Making a Stethescope Amplifier Circuit

The post explains how to make an electronic stethoscope amplifier circuit for enabling a loud audible reproduction of the heart beats which is being diagnosed. The article also reveals how the same can be applied within a cellphone through a wireless circuit. The idea was requested by Dr. Ankit.

Main Requirements

1. I would request you to help me with the following circuit "An Electronic Stethoscope".
2. Significance- An ordinary stethoscope is a device used to listen to breathing and heart sounds. A hollow rubber tube is connected at one end to a disk shaped diaphragm (placed over the patient) and other end connected to as Y shape to the ear of listener.
3. As breathing and heart sounds create slight vibrations, these make the diaphragm vibrate and then the sound is amplified in the disk and audible through the tube to the other end.
4. In hospitals, often there is noise of other equipment hence the weak sounds transmitted by stethoscope are sometimes inaudible and important diagnosis missed by the listener.

The Objective:

• A circuit is requested that picks up sound vibrations from the diaphragm of the stethoscope and convert it to electronic signals that are then amplified and can be heard through a speaker loud enough that connecting to ears is not required and no sound is missed (even by less experienced practitioners).
• The battery utilised may be small lightweight 4.5V or 6V (like ones used in rechargeable led torch) OR through mobile power banks since stethoscope must be portable and easy to carry at the same time avoiding wall socket connections for power supply.
• As an improvement of this circuit - If possible the circuit may derive power through an android phone directly AND again if possible the output signals may be visualized as a graph in the android screen.
• As there is no direct contact with ears, this will also prevent cross infection of ears as sometimes happens when one stethoscope is used by multiple users.

The Design

The sound of a heart beat can be extremely weak and therefore it cannot be heard without a minimum suitable device such as a stethoscope.

A stethoscope is a basic device which relies on picking and transferring the air vibrations through a tube into the ears of the user.

The vibrations are caused by the heart beats on the stethoscope's sensing diaphragm when it is brought at a close proximity to the chest where the heart is situated, and the diaphragm movement sets the air column inside the tube into a correspondingly push-pull vibrating motion

This surely means that even though the air vibration or the sound vibration generated by the heart could be small enough but it's loud enough to be heard without the aid of electrical device, which implies that the sound may be sufficiently strong to be amplified using an audio amplifier, because if a naked ear can hear these minute vibrations so can the amplifier MIC.

Producing Heartbeat in Loudspeaker

In order to reproduce the sound over a loudspeaker, the signal needs to be amplified significantly and also in the course it must be suitably processed to remove any associated disturbances.

The circuit diagram of the proposed electronic stethoscope amplifier is designed using two stages, one consisting of the opamp based tone control circuit, and the integrated proper amplifier stage.

The tone control stage is built around the opamp 741, and with the help of the associated RC networks and the pots. The upper pot controls the low frequency limit, while the lower pot is used to control the upper frequency limit. Both these pots can be appropriately set for achieving the best possible sound clarity.

In addition to the sound processing, the opamp stage also acts like a preamplifier for elevating the very low amplitude of the heart beat pulses to a suitable level for the power amplifier input. This enables the power amplifier to pick the signals at above the required minimum detectable level and amplify it on the loudspeakers optimally.

MIC as the main Sensor

The main sensing stage of this electronic stethoscope circuit is formed by an electret MIC which can be seen configured across the input of the tone control stage via an RC network.

In order to enable the MIC to sense the minute heart beat signals, the mic is enclosed within a rubber pipe with a rubber funnel like mouth opening.

The funnel like opening is supposed to be stuck over the chest of the patient just above the heart area for allowing the MIC to detect the concentrated heart rate sound and convert it into minute proportionately pulsating electrical pulses.

The opamp circuit responds to these signals and processes it appropriately as per the setting of the low pass and the high pass filter pots.

The finalized signal is applied to the input of the power amplifier configured around the TDA2003 amplifier circuit which is capable of generating a strong 10 watts of amplification over a 8 ohm loudspeaker.

The pot between the 741 output and the TDA input determines the volume of sound and can be adjusted for the same.

A Simpler Alternative (using a Wireless FM Transmitter)

In the request we also see the mentioning of a android phone compatible unit, which is difficult to achieve using the above circuit since the minimum operating voltage of this circuit can be over 12V so it cannot be operated easily using a cellphone existing battery

A simpler yet a more advanced method for achieving an electronic stethoscope amplifier functionality with a cellphone is to go wireless.

A small FM transmitter circuit can be used and positioned near the chest of the patient, and the heart pulses can be heard or recorded loud and clear over any cellphone equipped with an FM radio, which is commonly included in all standard cellphone regardless of its sophistication level.

The mic will need to be encapsulated appropriately inside a pipe/funnel kind of enclosure as suggested in the previous discussion, so that other forms of disturbances become undetectable for the MIC.

Once the heart beats are recorded inside the android phone, this can be easily used with a suitable app for converting the same into a graphical format and for enabling a more scientific assessment of the patient heart condition.

The wireless stethoscope amplifier circuit set up can be understood from the following diagram

Parts List

R1 =1M,

R2 = 2K2,

R3 = 470 Ohms,

R4 = 39K,

R5 = 470 Ohms,

R6 = 4k7

R7 = 270K

C1 = 0.1 uF,

C2 = 4.7 uF,

C3, C6 = 0.001uF,

C4 = 3.3pF,

C5 = 10pF,

C7 = 100uF/16V

D1----D4 = 1N4007

L1 = See Text

T1, T2 = BC547B,

T3 = BC557B

TR1 = transformer, 0-9V, 100mA

Feedback from Mr. Jan

I have built this project and it works well as a normal amp, but it is not sensitive enough to pick UP any heartbeats.

Any suggestions as to how I can make this more sensitive? Your assistance will be much appreciated.

Solving the Circuit Query

My Response: The design explained above needs to be correctly optimized in order to get the most favorable results, however in order to enhance the outcome to maximum, a transistorized MIC preamp could be introduced at C5, as illustrated in the following diagram, this should hopefully make the proposed electronic stethoscope circuit extremely sensitive and enable the heartbeat to become loudly audible.

Jan:

Thank you for the update. 

I have made the changes and must admit that it is much more sensitive, although I still cannot pick up a heartbeat clearly. I think the problem might be with the microphone.

Question: Are all electret microphones more or less the same or do you get some that are more sensitive? 

Analyzing the Circuit Results

Thank you Jan,

Electret Mics are all similar with their specs according to me, they will behave identically unless the device is faulty or accidentally a duplicate low quality piece.

I think you will need to fine-tune the circuit for getting a proper optimal response from the output.For this first you must replace the speaker with a headphone so that the initial low un-optimized sound becomes slightly audible in our ears.

Once you get hold of the sound you can begin adjusting the bass treble pots until the most favorable sound becomes available in the headphones, later on once the audio is perfect the headphones can be replaced back with loudspeakers.

If you find the existing bass treble stage inadequate, you can replace it with the following 10 stage equalizer and get an access to a 10 level optimzation control.

https://homemade-circuits.com/2013/06/10-band-graphic-equalizer-circuit-for.html

Best Regards.

Warning: The concept has not been verified for its accuracy and credibility and the author does not in any manner endorse the use of this circuit for serious heart diagnosing. Consult a qualified medical personal before using the explained circuit practically on a patient.

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!