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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
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