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Electronic Stethoscope


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This original circuit doesn't work here, and doesn't work on the original author's web site. The same complaints about it are there too.
Why not make a simple correction to make it function and make a few more corrections and changes to make it improved?
The professional doctors who use it don't need it sounding like a guitar fuzz distortion circuit!

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Emad, this is not directed at you...

Here is what I posted in April:


If you cannot put forth enough effort to build a project, your corrections will fall on deaf ears.

On the other hand, if you build a project and find a correction, then I want to know about it. No one goes around posting a project that does not work for the fun of it. A line or lines in the wrong place or a mis typed value or two usually correct most problems. Replacing the ICs and other parts is not a correction. It is a different design which accomplishes the same thing. It is harder to find the error in a present design than to redraw it. If you take the easy road, don't expect me to recognize it as a correction. Post your "new" design as what it is. A new design.

All members are welcome to post a design to the forum. I expect that it will be something you have built and tested and have a working model.


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Hi Guys,
As a review, I have posted analysis of faults with this project and my corrections in the Theory Section of this forum, here:

Opamp or Power Amp? http://www.electronics-lab.com/forum/index.php?board=15;action=display;threadid=1630

Low-pass Filter http://www.electronics-lab.com/forum/index.php?board=15;action=display;threadid=1661

Extra Parts http://www.electronics-lab.com/forum/index.php?board=15;action=display;threadid=1663

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Hi all . I constructed the corrected circuit and I have good amplification when I make clicks with my tongue.,but heart sounds are distorted . there are many cracles and crepitaions with the soundto the extent that no heart sond could be heared when increasing the volume. I used anon shielded wire with the mic and I forgot C7 and C8. Also I put R9 as it is and making short at it does not make a differance. Thats what I did--------------------------------------Thank youuuuuuuuuuuuuuuu

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Hi Doctor Emad,
I am sorry to hear that your corrected circuit doesn't work properly yet.
The first thing that should be done is to use shielded cable to the microphone. It is very important. Non-shielded wire would pickup a lot of mains hum that can overload U1 due to its high gain. Whenever I run out of shielded cable, I just use a piece of shielded RCA-plugs cable, which is the kind that connects the components of my stereo system together (but don't use speaker wire).

The loud crackles that are heard are probably caused by the microphone being in direct contact with your stethoscope head (cone). All applications (hand-held mic, tape recorder and telephone) of microphone elements have them isolated with a rubber mount or grommet. Can you isolate the microphone from the cone but still have it sealed to it, so that movement (friction or handling) of the cone or the patient is not too loud?

C7 and C8 are also important to have, especially with battery powered amplifier circuits like this one.

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Hi Prateek,
The TL071 and TL072 are very common and cheap in North America. They have been used in audio equipment for about 20 years.
A TL071 costs a little more than a 741. A TL072 dual costs less than a 741.
An LM833 dual is also low-noise, and costs a little more than a 741.

Can you get Japanese ICs? A list with a cross-reference has an NJM072, which is the same as a TL072. The list is here:

Just ask your supplier what they have for a low-noise single or dual opamp that is used in audio equipment.

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Hi Guys,
I just finished making a simple test to hear what these noises are all about:
1) 2-wire electret mic from a cell phone. It is mounted in a rubber sleeve.
2) Drilled a hole in a plastic jar lid and pushed the mic into the hole.
3) Biased the mic with a 10K resistor from a 9V battery, like our project.
4) Connected it with shielded cable to an LM386 circuit (my little portable test amp with a low-frequency roll-off of 20Hz).
5) The LM386 circuit has a volume control and a 10uF cap added between pins 1 and 8 for a gain of 200.
6) Stereo headphones from my portable CD player with both 32 ohm channels connected in parallel.

When I talked about 40cm from the mic, it was too loud and I had to turn-down the volume control on the input of the LM386. No distortion then but still pretty loud.
When I held the jar lid with the mic in it against my chest and over my heart, I heard my heartbeat loud and clear! I had to hold the jar lid still to avoid loud friction noises. No hum whatsoever, even with a flourescent light hanging above and electrical stuff all around. The LM386 made a noticeable hiss because it has a gain of 10 times more than what it will have in our project, and is not low-noise with such a high gain. When I talked with the jar lid against my chest my voice was quite loud but muffled, so my isolation wasn't very good. I could also hear my dog barking (she was annoyed by the feedback squeal that I got while moving the mic)and heard the phone ringing in the next room.
If the jar lid is more rigid then it will probably provide better isolation. A "stethoscope head" (cone) should be ideal. The low-pass filter in our project will also attenuate those background noises. I'm glad that our project will have enough gain to do its job.

On my 'scope the LM386's output showed from 1 to 3 cycles of about 25Hz for each heartbeat. Sometimes the single cycle was a rounded pulse of one polarity, and always the same polarity.

So our "corrected" project should work fine.

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Hi Doctor Emad,
U5 is the LM386 audio power amp IC. It drives your earphones.
U1 and U2 should be low-noise opamps such as the TL071.
U4 can be just about any opamp.
C3 and R7 connect together and to pin 6 of U2, the volume control and to pin 3 of U4.
Please see my corrected schematic of this project on page 7 of this web-thread.
I'm glad to help you with this project.

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hi Audiguru. I had no TLO71 in my area so I tried to use to use just one IC as apreamplifier and another one as an amplifier------- Just two . the preamplifier is LM358 and then the amplifier is the LM386 as that in your corrected circuit-------I hear my heart beats very well------- Ijust need trying another headphone as I expect sound to be better if I use an earphonr other than my mobile headphone. THats what I did . Thank you audaguru

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Hi Doctor Emad,
Did you use the KIA358 for both U1 and U2?
I checked, and a KIA358 is the same as an LM358, which is a dual opamp. It is noisy like a 741 and has crossover distortion because it uses very low supply current.
Can you get an NJM072B which is made by New Japan Radio? It is a dual low-noise opamp almost the same as a TL072. Don't use an NJM072 without the "B". They admit that they made a big mistake when they made that copy.
I do not recommend using only one battery because then you will need to add resistors and a capacitor to make a mid-supply voltage for R3 and R8 to connect to, as their voltage reference.
Use 2 batteries like my corrected schematic and hope that the noise and distortion of your KIA358 aren't too high.

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  • 4 months later...

Hi Singhal,
Welcome to our forum.
That project has errors and doesn't work. I have corrected and improved it and posted it here:

1) The bi-colour LED and the opamp driving it aren't necessary, it just looks nice. The bi-colour LED was chosen because it has two LEDs in one case with the LEDs connected in opposite polarities. Two separate LEDs of any colours would work the same. Each heartbeat causes the colours to indicate differently. Sometimes the colours alternate quickly and flicker, other times only one colour lights during each heartbeat.
2) Connect a "phone jack" properly by soldering short wires to it and soldering the wires to the circuit.
He, he. Yeah, the heartbeats make a phone dial for help! He, he. ;D
It isn't a jack for a telephone, it is called a "phone jack" because it looks like the jack telephone operators used long ago when they plugged-in your connection. Use a jack for home stereo headphones or the smaller jack for headphones from Walkmans or CD players. I recommend using cover-your-ear headphones to reduce hearing background sounds and feedback.
Don't use a speaker. It would produce loud squealing noise from acoustical feedback.

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To all users on this thread....

...Actually some users reported that the Electronic Stethoscope worked and others reported that they could not make it work. I see there are several posts in the Electronic Stethoscope 2 topic now going both ways as well. It is normal to have users with different capabilities on a forum. Many will not be able to make a project, even with much help.

It should be noted that Audioguru designed the electronic Stethoscope 2 but he did not ever build it. It should also be pointed out that Audioguru did not build this project to verify that it would not work. Audioguru is a "Theory Expert" as it states in his signature.

There will be those who can build both and there will be those who cannot build both.


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