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Electronic Stethoscope 2


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Hi Loveguru,
I'm glad and relieved that you finally got your project to work very well.
1) A 47K pot will work exactly the same as a 10K or even a 2.2K pot, no change in volume. Pots don't amplify, and it certainly won't load-down the very low output impedance of U1b unless it was 2K ohms or less when it would cause distortion, not a change in level.
2) I have discussed adding a second filter stage to attenuate background voices more. Look here on the previous page:


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hi audioguru.............
i have no words for your great help and supporting us..........
really thanx to you from bottom of the heart........
but still i have question ralted to ckt operation....
1)what is the exectlly difference b/w HPF & LPF?
2)you have answered some one that without filter(c1) , circuit might be "motorboat"when the battery runs down...what does it actual mean?
3)why we have used only tl072 ope amp...why not anothers.....
give me clear idea about this operatinal amplifier........
4)ic lm386 is used for driving headphone ........has it some another function in circuit operation..........?

plz reply fast as posiible as early......

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Hi Loveguru,
I am glad to help you.
1) A High Pass Filter passes high frequencies down to its cutoff frequency and attenuates lower frequencies. A LPF does the opposite.
2) An amplifier can oscillate at a low frequency if it doesn't have adequate supply bypassing in stages, to compensate for the rising internal resistance of its battery as it runs down.
A noise at the mic is amplified by the circuit. The amplifier draws current from the supply to drive the load. The current from the supply causes the battery voltage to drop, which returns to the mic preamp through the supply and it is amplified. Around and around it goes. It sounds like a motorboat, "put, put, put," etc.
3) I have chosen the TL072 because it has low noise, low distortion and wide audio bandwidth. It is manufactured by many manufacturers. It is used in many audio devices and is inexpensive. Many other similar ICs are available.
4) The LM386 is a small-power amplifier with an output current capability about 32 times more than an ordinary opamp. In the project is has a voltage gain of 20.

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u r telling that the heartbeats freq.is 20-3- hz./................
but i counted it as 1.2 hz...........
as the equation................
so..which one is true???????????????
tell me soon...i am in my clg project festival..........so i have to reply it...............ok

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in e-scope-2 ckt u have told that tha cut-off frequency of lpf at u1b is 103 hz.
now what is the frequency at hpf before u1a?/????????
we have used band-pass filter=lpf+hpf.
i wann the frequency responce of that.........i mean the lower limit..............please tell me fast..............
and as per my counting the freq. of heart-beats is 1.2hz.........then our hpf will be able to cut-off the 1 hz freq????????????
reply me soon.
and what is the funda behind led color changing?
please write in brief abt lpf and hpf operation in our circuit.........
reply me soon.................

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Hi Loveguru,
Don't use a HPF before U1a and don't use an audio bandpass filter. You need the frequency response to go very low.
You are correct that heartbeats repeat about once per second when slow. But the sound is low frequency audio, about 20Hz to 30Hz like a bass drum. I don't know what is vibrating at the low audio frequency, maybe the patient's skin under the stethoscope head.
In the thread of the original project, doctors posted pictures of professional electronic stethoscopes showing a low frequency vibration. I tested an elecret mic connected to an LM386 and heard in my headphones and saw on my 'scope a low frequency vibration . See my attached sketch:


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Hi again Loveguru,
I just finished measuring the output impedance of my electret mic with its 10K power-feed/load resistor. I measured 3123 ohms. Yours will probably be different.
Therefore the low frequency cutoff including the project's 2.2K R2 and 4.7uF C2 is 6.4Hz. The response at 32Hz will be down a little, the response at 3.2Hz will be down about 5 dB's and 1.6Hz will be about -12dB (one-quarter). You can call it a HPF.

The microphone itself has a HPF caused mechanically by the tightness of its diaphragm's support. It also has an electronic HPF caused by the capacitance of its diaphragm and the leakage current of its FET's gate.

I didn't design this project. I just fixed and improved the original defective one. I don't know why the original designer used a two-colour LED, I think it would just flicker for each heartbeat and waste battery power.

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  • 2 weeks later...

Hi all
I made these circuits many times without the part involving the bipolar diod and they were operating.
About three times they suddenly become badly operating.they respond to knocking on the diaphragm of the stethoscope but not to the heart sound.I dont knoww the cause.
I hope you give me advice.
Thank you.

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Hi Doctor Emad,
How many did you make?
There is nothing in the circuit to fail except the batteries.
Maybe the input or output coupling capacitor is installed with incorrect polarity. After a while, it won't pass low frequencies.
I recall that a few years ago many Chinese electrolytic capacitors failed, maybe you got some.

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Hi all
I made these circuits many times

Who simulated it?
Not Doctor Emad, he was too busy making them.
Not me, I only fixed the original defective one. I don't have a simulator.
Many people made the Electronic Stethoscope 2 project.

It needs only 2 small ICs so should be simple to make without any pictures or parts layout. If I needed one, I would post pics and a PCB. ;D
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I was refering to you, the author of the circuit...My mistake, then. I guess I figured you would not post a project without making a prototype or at least simulation because you made such a fuss about there being a project on this site that Aaron Cake did not actually build and test himself.

xoy: The circuit was neither built nor simulated by the author, nor are there any pictures of a working model. You are on your own.


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Hi May,
Welcome to our forum.
The project's Circuit Description clearly describes the functions of its 4 parts: Microphone Preamplifier, Low Pass Filter, LED Driver and Audio Power Amplifier. Can't you make your own block diagram with these boxes on it? You could also add 2 more boxes: a microphone at the input and headphones at the output. ;D

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Hi Audioguru...I'm again..I would like to know the advantages of this electronics stethoscope (at least 10) in brief ..because what i can see is just could hear our heartbeat...
Moreover, I would like to know what is the disadvantages of electronics stethoscope?Thank you for your help..

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Hi May,
I can think of only a few advantages of an electronic stethoscope:
1) Its output can be viewed on an oscilloscope or computer.
2) Its output can be recorded.
3) Its gain can be turned up to hear faint sounds not heard with a mechanical stethoscope.
4) The cutoff frequency of its low-pass-filter can be raised with a switch to hear breathing sounds or maybe even sounds from joint problems.
You should hear the noises from my ankle that a Chiropractor "adjusted" and sprained. It sounds something like a garbage can being dragged along the street! Sorry, I hope you are not studying to be a Chiro quack.

1) Its might be left turned-on then its batteries will go dead.
2) You might lose its headphones.
3) Its mic and/or headphones cables might break or wear-out.

Mechanics use electronic stethoscopes to locate noises in a car engine or general squeaks by moving it around to find where it is the loudest. This project would probably work better for mechanics if the cutoff frequency of its low-pass-filter was raised as explained in the text for the Electronic Stethoscope-2 project for raising the cutoff frequency for doctors to hear breathing sounds. ;D

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