Electronic Stethoscope

[audioguru2]

Hi Emad,
Welcome to our forum and greetings to Egypt.
The negative power supply terminal is not connected to ground. The 0V junction of the positive and negative power supplies connect to the project's "ground" points at many locations on the schematic, including the shield of the cable to the microphone which provides its ground.
If you power the circuit with two 9V batteries, it may not need an external "earth" ground.

 

[prateeksikka]

i simulated this ckt of stethoscope on circuit maker and orcad latest edition .both gave amplitude modulated kind envelope at o/p.please help /comment on this

 

[audioguru2]

Hi Prateek,
The simulator shows an amplitude-modulated kind of envelope because the microphone picks-up the hearbeat sound through the air, and feeds it to the circuit through a capacitor.
This circuit won't produce a DC single-polarity pulse like a DC-coupled ECG. The output of a professional electronic stethoscope was posted earlier. It is again, here:

View attachment 35781

 

[audioguru2]

Hi again,
Another discussion about how a heartbet looks like was here:
http://www.electronics-lab.com/forum/index.php?board=22;action=display;threadid=1274

 

[rg]

hi guys!
we are now having a big problem regarding this e-scope... actually we are building it for the school project but sadly it doesn't produce any output... all that is produced is noise!!!
it triggers us... we are almost a month making it function normally but it definitely doesn't work properly...
i've read that somebody had it functioning correctly... can you please post the diagram that really works so we may be able to build it correctly??? we are asking for your help pls... it would be a great help!!!

thanks!
GODBLESS!!!

 

[audioguru2]

Hi RG,
Welcome to our forum,
You are certainly not the only one with your problem. Just read this post and the one on the author's web-site.
This project's schematic and parts list have errors. A simple dot on the schematic fixes the noise problem (where certain parts should be connected together), and changing U5 to a proper IC type helps too. The detailed fixes are discussed here.
Good luck and let us know how you do.

 

[rg]

hi guys...

can anybody help me plsssss.... :'(

i have only a week to finish doing it, i'm terribly on the rush but still whatever i executed to make it function, i receive no response from the equipment... can anybody please send me your lay-out?? and even corrections with the parts?? ???


thank you so much!!! :-*
you're surely appreciated!

 

[audioguru2]

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.
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.
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.

Hi RG,
Time is running out and you have not given us any details about what you executed.
The above mods should make it work well.
Of course, it is designed for a 2-wire electret type microphone that is connected to the circuit with shielded audio cable.
Recently, people are confused about the batteries connections. The two 9V batteries are connected in series, with their junction connected to the circuit's ground.

 

[audioguru2]

Hi All,
Here's the corrected schematic for this project. ;D
Its Parts List will also be corrected soon.

View attachment 35824

 

[audioguru2]

OOOps,
Pin 3 of the LM386 was deleted and some wires didn't show on the small JPG. I've fixed them here:

View attachment 35826

 

[prateeksikka]

dear sir;
i know that it wont give a perfect d.c voltage at the o/p.but if we see the actual doctor's non electronic stethoscope,there we do not see any a.m wave kind of thing.do you mean to say that doctor actually listensto electrical signal corresponding to an a.m wave?
i have tried both speakers and earphone.when i did not get anything i simulated it on c.r.o and orcadlite software.both gave me this a.m waveform.please comment.
thanx

 

[audioguru2]

Hi Prateek,
I don't know why an electronic stethoscope produces "an AM kind of waveform" which has many cycles of a low frequency. Since the heart produces pressure pulses, I would expect an AC-coupled stethoscope circuit to produce a single half-cycle at the beginning, and the other half-cycle at the end of each heartbeat.
Even if you used a DC-coupled amplifier and a dynamic (coil and magnet) microphone, the microphone won't hold its voltage at the top of each pulse, therefore its output would be the same as AC-coupling.
Perhaps if you used a DC-coupled circuit with an "absolute pressure sensor" that is fast enough, then its output would be the same as a DC-coupled ECG.
An electronic microphone makes a low frequency "boom-boom, boom-boom" sound, and I think that a doctor's mechanical stethoscope makes a "thump-thump, thump-thump" sound, which is similar. Maybe doctors get used to the sound.
Please see my sketch:

 

[Emad1]

Hi--I made the 1st schematic ----it acts well------- i put the pin 4 of U5 to negative voltage ,also i put 2 light diodes in opposite instead of the bipolar diod as i did not find it ------and a variable resistance of 2.2k ------the circuit is working fine -----now I just need a cone to compare it to the real one------thank you for advice

 

[MP1]

Emad, are these changes you made to the original schematic which is posted in our projects section? I am a little confused with all of audiogurus theory comments in here.
Thank you for the update.

MP

 

[Emad1]

yes ,I mean the original schematic-----thank you

 

[audioguru2]

Hi Emad,
That is good that you modified the original circuit to make it work fine. You are clever to change it so that U5 is properly biased and powered. How is its noise level?
Where did you add a 2.2K variable resistance?
What impedance are your earphones?

 

[Emad1]

i have put 2.2k variable risistance instead of the 2.5k variable R simply because i did not find the 2.5k-------I made clicks with my tongue near the mike and i hear it as sharp clicks,so Ithought the circuit is good but it wasnt--------- sounds other than the clicks are not heared-----------Idont know if i made a mistake in the circuit or not as i am a doctor and i make circuits as it is -------------------except for clicks the sound is distorted ------i put the mike in the cone of an old stethoscope but i heared nothing-----I am sorry for giving false impression about the effectiveness of the circuit but I thought it good before trying the cone. Iused mobile headphone. I dont know its impedence. THANX ALOT

 

[audioguru2]

Hi Doctor Emad,
The original project has some errors that you can easily correct on your circuit:
1) Connect pin 6 of U2 to the junction of C3 and R7.
2) Use an LM386 for U5 and swap its pins: pin5 is output and pin6 is +9V. Do not connect pin 7. The LM386 has built-in feedback and biasing to be used with its inputs as shown, and can drive an 8 ohm earphone. Connect pin 4 to ground, not your change to -9V.
3) Change C2 to 4.7uF (the + lead to the mic) so that it can pass a low frequency heartbeat sound.
4) Disconnect the junction of R1 and C1 from +9V and add R15 (1K) from their junction to +9V.
5) Add C7 (470uF to 1000uF) across the +9V battery connection.
6) Add C8 (10uF to 100uF) across the -9V battery connection.

I have corrected the schematic as above and have also removed U3 and R9 which are not needed. They don't affect the sound so your circuit will work well with them.
Later, you can replace the noisy 741 opamps with modern low-noise opamps. My corrected schematic is posted here again:

View attachment 35934

 

[MP1]

Emad, someone else posted the same fix. The other post also claimed that the circuit will work with the -9v change. This circuit "as is" is obviously highly dependent upon the microphone and the earphone used. It is also highly dependent upon shielding your cables well.

MP

 

[audioguru2]

No, I wasn't wrong:
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?

That's right, his circuit "worked", but not very well.