Electronic Stethoscope

[audioguru2]

Cdak,
I am glad that you got your LM387 (obsolete) pre-amp working so well with an external 7th-order HPF used to reduce 50Hz hum pickup. Have you considered a simple notch-filter?
Are you still using the 9th order LPF that you mentioned earlier?
What frequency does the LPF cut-off at?
Do you think that an available TL071 opamp will work as a preamp here?

 

[alhilaly]

Hi bros.
I need a description about this circuit and how it works?
becouse ihave this cicuit as apart of graduation project as apre amp by using 741 op- amp to transmitte the voice signal and hert sound then receive it at(88-108Mhz) can you help me?

 

[audioguru2]

Alhilaly,
As you can see the many replies about this project, the schematic and parts list have errors. Until the errors are corrected it is awkward to give you a detailed circuit description.

 

[CDAK1]

MP,
I haven't yet started further work in the project. It may take a few weeks to restart. (I finished my course in the current institution and may go back start working in another environment. So it may take few weeks to come back to the project again.) I shall inform every step in the enhancement work. The first thing I would try will be ur expander circuit. Then a notch filter ( audioguru also suggested it, thanks). I had this notch filter idea, but since I had a long signal processing left in that work, I left the hardware there.
Reagarding the HPF, I haven't used any amplification and even without that it worked well in MATLAB ( .wav format keeps the value between +1 and -1).
Audioguru, I don't know why it is obsolet if somebody uses an available chip ( atleast it was available to me)...
CDAK

 

[MP1]

It is still available from Digikey.

MP

 

[MP1]

LM387 is still available through digikey. I have not tried other sources, but I am willing to bet that they are also available through other vendors. Also, many parts which go obsolete in the US and Canada are still widely available in other countries. Obsolescence of a part is many times dependent upon profit margin of a vendor.

I see two changes made here by CDAK:
1. HP filter added, but CDAK states that the signal was not very different from it.
2. Pre-amp was not used.

If this is so, I do not see why the circuit in it's original form would need to be replaced. Perhaps a second circuit which is different.

MP

 

[audioguru2]

MP,
Digikey won't sell me an LM387 (quantity available = 0) and neither will Newark Inone. Why would anyone want to use it now, without any manufacturer second-sourcing it and with its non-standard pin-out?
There is nothing special about it in this circuit, a standard low-noise opamp is fine instead. Cdak used it only because he had some.
Answering your last post:
1) His external HP filter made a BIG difference because it attenuated hum pickup, as is shown in his graphs.
2) He didn't use a preamp? Of course he did, he used his LM387.

I mentioned replacing the original circuit because it has errors (which are easily corrected) and therefore does not work. Nobody is correcting it.

 

[MP1]

audioguru, there are different ways to look at this. From a manufacturer's point of view, yes, you would not build a product with questionable parts availability. However, a hobbyist would have no problem with it for a one up circuit. As CDAK has pointed out, in his application, he had availability to several.
I took his previous comment that he did not use amplification to mean that he did not use the preamp ckt. As I read it again, I think he means he did not use post amplification.

Digikey will not sell the LM387N-ND but they do still offer the LM387AN-ND pin for pin compatible last time I checked.
Partminer gives me an availability of just under 400 pieces.
Dial Electronics in UK has both versions LM387N and AN with different prices. I do not know how many they have.
I am sure a serious search will bring up many possibilities.

MP

 

[audioguru2]

MP,
Never mind the special LM387 "phonograph preamp", how about using a standard, available and low noise opamp?

How do we get the project corrected?

 

[MP1]

someone builds it, fixes the part that does not work, posts the fix. This has been done with other projects such as the voltmeter project.

MP

 

[audioguru2]

Cdak,
The electret microphone preamp circuits that I build don't pick-up hum, so why does yours? No shielded cable?

 

[audioguru2]

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)

 

[shiva garg1]

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.

 

[MP1]

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

 

[audioguru2]

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.

 

[MP1]

audioguru, CDAK made the project work without making this list of changes. I hope that we can hear from him soon.

MP

 

[audioguru2]

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?

 

[MP1]

...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

 

[shiva garg1]

Thank you audioguru for your corrections.I will try to implement them,but i do not understand why this circuit work well on computer.

 

[audioguru2]

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.