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Electronic Stethoscope


t_ang4
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Ante,

I think amplification and filtering is "How" it works. Perhaps you are correct that he is looking for the theory behind the circuit or a more in-depth explanation of how op-amps work. What I think is pretty clear is that he has already read the description and still came here to ask how it works. Someone simply telling him to go back and read the description is rather insulting.

MP

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hi.. i am an engineering student from india. i started this e-steth project recently..i coudn't  understand the phone jack connection.. and where should we connect the ear piece of steth?or can i replace the earpiece with thehead phone connected from the phone jack? more over here i coudn't get a phone jack...iam sending a photo of what they ive as phone jack here in india...please reply me whether it is correct or not...

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The 1/4 inch phone plug and jack were invented for use by telephone operators to plug-in one person calling another person. The operators used cords with phone plugs on each end to plug-in to hundreds of phone jacks in front of them. The operators did the switching.

Now, 1/4 inch phone plugs are used to connect guitars and speakers to guitar amplifiers and for stereo headphones.

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To All:
Keep the posts on topic so that they will remain.
If you want to discuss anything else, start a new topic or if it has nothing to do with a project on this site, send someone a Private Message.

I think I have made it very clear over the years why posts will be deleted here.

MP

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PM from Narmadha:
hi
can u send me the picture of phone jack used in the e- stet 2 ..
Hi Narmadha,
I am sorry that the pic I posted of stereo headphones dissappeared from this thread. I don't know what happened to it. I found it in my recycle bin.
You should use headphones that completely cover your ears which will have bass response that goes low enough so that you are able to hear the very low frequency heartbeat sound. This type of headphones will also prevent the microphone from picking-up their sound which would cause acoustical feedback howling.
Here is the pic again:

post-1706-14279142618678_thumb.png

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  • 1 month later...

Hi everyone
Tanks for the interesting reading (20 pages is A lot ! )

Like many others I have, chosen this stethoscope as a school project.
My particular study in the team is about different heart frequency, and thus i need to have multiple optional filters... e.g. No filter, low pass, band pass.

After recording different stethoscope song I have arbitrary found those values:
Low pass 640 Hz (song in higher frequency sounded too much like noise)
Band pass Fc = 160Hz, Width = 160 Hz. (about 90 to 300 after rounding pieces value)


I have found that a professional 3M Littman e4000 have those frequency:
Extended range 20-1000
diaphragm 200-500
"precision" 20-100 <- the current circuit fit this category which is good ;)


I initially tough I could do this using only opamp but after reading I

post-17995-14279142711108_thumb.jpg

post-17995-1427914271126_thumb.jpg

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Thanks you, well i'll see for my quad opamp ... i may change it latter.
Is the noise twice as it's a quad versus a double ?
If not do you have any quad you recommand ?
What do you mean exactly by inbut bias ?
I see that you are using some electrolytic capacitor in your circuit.
Aren't they polarised to only one direction, a bit like a diode ?
If so ... doesn't the sound need to go in both direction as it have +- v
Also considering other chips i have my input source is more likely to be 2*AA (3V)
Than 9V .... does it makes any major difference ?

I need to put a recorder chip in the stetho... Is there any place you suggest me to *plug* the recorder input and the playback output ?

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Is the noise twice as it's a quad versus a double ?

It is noisy because it is not designed to be a low noise preamp. All of its opamps are noisy. All opamps in a TL071 single, TL072 dual and TL074 quad are quieter.

If not do you have any quad you recommand ?

The TL07x opamps are designed for low noise audio, but they don't work well when a 9V battery's voltage runs down to only 6V. An NE5532 dual is very low noise and operates fine when the supply drops to only 6V, but has a fairly high supply current so a 9V battery won't last very long.

What do you mean exactly by inbut bias ?

The input bias voltage of a transistor or opamp circuit is its most important requirement. Opamps usually have their input bias voltage set at half the supply voltage which is ground when a positive and negative supply is used.

I see that you are using some electrolytic capacitor in your circuit.
Aren't they polarised to only one direction, a bit like a diode ?

Yes, they must be installed with the correct polarity in the circuit.

If so ... doesn't the sound need to go in both direction as it have +- v

C2 and C6 are polarized and couple audio in this circuit. The audio never causes the polarity to reverse across the capacitors so they pass audio fine.

Also considering other chips i have my input source is more likely to be 2*AA (3V)
Than 9V .... does it makes any major difference ?

It won't work with a 3V battery that drops to only 2.0V over its life! A 9V battery voltage drops to 6.0V.
The electret mic won't work from only 2V.
The opamps won't work from only 2V.
The LM386 won't work from only 2V.
Even if they do work at only 2V, the voltage swing will be reduced to nearly nothing due to losses.

I need to put a recorder chip in the stetho... Is there any place you suggest me to *plug* the recorder input and the playback output ?

The recorder probably needs an input level control, so can record the output of the LM386.
The recorder can playback into the circuit's volume control if a switch and attenuator for it is added to the circuit.
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The input bias voltage of a transistor or opamp circuit is its most important requirement. Opamps usually have their input bias voltage set at half the supply voltage which is ground when a positive and negative supply is used.



Now, how exatly i assure that the input bias is correctly configured ?
If i have ground and 9V... the input bias is 4.5V ? meaning that i need to keep at least 4.5volt anywhere in the circuit ?
If so ... i may switch to your positive / negative power source.






C2 and C6 are polarized and couple audio in this circuit. The audio never causes the polarity to reverse across the capacitors so they pass audio fine.


Sorry to be a noob but what exatly you mean by couple the audio ?
You mean that between C2 and C6 the sound alwais go in the sema direction ?
Will this still be true if we change the central filter to target otehr frequency ?
What are the advantage of using such polarised capacitor instead of normal ones ?
Is it only a matter of electrolytic capacitor being easier to find at high value (1000uF)?




Also considering other chips i have my input source is more likely to be 2*AA (3V)
Than 9V .... does it makes any major difference ?

It won't work with a 3V battery that drops to only 2.0V over its life! A 9V battery voltage drops to 6.0V.



Well the electret work from 1.5V and so does the current opamp i have choosen.
one of my problem is that the recorder chip work at a very low voltage and it's the more expensive part of the circuit so i tougth i'd keep everything low.


Even if they do work at only 2V, the voltage swing will be reduced to nearly nothing due to losses.


Please explain this.  For the headphone at the end is it the voltage or current wich is important ?



The recorder probably needs an input level control, so can record the output of the LM386.
The recorder can playback into the circuit's volume control if a switch and attenuator for it is added to the circuit.


a switch will be added .... what do you mean by attenuator .. resistance, opamp used as a buffer ?

Thank you for your quick reply
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Now, how exatly i assure that the input bias is correctly configured ?
If i have ground and 9V... the input bias is 4.5V ? meaning that i need to keep at least 4.5volt anywhere in the circuit ?
If so ... i may switch to your positive / negative power source.

The circuit must be re-designed to use a single supply voltage.

Sorry to be a noob but what exatly you mean by couple the audio ?

C2 couples AC audio from the approx. +6V microphone to the 0V opamp input, so C2 always has the correct polarity.
C6 couples AC audio from the +4.3V output of the LM386 to the 0V connection for the headphones, so C6 always has the correct polarity.

You mean that between C2 and C6 the sound alwais go in the sema direction ?
Will this still be true if we change the central filter to target otehr frequency ?

A coupling capacitor passes higher frequencies and blocks DC and lower frequencies. The frequency of the filter has nothing to do with it, except the reactance of the coupling capacitors must be low at the desired lowest frequency.

What are the advantage of using such polarised capacitor instead of normal ones ?

Electrolytic capacitors are with a very high value, much higher than any non-polarized capacitor, and they are inexpensive and small.

Well the electret work from 1.5V

Electret mics have a range of operating current. If your mic has a current that is higher than your circuit's series resistor can feed it then the mic's voltage will be much too low.

and so does the current opamp i have choosen.

The quad opamp has a minimum supply voltage of 2.7V. Two AA alkaline cells will quickly drop to 2.4V the slowly drop to 2.0V.

Please explain this.  For the headphone at the end is it the voltage or current wich is important ?

A low impedance headphone needs a fairly high current and a faily low voltage swing. A high impedance headphone needs the opposite.

a switch will be added .... what do you mean by attenuator

You have a capacitor in the LM386's circuit which boosts its gain to 200. A two-resistors attenuator is needed to reduce the output level of the playback chip to match the very sensitive input of the LM386.
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The circuit must be re-designed to use a single supply voltage.


I am wrong to beleive that the only place where -9V is needed is at the opamp ?
Then the redisign would be in choosing resistance / capacitor values ?

In splitted voltage.. the chip i use can work at +- 8V Max recommanded
Is there an easy way to get +- 8V from a 9volt battery ?


C2 couples AC audio from the approx. +6V microphone to the 0V opamp input, so C2 always has the correct polarity.
C6 couples AC audio from the +4.3V output of the LM386 to the 0V connection for the headphones, so C6 always has the correct polarity.


Those two, i understand now...
I guess C5 is like C6
C4 ... is grounded so i understand also
but what about c3 ?



You have a capacitor in the LM386's circuit which boosts its gain to 200. A two-resistors attenuator is needed to reduce the output level of the playback chip to match the very sensitive input of the LM386.


so the attenuator is basicly a voltage divider to fit the need of the LM386 input. good to know.

---------

thank you for your help
I have one last question....
Do you have anny comment on the two "filter cricuit" I decided to adopt ?
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I am wrong to beleive that the only place where -9V is needed is at the opamp ?

Correct. But the max total supply for your opamp is only 16V.

In splitted voltage.. the chip i use can work at +- 8V Max recommanded
Is there an easy way to get +- 8V from a 9volt battery ?

it would be much easier to use a "normal" quad opamp. Suppliers deliver parts the next day.

I guess C5 is like C6

C5 prevents R14 from drawing 1.1A continuously from the output of the LM386 which would destroy it and kill the battery very quickly.

C4 ... is grounded so i understand also
but what about c3 ?

C3 couples AC audio but blocks DC and low frequencies.

so the attenuator is basicly a voltage divider to fit the need of the LM386 input.

Yes.

Do you have anny comment on the two "filter cricuit" I decided to adopt ?

It is a bandpass filter that cuts the low frequencies that you need to keep. You want a lowpass filter not a bandpass filter.
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You want a lowpass filter not a bandpass filter


Well ... i sort of know what i want.
I'm building this to identify certain heart song and enphasis them thus a bandpass


Oh well i was proud of the opamp i choose as i figured it was good quality etc.
I'll wait before doing another digikey order as last one was quite expensive and it's many trouble.
The main reason i should switch is what you call the  " input bias voltage "
what are the consequences of that being unregulated ?


Well i guess you conviced me, the cricuit is mostly what i need
Do you have any reference / part number for affordable shielded cable ?
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Well ... i sort of know what i want.
I'm building this to identify certain heart song and enphasis them thus a bandpass

A heartbeat is a DC pulse of energy. Sometimes something is caused to vibrate at a very low frequency by the pulse. I don't know what vibrates, the skin maybe?


Oh well i was proud of the opamp i choose as i figured it was good quality etc.
I'll wait before doing another digikey order as last one was quite expensive and it's many trouble.
The main reason i should switch is what you call the
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My friend could assemble the exact circuit at this link:
http://www.electronics-lab.com/projects/science/019/index.html

Here's it on the breadboard:
stwork.jpg

Here's the sound "Puffing":
See the Video

The heart sound was very strange:
Here's an mp3 amplified file

Changes in that circuit, I didn't find 3.9 Ohm, so I replaced it with 4.7 and I've not applied a shielded cable yet  :)

Was the 1st link about the circuit in this site working, or there was another one?  ;D

Thanks everyone, specially Audioguru  8)

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Hi TriGerminal,
Welcome to our forum. ;D
I couldn't make your video play but the sound file is very strange, it is missing the very low frequencies that are produced by heartbeats.

The original project has errors on its schematic and in its parts list and might sound something like yours:
http://www.electronics-lab.com/projects/science/014/index.html

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