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Electronic Stethoscope


t_ang4
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Hi Soly,
Welcome to our forum. ;D
The headphones output of the Electronic Stethoscope-2 project can be connected to the Line Input of a computer by using whatever plug that fits your computer. If the computer doesn't have a Line Input but has only a Microphone Input jack then an attenuator must be made with a couple of resistors.

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hi audioguru,

i am a biomedical engineer student working on a project on heartbeat sensor ive to build an application that can store the bpm of a patient...i plan to use a mod 60 counter as timer can u give me an idea of how do i store the heartbeat pulses? thx

uhm need a reply urgently :\

ill try to post my sensor circuit

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Hello Everyone.

This is my fist message to the group. I am currently workin on builiding a blood pressure monitor as my final year project. For the first sem i had used oscillometric method to measure the blood pressure. Luckly i was successfull in doing that...and got very good marks for that...  :)

for my second semester, I am now working on auscultatory method, and task is to capture Korotkoff Sound(Acoustic sounds from radial artery) and display the waveform on PC..project is exactly similar to Electronic Stethoscope...

I am currently going through 24 pages of discussion on the forum, trust me there are so many different circuits and suggestions ....well ill have few sleepless nights but ill try figure it out...

let me know if anyone wannts my report for oscillometric blood pressure monitor method...

ill keep u guys posted...WISH ME LUCK! ;D

pri

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G'day guys...

Hi audioguru, hello everyone...

As per ur suggestion i will try with circuit2. I was getting bit confused with so many circuits flying around in this forum..iv got test this week but i am confident that by next monday i should be able to breadboard the circuit ...i will keep u guys posted.

Audioguru i appreciate ur help. .. :)

pri
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Hey Guys..hi audio guru ;D

Iv just completed my tests for this week. last night i was simulating the circuit#2, i have simulated exactly according to ur modified circuit..i.e circuit#2. The only difference was during simulations i have used LM741 instead of lm386. I provided a sinewave signal of 10mv,10hz. The ouput which i got from the circuit(U5 at R15 of ur circuit diagram) was a sqaure wave with a amplitude of around 10-12 volts.

I am not sure if that is the output which i should be getting, i feel the output is clipping and thats the reason im getting bit distorted sqaure wave. Following is the picture of my simulated circuit and the output which i get.

please find attached screen shots of the simulated circuit and the transient response....(files are *.jpg files)....if u wannt i can also give all the dc opearting points of the simulated circuit. i am intending to start breadboarding the circuit from monday..before i do that i wannt to make sure i get everything correct on the simulations...

cheers
priyank :)

post-21720-14279143082944_thumb.jpg

post-21720-14279143083222_thumb.jpg

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Your 741 opamp at the output doesn't have any negative feedback and therefore has a voltage gain of about 200,000. The LM386 that it is supposed to be has built-in negative feedback and therefore has a voltage gain of only 20. The gain of the 741 opamp is 10,000 times too high, sure it will clip.

The 741 won't work properly with its inputs DC at its negative supply (ground) but an LM386 is designed to work that way.

Your circuit has C3 connected to the wrong place, messing up the lowpass filter.

Fix C3, add negative feedback to the 741 opamp so its voltage gain is 20 and disconnect its negative supply pin 4 from ground and connect it to -9V instead. Then the sim should work correctly.

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Hi audioguru, :)

I have made the changes which u had recommended, i have breadboarded the circuit exactly according to ur circuit diagram.  My professor at uni has given me a OMNI-DIRECTIONAL ELECTRET MICROPHONE INSERT.. he is suggesting me to insert microphone in the bell of the sthehoscope.

The issue i have is the specification of the microphone insert. On the specfication  it says its operatin frquency is 50-12,500 hz.

My concern is that in order to obeserve korotkoff sounds in branchial artery,frequency of interest is 2- 100 hz. I am not sure if the operating frquency of
50- 12,500 hz will be of any good.

i tried search other microphone inserts but coudnt really find one. Audioguru would u recommend any microphone or microphone insert?

Following is the link to the microphone insert which my professor has given to me.

http://www.maplin.co.uk/SEODynamicPages/module.asp?ModNo=29846

cheers  ;D
pri

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Hi Priyank,
Maplin don't have a detailed spec on that page for the response of the mic. The FAQ has a question, "How flat is the frequency response?" but I couldn't find the answer. Ask them.

Panasonic makes many electret mics which were available somewhere in North America. Their cheap one has a detailed frequency response graph showing that the response is absolutely flat down to 25Hz and probably to much lower frequencies.

post-1706-14279143096928_thumb.png

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  • 2 weeks later...

After a while I decided reloaded the project again, I still use the breadboard but just re-connected the wires and get longer wires with clips for the batteries and speakers ..

I connect it to the audio card in my computer and I get this noise all the time (even if I removed the entire mic!)

Is it receiving signals from air or something? :)

Thank you

problem.zip

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Hi TriGerminal,
The circuit has high gain and could pickup interference from the air if it is not built compact on a pcb or on Veroboard or does not use a shielded audio imput cable.

Your circuit produces a static noises and a low frequency oscillation called "motorboating".
Does the LED oscillate slowly like the sound?

Did you substitute any parts?
Are the 9V batteries fairly new? What voltage are they while the circuit is powered?

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Does the LED oscillate slowly like the sound?
It's doesn't give any light (maybe hardly I can see it!)

Did you substitute any parts?
yes, R12 instead of 330k I put 270k because I lost my 330k .. also I connect solid single wires with standard several fine wires ones :)

Are the 9V batteries fairly new? What voltage are they while the circuit is powered?
They're the same I used the first time, but I didn't measure the voltage :)
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Hi TriGerminal,
Your circuit motorboats and the LED doesn't light. So I think one or both of your batteries are dead.
The circuit makes a little amount of noise that is amplified. When the opamp tries to drive current into the LED then the battery voltage collapses and the noise stops, then the battery voltage slowly builds up again and the problem repeats over and over.

Measure the battery voltage on the circuit while the circuit is making the slow oscillation. Maybe the clips from the batteries are making a poor connection.

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I think the project should work fine when the batteries measure a minimum of 6V while they are powering it.

What load resistance do you have at the headphones jack? If it is 8 ohms or less then the positive battery has a high current drain when the sound is loud, possibly dropping its voltage too low. That would cause the circuit to motorboat.

Try new alkaline batteries.

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Hi all
Ihave made the original ,the corrected and the elect. steth. 2 about one year ago
The original didn t work and the other two worked.

The  best one was the corrected circuit.

The stethoscope should hear the sounds between the 1st and 2nd , and the 2nd and 1st heart sonds , like murmours as systolic and diastolic , also clicks and so on.
So there shoud be no noise .
I hope to find a new project using 3v supply and a small head phone like that used with mobile . This will be more practical and I can use it in my clinic.
Thanks to all especially audioguru.

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  • 4 weeks later...

Hi all, Hi audioguru...

I have tried the modified circuit, but sadly it didnt work for me. I need to clarify one thing in the circuit. IS pin 4 of u5 (lm386) ground or connected to the -9 volt  supply. Some of the post in forum have suggested to connnect pin 4 to the -9 volts. why would that be? I went through the datsheet of lm 386 which suugests to ground pin 4. i have built the plastic diaphragm for the microphone insert and have breadboarded the circuit according to the circuit diagram. WIth me i dont get any out put when i connect output from pin 5 of u5 to my a/d converter of the dspace card . i have also used shielded cable for the microphone part.

ANy suggestion please....have attached pics of breadboard and the plastic diaphragm for the microphone insert

cheers
pri [attachment deleted by admin]

post-21720-14279143181091_thumb.jpg

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I have tried the modified circuit, but sadly it didnt work for me. I need to clarify one thing in the circuit. IS pin 4 of u5 (lm386) ground or connected to the -9 volt  supply.

Pin 4 (and pin 2) of the LM386 should be connected to ground (0V).

Some of the post in forum have suggested to connnect pin 4 to the -9 volts. why would that be?

Because the original project has an error in its parts list and says to use a 741 opamp for U5. A 741 won't work with its pin 2 connected to its pin 4 but works a little with -9V on its pin 4.

i dont get any output when i connect output from pin 5 of u5 to my a/d converter of the dspace card . i have also used shielded cable for the microphone part.

Pin 5 of the LM386 has a DC voltage that idles at half the positive supply voltage. Then its output voltage swings down to about +1V and up to about +8V. The +8V is too high and might damage your a/d converter. You need to convert the signal to a voltage range of from 0V to +5V for the a/d converter.

If you cannect a speaker or headphones to C6 do you hear anything?
Do the LEDs flicker when there is sound at the microphone?

post-1706-14279143182524_thumb.png

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