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Mashavu Stethoscope 1

Contact Information

University and Department: Penn State

Team Members: Katie Workinger

Primary Email Address:

Project Information

The challenge the project is trying to solve:

The initial specifications of the stethoscope, and all eight of the biomedical devices in Mashavu, were that the devices needed to be low-cost, low-power, rugged and use materials native to Kenya. Also, fun, with respect to the patient and Kiosk Operator, had to be incorporated into the design as much as possible.

Initially, $20 USD was allotted to the development of the stethoscope with the intent of the final steady-state production cost of less than $10 USD and ideally less than $5 USD. The reason that the cost of the final stethoscope needs to be so low is to reduce the overall cost of the Mashavu Station so that it is affordable for orphanages, community based organizations (CBOs), schools, clinics and entrepreneurs.

    The low-power specification implies that the device must run on less than or equal to 5V and draw less than or equal to 500 mA if using a USB 2.0 or less than or equal to 900 mA if using a USB 3.0. This is the maximum voltage and maximum power that a USB port can deliver from the computer to power the device. However, the stethoscope needs to be used in conjunction with the blood pressure cuff in certain applications and the blood pressure cuff needs to run off of the same power supply from the same USB port. This means that the stethoscope and the blood pressure cuff can both use 5V in their respective operational modes, however they can only use a combined current  of at most 500 mA or 900 mA and they needed to be designed with this in mind.

    All of the components in the stethoscope needed to be rugged so that they could withstand high temperature differentials and weather conditions in Africa or more specifically Kenya and Tanzania. All of the components also needed to withstand lots of use so that little or no repairs ever would have to be made.

However, if repairs would be needed it is critical that the materials required for repairs be readily available at or near the location of the Mashavu Station. In this instance it is necessary for the stethoscope, and all of the other biomedical devices, to be made out of materials which are readily available on the ground in Kenya. While it is possible to get an idea of what is available in Kenya while still in the United States, the final design of the stethoscope will not be known until material adaptations can be made on the ground in summer 2009.

    The development of the stethoscope was broken down into three main areas: research, hardware development, and software development. Upon researching computer or digital stethoscopes, also called stethophones, it was found that the products already on the market were too expensive to be incorporated into Mashavu, however the concept behind them needed to be emulated. Therefore, the hardware component was scaled down to include the minimal amount of raw materials necessary and all of the signal processing was done by software.

List of Tools

Throughout the development and testing phases of the stethoscope the tools found in Table 1.2.1 were needed. All of these tools, except the oscilloscope,  will also be required on the ground in Kenya in summer 2009 to test the modified versions of the stethoscope. While it would be nice to have the oscilloscope for testing purposes in Kenya, transporting one there is unlikely and buying one there is out of the question. Therefore, anticipated changes in the testing procedure occur and are documented in Chapter 2, and any other tests deemed necessary once on the ground will have to be added to the testing procedures.

Table 1.2: List of Tools for Stethoscope Construction and Testing

Hardware Construction

The stethoscope for the Mashavu Station had three different hardware designs. Each of these iterations were very close to one another, with only a few minor changes each time. When designing all of the iterations of the stethoscope, a block diagram, similar to the typical digital stethoscope block diagram, was developed and can be seen in Figure 2.3. Using this block diagram as a model for the stethoscope meant that the only things necessary in the hardware portion of the design were the stethoscope head and the microphone and eventually a pair of. Also, the final iteration of this design will not be the one implemented in Kenya in Summer 2009. There will be several prototypes of the third iteration taken to Kenya, however the design will again be modified on the ground so that it is constructed out of materials native to Kenya as much as possible.

The stethoscope head is typically a round conical shaped piece with a bell and a hard diaphragm. The diaphragm helps to amplify the sound of the heartbeat and the conical shape of the headpiece helps to focus the pressure and sound waves to a focal point. While this is the ideal design, there are others which cost a lot less and that work. For instance, there are toy stethoscopes made of simple plastic cylindrical heads filled with foam that allow children to hear their heartbeat. Therefore, the headpiece for the initial stethoscope was a Gatorade bottle cap filled with foam. A hole was drilled in the top of the Gatorade cap so that the sound could escape out the top into a tube connected to the microphone.

The microphone was not developed in-house, but was instead purchased. The microphone use was the Radio Shack 270-092 Electret Microphone with Leads. This particular microphone has frequency response between 30Hz—15kHz and runs on a 4VDC—10VDC power supply. Even though the microphone can work off of a range of voltages, in this application the nominal voltage will be 5VDC because that is what is supplied by the DAQ. It also contains three leads, one for power, one for ground, and one for the output signal.

The microphone was attached to the Gatorade cap via a piece of 1/2 inch plastic tubing. This size was chosen because the microphone fit snuggly into this tubing. A longer piece of tubing was then used to house the wires coming out of the microphone. The stiff but malleable tubing allowed enough flexibility for the stethoscope head to be placed at various locations on a patient’s body, but was stiff enough to keep the wires from bending too much and breaking prematurely.

The stethoscope itself was connected to the DAQ by a BNC connector. While this worked, it was not an optimal way to read a sound file into LabVIEW. Therefore, in the second iteration, this connector was changed to 1/8 inch mono phone plug. This provided an easier way to filter, amplify and then store the data as a .wav file in LabVIEW and this process will be explained more in the next section.


As stated before, the second iteration of the hardware design for the stethoscope was just replacing the BNC connector with a phone plug. This allowed the hardware to replay the signal to the software much more fluidly and allowed for a clearer audio signal quality.

    The third iteration of the hardware had to do with a revision of the stethoscope head. It was discovered that the foam within the head of the stethoscope in the first two iterations was not readily available on the ground in Kenya. However, musical instruments are very popular in the Kenyan culture as is drum making. This led to the use of leather being stretched over the bottle cap and secured using a zip tie. This creates a drum-like head for the stethoscope and basically acts as a crude version of a diaphragm. Another issue with the first and second iteration, was that the head was too big to use with infants and small children. Therefore, a smaller stethoscope head was constructed out of a water bottle cap in the exact same way that the larger head was constructed in iteration three. Switching between the two heads during operation is as easy as removing electrical tape and attaching the new head and then replacing the electrical tape.

During the testing phases of the stethoscope, a problem arose because with the extended use of the stethoscope, the leads of the microphone pulled out of the microphone body. Therefore, on the third prototype some extra wire was put inside of the plastic tube to protect the wire, and then the wire was secured to the plastic tubing by hot-glue.

    While this is the final design of the stethoscope in the United States, the stethoscope used at the station in Kenya will differ slightly in the materials that most of the materials will be native to Kenya. Step by step instructions on making this stethoscope can be found in Section 1.3.1.

The software used to condition the signal is LabVIEW. LabVIEW, which is short for Laboratory Virtual Instrumentation Engineering Workbench, is a visual coding platform which was chosen because it is the platform in which the Kiosk software will be created, LabVIEW has the feature to create .exe files, and it is a very strong data processing language. In this description, the stethoscope is assumed to be a lone entity and is not yet connected to the Mahsavu Kiosk software in any way.

The stethoscope software consists of three main phases including initialization, signal conditioning and exportation. During the initialization phase the sound input is configured, the speaker output is configured and the file to save the filtered data is configured. The sound input device ID is set to 0, this activates the audio-in jack of the computer where the microphone is connected. The signal from the microphone is then set to be sampled at 22050 samples per second in one channel with 16 bits per channel. The speaker output must contain these same specifications so that no data is lost internally after it is sampled. The final configuration in the initialization phase is the creation of the file to save the data. Once the program begins to run, before data collection, the program will prompt the user for a file name and location, but will provide the format of the file. This is done by configuring the file dialog to recognize the data as a sound file, .wav, and by setting up the file name as a control. Eventually this process will be automated in the Kiosk software.

The next phase, signal conditioning, is the phase where the data is cleaned up and amplified so that it can be heard through the speaker of the computer as well as saved to the sound file to be packaged and sent to the local and international doctors. When the data is read by the software from the microphone it is read as an array. Before the data can be conditioned, the data needs to be taken out of the array and made into a series of elements. Therefore an indexing array is used to break down the initial data array. This data is then fed into a forth order Butterworth bandpass filter. The Butterworth filter was chosen because it is important to have a flat magnitude response in the passband so that the signal does not get distorted. The signal is then multiplied by a constant so that it is in an audible range. The signal is then recombined with the time stamp and continues to the exportation stage.

During the exportation stage the filtered signal is exported in two different ways. First, it is exported in real time the speaker output of the computer. This allows  the patient and/or the Kiosk Operator to make sure that a good signal is being collected to send to the doctors. Second, it is exported to the file that was created in the initialization phase. This file is then collected by the Kiosk software and digitally packaged along with the data collected by the rest of the biomedical devices and sent to doctors around the world for analysis. The commented code identifying the main components in each stage can be seen in Figures 1.8 to 1.11

Final Device Specifications

The final device specifications are summed up in the following tables:


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