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Electronic Stethoscope


t_ang4
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hi audioguru!!!

long time no hear!!! hehe... :D :D

i just want to ask if the counter in (http://www.aaroncake.net/circuits/counter.htm),
would be compatible to the electronic stethoscope2 when tapped to the bi-color LED.

tnx!!!
have a good day!!!!

---------xoy------ ;D ;D

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Hi Xoy,
That old counter circuit will work if you add another very low frequency lowpass filter so that it receives only heartbeats, I think I discussed it before. You also must use a Schmitt trigger device to produce pulses with rise and fall times much quicker than a 741 opamp can provide. You must rectify the signal and limit its output voltage to 5V so that the TTL counter isn't damaged. You must provide a regulated 5V supply for TTL.

You won't need to rectify the signal, nor use a Schmitt trigger nor limit the output voltage if you use a modern Cmos counter circuit operating on the same 9V supply as the Stethoscope-2 project. Of course you still need an additional very low frequency lowpass filter. Just feed the 741's output through a resistor to the input of the Cmos counter circuit. A suitable Cmos dual BCD counter is a CD4518. Suitable Cmos 7-segment display drivers are CD4511 or CD4543. ;D

http://focus.ti.com/docs/prod/folders/print/cd4518b.html
http://focus.ti.com/docs/prod/folders/print/cd4511b.html
http://focus.ti.com/docs/prod/folders/print/cd4543b.html

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hi audioguru!!

tnx!! for the help....

but at as usual... pls help me again...

i cant find bi-color LED that has only two leads, what i only see is an LED which has 3 leads, it seems its not available here in manila, philippines.

is there any replacement for IC TL072?

i have thought that the kind of capacitors (C3 nad C4) used in the filter is an adjustable.. but it wasn't... is there anything to use which is an adjustable?

Maraming salamat po talaga!!!!!!! (I'm very thankful to you!!!!)

----xoy------

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i cant find bi-color LED that has only two leads, what i only see is an LED which has 3 leads, it seems its not available here in manila, philippines.

is there any replacement for IC TL072?

Use two LEDs connected back-to-back as shown in the schematic.
New Lapan Radio have some low-noise dual opamps. Don't use their 1st copy of the TL072 because it doesn't work and they tried to recall them all.

i have thought that the kind of capacitors (C3 nad C4) used in the filter is an adjustable.. but it wasn't... is there anything to use which is an adjustable?

If you want the lowpass filter to have an adjustable frequency then just use a dual (stereo) pot for R5 and R6. Connect 1k in series with each section so they can't be adjusted to zero. ;D
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  • 2 weeks later...

WOW THANKS AGAIN AUDIOGURU!!!!

YOU HAVE TOLD ME BEFORE TO CHANGE MY 74LS90 TTL BCD COUNTER AND 74LS47 TTL SEVN SEGMENT DISPLAY DRIVER OF MY COUNTER,  TO A Cmos dual BCD counter CD4518 AND Cmos 7-segment display drivers WHICH ARE CD4511 or CD4543.

BUT HOW ABOUT THE 7-SEGMENT LED DISPLAY?? DO I HAVE TO CHANGE IT ALSO TO HAVE A Vcc INPUT OF 9V?? ???
DOES THE RESISTORS NEEDED IN THE COUNTER IS ENOUGH? ???
IF I WOULD CHANGE THOSE IC's, DOES THEIR PIN CONFIGURATION ARE JUST THE SAME TO BEFORE?? ???

THANKS!!!!!

------XOY------- ;D ;D

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  • 2 weeks later...

Hello Audioguru,
  I'm new to this forum (and most forums) so please bear with me. I've built your circuit for the modified (corrected) stethoscope but am having problems similar to Dazza's reply #132.  That being I can hear my heartbeat faintly with the pot turned down low but I can't turn it up very far before I get loud noise then nothing.  I checked the voltage levels you suggested in reply #135 and they are good except for the 3rd check.  Pin 5 of U5 is at 4.3 v when pot is turned down. But as I turn it up it gradually increases as it should until it jumps way high, then bounces all over - oscillating. The circuit is breadboarded and I checked the ground paths as you suggested to keep them separate for the in and out. I suspect I wired my pot wrong. I'm very rusty and unsure of it.  The middle contact (wiper) I connected to pin 3 of U5.  One of the other pot contacts I connected to pin 7 of U1b and the 3rd pot contact I connected to pin 3 of the 741.  My headphones are over-the-ears with 32 ohms.  I'm using a stethoscope head.  Put in a Knowles NR-3160 electret mic into the steth head.  I think I'm close to getting this working correctly but not quite there.  Will you offer some more assistance?  Or are you getting tired of this thread?  :)

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Hi AAronopolis,
I like tis circuit, the modified one should work well.
You have the pot wired incorrectly without ground on one of its terminals.
Pin7 of U1b and pin3 of the 741 are the same piece of wire, the input to the pot's single input terminal when the volume control is at maximum, the pot's slider connects to this terminal. With the volume control at minimum, the pot's slider should connect to ground. ;D

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thanks for the quick response, Audioguru
I made the correction but then there was a slow steady oscillation going on that leads me to believe I made other errors.  So I dismantled it and am in the process of putting it back together with a much cleaner layout.  I had long leads and wires poking out all over the place.  Gotta take my time and do it right.  Soon as its done I'm sure I'll have more questions.
Thanks again!  ;D

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Hi Audioguru,
  well I have a much cleaner looking circuit on my breadboard now. And I removed the optional 741-LED section.  I have the wiper of the pot connected as you described but something is wrong somewhere  :-[.  So I made a pic of my circuit showing the DC voltages hoping you'll help me debug it!  file:///D:/stethoscope/Steth2-aaron.jpg

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Hi Audioguru,  yes - you're right about that diagram.  I made it too quickly. But I didn't wire it according to that diagram.  For building the circuit I used a printout of your diagram (its much more clear) and I made this erroneous diagram after the fact.  I thought it would provide a better visual aide in the debug process if I put the voltages on the diagram itself.  So with that in mind can you tell what might be wrong?  :-\

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Hi Audioguru,  I just posted a long reply with an updated diagram but either the server hasn't had time to update it or it got lost.  Having a rough night here  :P.  Think its time for bed. 
My last comment was the suspicion of an opamp being blown which I see you have just suggested.  I hope that is the case!  I'll check tomorrow to see if my updated diagram was posted.  And I'll check out those opamp pinouts you posted.  Thanks, again.
Talk to you soon.

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Hi Xoy,
The Digital Stopwatch project you posted shows how to use 4511 ICs to drive common-cathode displays. But it counts seconds, not events. You need a counter circuit to count heartbeats and is gated and latched every 6 or 60 seconds.
So you need a circuit like the 4018 and 4511s part of the Digital Stopwatch project to count heartbeats, a circuit to prepare the heartbeat sounds for counting then a timer circuit to latch the display driver every 6 or 60 seconds.
I don't know why you want to use a mechanical relay in this electronic circuit.

The events per minute counter circuit that you need is not in any project in our projects section but there are many circuit sites on the web that would have some. Try http://www.discovercircuits.com/list.htm . 

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Hi Xoy,
You don't need a relay. A simple logic gate or transistor can cutoff the counter.
It would be best to latch the display driver (a high at the latch pin of a CD4511) when the time has elapsed then reset the counter and let it continue counting for the next update.

A 1 minute update on a stethoscope is a long time to wait and to see any change. You could use a 6 seconds timer for a quicker update, then add a zero to the display to show beats in 60 seconds.
You could add a phase-locked-loop with a digital divider to multiply the frequency of the beats per minute, then display them accurately in beats per 0.6 seconds, updated every 0.6 seconds. ;D

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Hey Adioguru - whoo hoo!!  ;D  Finally got this circuit working!
Got new opamps and carefully replicated my wired circuit into a Visio diagram thinking it would help you, help me, find my problems... and that process pointed out a couple of mistakes I missed!
Now I think I need to work on isolating the mic better in the stethoscope head because slight movements of it hurts my ears!
A little background is in order here...  I too am expecting my first child and my wife has been very supportive (and eager) for me to complete it.  She has just started her 3rd trimester.  I can hear my heartbeat easily but I can't hear hers. I'm sure her heartbeat is faster and of course our baby's would be faster yet.  Didn't try long because of the loud friction noises from the steth head but I wonder if I should do as you suggested to Dazza in reply #144?  Is 3.3 uF for C3 and C4 a good place to start like Dazza did or are there better values that you'd suggest?
Thanks again for your patient help!  :)

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Hi Aaron,
It is great to hear that you got it working. ;D ;D
But it is too bad that it passes friction noises.
In my test setup, I used a soft, thick rubber grommet around my electret microphone to isolate it from friction and handling noise conducted from the stethoscope head. I used a very flexible shielded cable to stop noise from being conducted down the cable.
I didn't use a proper head, instead I used a plastic jar lid that made many noises because it wasn't heavy. With a solid heavy head that is used with some lube, you shouldn't hear many friction noises.

The circuit's lowpass filter passes 25Hz to 30Hz heartbeat sounds and their lower harmonics up to 103Hz, and reduces the level of higher frequencies caused by noises. The circuit's cuoff frequency is 103Hz if C3 and C4 are 47nF. If you use 3.3uF then the circuit will have a cutoff frequency of only 1.5Hz and the important heartbeat sounds would barely be heard and lower harmonics would not be heard at all. :(

Be certain that your headphones can reproduce the very low frequency of heartbeat sounds. Some headphones have a boost at 100Hz to give a false effect of bass, but cutoff lower frequencies.

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