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t_ang4

Electronic Stethoscope

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To All,
How come, OOOps, why is it that Cdak's professional and even his own graphs show a heartbeat with a low frequency vibration?
If a heart beated with those vibrations then I think that its owner wouldn't be alive for very long.
Is it the skin under the stethoscope head that is vibrating?
Is it the electret mic's diaphragm? I doubt it.
Is it the preamp almost motorboating?
Is it the low-pass filter ringing?

Has anyone seen a live heartbeat scope? (Never mind those simulated ones on TV)

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I MADE THIS CIRCUIT THRICE BUT IT IS NOT WORKING.WHEN I USED SIGNAL GENERATOR INSTEAD OF MIC IT DID NOT GIVE OUTPUT, WHEN INPUT IS GIVEN AT INPUT OF 2ND OP AMP IT GAVE OUTPUT VARYING WITH FREQUENCY.HOWEVER OUTPUT WAS NOT IN ACCORDANCE WITH COMPUTER DESIGN(ACTUALLY I MADE THIS CIRCUIT ON COMPUTER).WHEN INPUT IS GIVEN AT FIRST OP AMP IT GIVES OUTPUT UP TO THE INPUT OF FIFTH OP AMP AFTER THAT THERE IS NO OUTPUT.WHAT COULD BE THE PROBLEM.

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shiva, another group member with user name CDAK also made this unit and also had some problems with it. He has made some modifications. I am awaiting the final changes. If you will look through this thread, he describes the changes he has made.

MP

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Shiva,
As you have found, this project has errors. Have you made corrections that I recommended? They are here:


Hi guys,
Sorry for this late post. This project has attracted a lot of attention but has also created much frustration and confusion, due to schematic and parts list errors in the original article. Let us stick to the original circuit using standard parts and having 9V batteries. Dpak showed a nice circuit but the LM387 is obsolete (discontinued in '98) and it cannot be powered with 9V batteries easily.
Corrections to the original circuit follow:
1) Connect pin6 of U2 to the junction of C3 and R7. This applies proper negative-feedback which dramatically reduces gain (and noise) and allows U2 to function as a 2nd order low-pass filter. Thanks, T_ang4 and Staigen.
2) Use an LM386 for U5 and swap its pins: pin5 is output and pin6 is +9V. The LM386 has built-in feedback and biasing to be used with its inputs as shown, and can drive an 8 ohm earphone. Thanks, Staigen and Mozikluv.
3) Change C2 to 4.7uF (the + lead to the mic) so that it can pass a heartbeat sound. Listen to Pink Floyd's "Dark Side Of The Moon" rock song since it starts with a 16Hz heartbeat. The original circuit's gain is 'way down at 16Hz.
4) Get rid of U3 which doesn't do anything. U2 can easily drive U4 and the volume control.
5) Get rid of R3 and R9 and replace them with wire . They also don't do anything.
6) Use a TL071 (or a dual amp TL072) for U1 and U2. It is low-noise and inexpensive. Or use a quad amp TL074 for U1 to U4.
7) Disconnect the junction of R1 and C1 from +9V and add a 1K resistor from their junction to +9V. This will filter the amplifier's input from the bouncing +9V power supply. Add an additional 1000uF capacitor fom +9V to ground. This helps the battery provide power to U5.
8) Add a 1K resistor across the ouput jack. This will stop a loud "pop" when you plug-in your earphone if the stethoscope is already turned-on.
If the mic is properly mounted in a stethoscope-head (jar lid or whatever) then it should reproduce a heartbeat sound well without much background sound. If breathing sounds must be heard then change R5 and R6 to 1K resistors, but background will be louder, and keep the mic away from your earphone to avoid howling. Add a switch to hear either sound properly.
Please reply if you make these mods and let us know how it works.

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MP,
50 days after I corrected it, the circuit in our projects section still shows opamps running open-loop, without feedback. This has frustrated Cdak, and now Shiva. The errors are still not simply corrected. The original author's site shows similar frustrations.
Cdak gave-up with this circuit before learning of its errors and simple corrections, and therefore made his own design.
When will our schematic and parts list be corrected so that our circuit can function as intended?

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...as soon as CDAK reports his successful changes.

Your changes are based upon your theory, which I have seen fail you in other posts. You have not built the circuit, and therefore do not have a grumble in my opinion.

MP

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Shiva,
You're welcome. I'm glad to help you.
Perhaps your computer simulation program doesn't know the circuit's required functions:
1) U2 is supposed to be a classic "Sallen and Key equal-capacitor, equal resistor (requiring its gain of 1.6), Butterworth 2nd-order low pass filter" with a cutoff frequency of about 103Hz.
Your computer thinks that U2 can work fine open-loop, without any feedback, and amplify very much the noise from U1.
Your computer doesn't know that U1 and U2 probably have a small DC offset voltage. With an offset voltage at U2's input, its output would be idling against a power rail since its enormous gain would amplify very much this offset voltage. With its output idling against a power rail, then it becomes a high-gain rectifier with severe distortion.
2) U5 is supposed to be an earphone driver.
Your computer thinks that U5 can work fine without any feedback, with an open-loop gain of 100,000 or more, amplifying its own and other circuit noise.
Your computer doesn't know that the 741 opamp that is specified for U5 in the parts list is not capable of driving an 8 ohm to 32 ohm earphone, which is like a dead short to a 741.
Your computer doesn't know that the 741 that is specified for U5 will not work with its inputs DC referenced to its negative power supply.
3) This project is supposed to amplify low-frequency heart-beat sounds.
Your computer doesn't know that with the small value that is specified for C2 in the parts list, then the desired low frequencies are cutoff.
Etcetera. The simulation program doesn't know all the details.

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I built this project for school. Its working very good. I guess people here dont know how to build if its dosent work for them.
Only change I did is instead of U5 leg 4 going to ground, its going to -9v.
Its working very good and i can hear my heart beep very good and high.

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Tomah,
Thank you for the confirmation. The project also simulates well in Electronic Workbench as Shiva has told us.
It is normal to see more posts from those who need help on a project than from those who have no problems with a project.
I really appreciate to hear from those who have success. Your feedback is very beneficial to others. It gives others faith that the project is workable.

MP

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Tomah,
That is wonderful that your project works well. ;D
You are clever to change it so that U5 has a proper negative supply voltage.
Try connecting the junction of C3 and R7 to pin 6 of U2, and change C2 to 4.7uF, with its + terminal towards the microphone. Please let us know the results.
What is the impedance of your earphones? ???

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Why would he want to change anything if it is working?

Tomah, I introduce you to audioguru, one who has not built this circuit but has made the statement that the project will not work without his specified changes. I guess he was wrong.

MP

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If U5 had .... its pin4 connected to -9V so that it could function as a 741 opamp, then it certainly would not be able to drive an 8 ohm earphone like the LM386 is designed to do.


MP,
No, I wasn't wrong:
1) Tomah's project didn't work until he made a change that I discussed above.
2) Shiva's three circuits had no output from U5 without this change, but his computer simulation did not catch this error and therefore worked.
I'll bet that Tomah's earphones' impedance is much more than 8 ohms so that the low-current-output 741 opamp can drive them.

Tomah's slightly modified circuit makes his heart sound like a "beep". I don't think that is a correct heartbeat sound since with another couple of changes then it will produce the correct low-frequency "thump" sound. Why not try a couple more changes to make it much better?

Note that I have not mentioned yet that Tomah's U5 is running "open loop" without any feedback, with AC and DC gain of 100,000 or more. What signal corruption does that cause? Maybe his "beep" is more like a high-frequency "buzz". Should a heartbeat sound like that?

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Terribly wrong!!! audioguru.
You posted a long winded page of 8 necessary changes, and you did not even build the circuit. Before you stated that this circuit could not work. Now you state that it does not work well. Others have made the circuit work without the 8 necessary changes. Why is this circuit able to work for others without your corrections? In fact, no one has built it "WITH" your 8 suggestions, so we do not know in fact that your theories work well (they might, but we do not see the working circuit). I will have to go with the proof, not the theory. From the replies, I will have to consider your changes unnecessary.

MP

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Only 3 of my changes are important and were also discussed by other members. The other changes only make it better (low-noise microphone preamp, etc).

Has ANYONE made this project work without any changes?

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sir isee it as very interesting topic, but i want to modify it.
Is it possible to use a digital display of no. of heart beat through it by modyfying sound signal and using counter ic with seven segment display?

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hi every body, i have spend 3 precious months of my college life working on this project of electronic stethoscope.i made it thrice with one of my friend's help but could not hear anything other than humming noise in the speakers.soldering is perfectly alright.when i tried to trouble shoot it using a c.r.o i got an amplitude modulated kind of waveform at the output . please help.
prateek

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Prateek,
Speakers?
Are you using speakers instead of earphones?
Maybe the microphone could hear the speakers and the circuit produced acousical feedback. When low-pass-filtered, it would make a humming noise in the speakers with an amplitude-modulated waveform.

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Hey guys

I have made this circuit but I am using a scope so I do not have a U5 stage yet. I get good audio signals when I look at the output using the elec mic when I speak into it. However when I place it on my skin I cannot pick up the heart beats.

What do u think is the cause of this? I have not used a stethoscope cap or bottle cap, could this be the reason?

Many thks

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Hi Damras,
Welcome to our forum.
To pickup a heartbeat sound, the microphone must not touch the skin, but should be spaced a short distance away. A stethoscope head (or jar lid) provides this spacing, surrounds the skin around the listening location and blocks background sounds. It is important to use one.
The original value of C2 is much too small to pickup low-frequency heartbeat sounds. That is why I recommended to replace it with 4.7uF, with its positive terminal toward the microphone.
The original article also has an error in the schematic: the connection of C3 and R7 should also be connected to U2's pin 6.
If you later add U5, then it should be an LM386 (not a 741 as shown), with its pin 6 connected to +9V, its pin 5 connected to R14 and C6 and its pin 7 not connected. It will have a voltage gain of 20 times.

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