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Electronic Stethoscope


t_ang4
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Hi Duke,
I heard my heartbeat loud and clear, too loud. See your doc because maybe you have a weak heart. ;D

Did you build the 2nd project? Did you substitute any parts?
Please describe the random noise you hear. I heard movement of the mic and my dog barking and a phone ringing in the next room with a flimsy plastic jar lid as the mic's holder. I used a rubber grommet so my hand moving on the holder wasn't too loud. It would be much better with a solid cast metal holder which docs call a "head".

Maybe your mic or headphones have poor very low frequency response. Use headphones that surround your ears. Mine picks-up the rumbling of jetplanes at an airport about 20km away. 

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I was also wondering if you might be able to post a picture of the mic and it's holder.  Where is the rubber grommet placed?  The noise I hear is a small intermittant bursts of static.  It's difficult for me to tell whether I'm hearing a heart beat or whether all I'm hearing is noise.  Is the gain of your entire circuit about 10*20 or 200? Thanks.

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Hi Duke,
The total gain of my project is 272. U1a has 8.5 which is more than I thought because the impedance of the mic is a little lower than I thought. U1b has 1.6 and U5 has 20.

I don't know where your bursts of static noise is coming from. On other circuits you could separate the stages but it would be awkward on this one because it is DC coupled. If you hear the noise with the volume control turned down then the noise comes from U5.

Maybe your mic or circuit doesn't work. You should hear your voice loudly if you speak with a normal voice level towards the mic's front with it held at arm's length away. It will sound muffled due to the lowpass filter. Keep the mic away from your headphones to avoid feedback.

My mic came from a cell phone and isn't supposed to have its very good very low frequencies response but it does. I think all ordinary electret mics are the same.

My mic and its rubber jacket are mounted in the center of a plastic jar lid and look exactly like my sketch on the schematic. The rim on the lid makes a space so that the mic isn't blocked by skin.

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Hey, Audioguru,

I have some questions regarding the original electronic stethoscope design
http://www.aaroncake.net/circuits/steth.htm
and the second one that you improved it to
http://www.electronics-lab.com/projects/science/019/

Why do you change the 741 op-amps to the LM386 op-amp and to the TL072 dual op-amp?  The circuit would still work with 741s, correct?  Please explain the need for the dual op-amp, could you, as opposed to 2 741s?

Thanks.

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The 741 is a pretty outdated slow opamp so audioguru decided to replace it with the more modern TL072 but this isn't needed since the bandwidth of this circuit is very low. ;D The idea behind using a dual amplifier is to both save money and space, I bet you could use the LM1458 which is a dual 741 and it wouldn't sound that bad.

Edit:
I do agree about the LM386 though, the old 741 could barely output enough power to drive normal headphones while the LM386 is specifically designed for the job.

Yes, do go for the TL072 or even the TL082 or old obsolete LF315 or LF451 rather than the 741 but it's ok to use it if you're totall stuck at it still will work.

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Hi UH,
The original project had errors that I corrected:
1) The original circuit shows a 741 for U5. A 741 is designed to drive a load of 2k ohms or more. Headphones are 8 ohms to 600 ohms. Two 8 ohm headphone "ears" in parallel are a load of only 4 ohms.
The original U5 is shown with its inputs grounded, and without any negative feedback, but a 741 opamp cannot work like that in this application.

The LM386 IC is not an opamp, it is a power amp designed to drive 8 ohm speakers to 1/2W or less. It is designed to have its inputs grounded and has negative feedback built-in. It is perfect for U5.

2) The original schematic is missing a junction at pin6 of U2 with C3 and R7.

3) I replaced the two noisy single 741 opamps with a low-noise dual TL072 opamp that is much better and costs less for the dual than for a single 741.

4) I increased the value of C2 which was about 100 times too small to pass low frequency heartbeat sounds.

5) I added very important supply bypass capacitors so the project can operate properly with its batteries.


Hi Alun,
I wouldn't use a dual MC1458 nor LM358 dual opamp as the low level microphone preamp in this project. They are much too noisy.

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Hi Alun,
I wouldn't use a dual MC1458 nor LM358 dual opamp as the low level microphone preamp in this project. They are much too noisy.



I never mentioned the LM358.

The 741 isn't too noisy, it's 23nV/root(Hz) and the TL072 isn't that much better at 18nV/root(Hz). The main source of distortion in the 741 is the slew rate rather than anything else and this won't affect things much in this circuit as it's only low bandwidth.
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Noise specs reallly don't mean much in a circuit that gives you a beep beep or a thump thump. Now if the person has swallowed a radio, and you must hear the football game or latest concert, then I agree 100% to the updated op-amps.  Make 2 such circuits for stereo effect.  ;D

Have fun uh3455!

MP

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Noise specs reallly don't mean much in a circuit that gives you a beep beep or a thump thump.

Real doctors are using this project. A doc used to using a mechanical stethoscope with no noise whatsoever, is in for a big surprise if he hears noise from a noisy old opamp.

I don't think docs use a stethoscope to hear if the patient's heart is beating, that is obvious. He is listening to the particular sounds from the heart that would be masked if there was noise from the opamps.
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How much noise can you hear from a TL072?

Even if you doubled it, it wouldn't sound twice as loud due to the ear's logarithmic response - I bet it wouldn't sound any worse than the background hiss on a cassette.

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Real doctors are using this project. A doc used to using a mechanical stethoscope with no noise whatsoever, is in for a big surprise if he hears noise from a noisy old opamp.


The low pass you have put on the front end of your revised circuit kills any noise specs that might show up. Noise is found in the higher frequency ranges. That is why many music noise cutting circuits employ low pass features. Many noise gates are high pass cut circuits and noise removal on analog bucket brigade delay lines are sharp cut low pass filters. High frequencies are easy to amplify. Low frequencies are not. You would have to highly amplify low frequencies to make them more noticable than the amplified sound signal.

Real Medical Doctors are using much more expensive equipment than this project......why would they bother with getting out a solder iron and making something like this? As far as I understand, Doctors spend their off-time playing golf. Not making op-amp circuits.

Oh, by the way....These circuits are for fun...the manufacturer of these parts does not allow for their use in medical equipment. You are supposed to purchase special ICs for this purpose. The manufacturer has a medical disclaimer on all their components except the ones which go through more rigorous testing and which are deemed as components worthy of medical use. If you see a doctor using one of these home made devices, he is violating some basic rules......run! He is not a qualified doctor.

MP
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The low pass you have put on the front end of your revised circuit ...

I didn't add the lowpass filter in this project. I just corrected the original circuit's filter that was missing a solder joint on its schematic. The author states in the text that his lowpass filter is used to remove background sounds picked-up by the microphone.

Noise is found in the higher frequency ranges.

Not true. The noise from opamps increases at lower frequencies used by this project. Manufacturers describe the "burst" noise as "popcorn" and "shot" noises which would distract and mask sounds of the heart.

Real Medical Doctors are using much more expensive equipment than this project

A few real doctors have enquired about this project and have built it and use it successfully using the recommended low-noise opamps.

the manufacturer of these parts does not allow for their use in medical equipment.

The disclaimer avoids law-suits if their part fails and causes a wrong diagnosis: "The heart stopped, give it a jolt! No it didn't, the opamp stopped".
;D

post-1706-14279142491687_thumb.png

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  • 4 weeks later...

Hi Xoy,
Mylar is a plastic film. A mylar capacitor is a metalized plastic film one. It is fine.
I spec'd metallized plastic film caps so people wouldn't use ceramic caps that have a very wide tolerance (+80%, -20%). My European plastic caps have a 5% tolerance. ;D

You could use Oriental "green caps" (I've also seen shiny reddish-brown ones) which are metallized plastic film, but if they are marked "M" then they are 20%. K is 10% and J is 5%.

A European metallized plastic film cap with a value of 0.047uF is marked 47nF. Oriental ones are marked 473.

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