Link at 4 Feet 25 20 Gain (dB) 15 10 5 0 -5 1 10 100 1000 10000 100000 Frequency (Hz) Figure 18. Frequency Response of the Device with the Wireless Link 22 50 45 40 Gain (dB) 35 30 25 20 15 10 5 0 1 10 100 1000 10000 100000 Frequency (Hz) Figure 19. Frequency Response of the Device without the Wireless Link module are 1, 4, and 6 feet apart. The signal-to-noise ratios of the wireless link for 1, 4, and 6 feet are shown in Figure 21. As the distance increases between the transmitter module and the receiver module, the noise increases in the signal, and hence the signal-to-noise ratio drops. The signal-to-noise ratio of the wireless stethoscope (assuming that the conditioning circuits of the transmitter module and receiver module contribute no noise) is ~31 dB at 1 foot and 4 feet, which is less than the signal-to-noise ratio of an electronic stethoscope, about >90 dB [17]. And the signal-to-noise ratio of the device is approximately the same as the signal-to-noise ratio of an acoustic stethoscope, ~30 dB [17]. The signal-to-noise ratio of the device reduces to ~23 dB when the distance is 23 increased to 6 feet. The frequency response of the device for the heart and lung sounds range, 30 – 1000 Hz, is not flat when the distance between the transmitter module and the receiver module is 1, 4, and 6 feet. The cost of the design (Appendix) is less than the average cost of the electronic and less than the wireless stethoscopes that are currently available, without considering the manufacturing cost. The cost of the transmitter module, i.e., chest piece used on the patient (consisting of the microphone, the conditioning circuit, and the transmitter), is $23.07. 1 foot 0 4 feet 6 feet -5 Gain (dB) -10 -15 -20 -25 -30 -35 -40 -45 10 100 1000 10000 100000 Frequency (Hz) Figure 20. Frequency Response of the Wireless Link for 1, 4, and 6 Feet 24 1 foot 4 feet 6 feet 35 Signal-to-Noise Ratio (dB) 30 25 20 15 10 5 0 10 100 1000 10000 100000 Frequency (Hz) Figure 21. Signal-to-Noise Ratio of the Wireless Link for 1, 4, and 6 Feet 25 CHAPTER 5 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS A prototype of wireless stethoscope was designed in this project. The aim of the project was to overcome the disadvantages of the existing wireless stethoscopes by reducing the cost of the device so that it is more affordable, the patients can be given individual transmitters modules, and perhaps the transmitter modules can even be disposable. The device developed was able to function but could not meet the requirements of transmitting clear sound for a wireless distance of 10-15 feet. The noise of the device is worse than that of the electronic stethoscope, and it increases as the distance between the transmitter module and receiver module increases. The cost of the device [Appendix] (excluding the manufacturing cost) is less than the price of the wireless stethoscopes that are available in the market, less than the average price of an electronic stethoscope, and more than the average price of an acoustic stethoscope. The frequency response of the device is not satisfactory, as it is not flat for frequencies 30 – 1000 Hz. From the results, the frequency response of the LM386N-3-ND audio amplifier used in this design is not flat for 30 – 1000 Hz. It is recommended to use an audio amplifier with a flatter frequency response, even though cost will increase. The RF digital transmission technique is recommended for use to improve the signal-to-noise ratio, though again cost will increase. The conditioning circuits used in the transmitter and the 26 receiver modules of this design are assumed to contribute no noise. It is recommended to measure the noise of the conditioning circuit to determine if this assumption is valid. It is also recommended to use low-pass filtering at the receiver output to attenuate frequencies higher than those in the heart and lung sounds and thus to improve the signal-to-noise ratio. 27 APPENDIX Prices of the Components Used Part no. P9925-ND 493-1036-ND 493-1001-ND P5112-ND P4521TB 445-2851-ND 2.2KW-1-ND CF14JT10R0CT-ND P3P5103-ND LM386N-3-ND EVAL-916-ES-ND Battery WM2582-ND Total Price Value 10 µF 33 µF 220 µf 0.047 µF 1 µF 2.2 kΩ 10 Ω 10 kΩ 9V Price/unit ($) Quantity Price ($) 1.92 0.58 0.88 0.20 0.27 0.43 0.18 0.08 0.76 0.95 99.00 1.98 19.99 1 2 2 2 2 2 2 2 2 2 1 2 1 1.92 1.16 1.76 0.40 0.54 0.86 0.36 0.16 1.52 1.90 99.00 3.96 19.99 133.53 All the components were bought from www.digikey.com. The evaluation kit, EVAL-916-ES-ND, has other components that are not required for the design. The price of the transmitter chip (TXM-916-ES-ND) is $13.84 and of the receiver chip (RXM-916-ES-ND) is $17.17, which is less than the price of the evaluation board used. The cost to manufacture a wireless stethoscope based on the prototype developed in this project is unknown. 28 REFERENCES [1] D. Callahan, J. Waugh, G.A. Mathew, W.M. Granger, “Stethoscopes: What Are We Hearing?” Biomedical Instrumentation & Technology, vol. 41, pp. 318-323, July/August 2007. [2] M. Grenier, K. Gangnon, J. Genest, Jr, J. Durand, L. Durand, “Clinical Comparison of Acoustic and Electronic Stethoscopes and Design of a New Electronic Stethoscope,” The American Journal of Cardiology, vol. 81, pp. 653-656, March 1998. [3] F. Yu, A. Bilberg, F. Voss, “The Development of an Intelligent Electronic Stethoscope,” Mechtronic and Embedded Systems and Applications. IEEE/ASME International Conference, Beijing, China, pp. 612-617, Oct 12-15, 2008. [4] [Online]. Available: http://en.wikipedia.org/wiki/Stethoscope [Accessed: October, 2009]. [5] [Online]. Available: http://www.allheart.com/Category.aspx?c=1114&ps=&pn=&sb=PRICE&sd=ASC&filter [Accessed: May, 2012]. 29 [6] [Online]. Available: http://www.coloradoheartrescue.com/sedation-stethoscopewireless/ [Accessed: June, 2012]. [7] [Online]. Available: http://www.allheart.com/product/p/ca7187003t/ [Accessed: June, 2012]. [8] [Online]. Available: http://en.wikipedia.org/wiki/Microphone [Accessed: April, 2011]. [9] Panasonic Corporation, “WM-61A Datasheet,” [Online]. Available: http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE22.pdf. [Accessed: April, 2011]. [10] National Semiconductor Corporation, “LM386 Low Voltage Audio Power Amplifier Datasheet,” [Online]. Available: http://www.ti.com/lit/ds/symlink/lm386.pdf. [Accessed: April, 2011]. [11] Linx Technologies, Inc., “ES Series RF Transmitter Module Data Guide,” 2008. [Online].Available:http://www.linxtechnologies.com/resources/data-guides/txmxxx-es.pdf. [Accessed: April, 2011]. [12] Available: http://www.fcc.gov/ [Accessed: April, 2011]. 30 [13] Linx Technologies, Inc., “ES Series RF Receiver Module Data Guide,” 2008. [Online]. Available: http://www.linxtechnologies.com/resources/data-guides/rxmxxx-es.pdf. [Accessed: April, 2011]. [14] Molex Inc., “WM2582-ND Datasheet,” 2010. [Online]. Available: http://media.digikey.com/pdf/Data%20Sheets/Molex%20PDFs/503700-0100.pdf [Accessed: April, 2011]. [15] Panasonic Electronics Components, “P3P5103-ND EVUE/EVUF Datasheet,” 2005. [Online]. Available: http://www.panasonic.com/industrial/components/pdf/aok0000ce4.pdf [Accessed: April, 2011]. [16] Panasonic Corporation, “WM-61A Catalog Drawings,” [Online]. Available: http://media.digikey.com/pdf/Catalog%20Drawings/Audio/MicCartridgeCircuit.jpg [Accessed: April, 2011]. 31 [17] M.V. Shervegar, G.V. Bhat, K.R.M. Shetty , “Phonocardiography - The Future of Cardiac Auscultation,” International Journal of Scientific & Engineering Research, vol. 2, Issue 10, October, 2011. [Online].Available: http://www.ijser.org/researchpaper%5CPhonocardiography-the-future-of-cardiacauscultation.pdf [Accessed: May, 2012].