Electronic Stethoscope

[audioguru2]

Do your headphones sound like the low frequencies are missing like your recording does?
I made part of the circuit and my headphones made my heart sound like thump-thump, thump-thump etc, with a very low frequency.

 

[f3x]

Hi ... I have exatcly two guess to make on that topic
1st (less likely ) is the quality of the microphone


2nd ... you have inversed R6 and C4 or C3 and R5 (or R7)
making it a highpass filter that cut at 100hz instead of a lowpass filter that keep 0-100hz
Pls make sure your capacitor are at the rigth place.

I can also see you are using one led instead of two or a dual ... coud this have any impact ?


--------
to audioguru:
finally about the input voltage, i have realised that two 9V battery is quite big for the "portable" project we are trying to make. So my guess is that i'll be ok with my current pieces and draining +- 4.5 V from a 9V battery. at the worst cases the 9V will get to 6V wich is +- 3V wich is still acceptable.

 

[f3x]

Hmm, i have one question

What do you mean by the TL074 behaving badly when the voltage dropdown to 6V ? Because i have to find some research question on the circuit, and the (Noise/distortion/whatever happen) Vs input voltage seem an interesting subject.

Or i can just test experimentally the curve Voltage/current vs input frequency wich is a bit less interesting.

Or maybee i can analyse what happen arround the LM386 without your "rectifier"

 

[audioguru2]

What do you mean by the TL074 behaving badly when the voltage dropdown to 6V ?
See the sales sheet from TI. They rate the minimum supply voltage at 7V.


The circuit doesn't have a rectifier. The LM386 is a power amp for driving low impedance headphones.

View attachment 38613

 

[f3x]

from reading the small footnotes and reading other TI datasheet, i am under the impression that 7V means (Vcc+) at 7V and (Vcc-) at ground or Vcc at +- 3.5 v


how long does it take to take a 9V battery to 6V ?

 

[audioguru2]

from reading the small footnotes and reading other TI datasheet, i am under the impression that 7V means (Vcc+) at 7V and (Vcc-) at ground or Vcc at +- 3.5 v

Correct.

how long does it take to take a 9V battery to 6V ?

Energizer's site used to have more detail about their batteries but still show how quickly the six AAAA cells inside a little 9V alkaline cell drop in voltage with current.View attachment 38616

 

[xoy]

HI EVERYONE!!!

JUST ASKING ONLY... HOW DOES THE CUT-OFF FREQUENCY OF 20-30HZ AT THE FILTER WAS COMPUTED??? CAN ANYONE PLEASE PROVIDE EQUATIONS FOR IT... THANKS....



-------------XOY---------------

 

[audioguru2]

JUST ASKING ONLY... HOW DOES THE CUT-OFF FREQUENCY OF 20-30HZ AT THE FILTER WAS COMPUTED??? CAN ANYONE PLEASE PROVIDE EQUATIONS FOR IT

The -3dB (half-power) highpass cutoff frequency of a capacitor and a series resistor is at the frequency where the reactance of the capacitor equals the value of the resistor.

C1 is in series with R2, which is also in series with the impedance of the mic (about 5k) in parallel with R1. So the total resistance is 5.53k and the 4.7uF of C1 equals it at 6.2Hz.

The only other series capacitor in the audio circuit is C6 which is in series with the impedance of the headphones. For two 32 ohm headphones in parallel making 16 ohms, the reactance of C6 equals 16 ohms at 10Hz.

The combination of 6.2Hz and 10Hz creates a droopy cutoff frequency of about 16Hz.

 

[mayzry81]

Hello Audioguru .!!!!!!! Long time no see you...coz i'm bz...thanks for all info in this forum..i successful done it..!!!! but one problem..please give me the comparison between the experimental and theoretical result of this electronic stethoscope...Urgent !!!! thanks

 

[audioguru2]

Hi May,
I think that the low frequency response and seal of the headphones determines the experimental result of this project. I tried pretty-good cover-the-ears headphones and heard my heartbeat loud and clear. Then I tried a pretty-good speaker and after turning down the volume to avoid acoustical feedback howling, I couldn't hear my heartbeat.

 

[TriGeminal]

My clear Heart sound  ;D
I got a semi-shielded mic + pluged the audio jack into my computer .. then recored by jetaudio program ..

The air noise (some bad noise like air noise) are still mysterious!! I think I've a problem in something at the breadboard, that's why I'm soldering another one

Those noise can be heard even without connecting a mic!

 

[f3x]

Yes, i had this question.
Can you really plug the output of the lm386 to Microphone input
Or you need to go rigth before the volume control to do that ?

Your noise sound like aluminium paper being folded near the mic.
I hope this isnt the pop and crack that is tipical to opamp audioguru last described

Good luck in your project

 

[audioguru2]

My clear Heart sound ;D

It sounds pretty good except for the extra noises.

The air noise (some bad noise like air noise) are still mysterious!! I think I've a problem in something at the breadboard, that's why I'm soldering another one

Those noise can be heard even without connecting a mic!

I don't hear much hiss (air noise?) in your recording. The extra noises are friction of the mic holder on the skin, or friction of the cable moving. It could also be noises caused by loose connections on your breadboard.

 

[audioguru2]

Can you really plug the output of the lm386 to Microphone input
Or you need to go rigth before the volume control to do that ?

The output level from the output of the LM386 is about 200 times too high for a mic input!
Right before the volume control is about 10 times too high for a mic input.
These signal levels are speaker level and line level, not mic level. An attenuator is needed to connect them to a mic input.

Your noise sound like aluminium paper being folded near the mic.
I hope this isnt the pop and crack that is tipical to opamp audioguru last described

The noises are from friction. Opamp noises are much lower in volume.

 

[f3x]

As you said....
I need an attenuator to reinject chipcorder output to the LM386
I need an attenuator to (optionnally) plug the output of the electronic sthetoscope to computer mic input

Both need to be somwhere arround the volume control potentiometter.
I somehow fear that there will be too much attenuator in that region
Do you need something special to prevent the normal flow from leaking in all those attenuator ?
Op-amp buffer ?

The last thing i want would be using an hardware switch to choose between mode:
- chipcorder playback
- realtime hearing
- realtime hearing + mic output


btw can the volume control be after the lm386 ?
So you can merge the chipcorder output and the lm386 output and after that, control volume ?

 

[audioguru2]

As you said....
I need an attenuator to reinject chipcorder output to the LM386
I need an attenuator to (optionnally) plug the output of the electronic sthetoscope to computer mic input

Both need to be somwhere arround the volume control potentiometter.
I somehow fear that there will be too much attenuator in that region
Do you need something special to prevent the normal flow from leaking in all those attenuator ?
Op-amp buffer ?

To avoid shorting outputs together and avoid a switch, you need an audio mixer circuit.

The last thing i want would be using an hardware switch to choose between mode:
- chipcorder playback
- realtime hearing
- realtime hearing + mic output

You can use electronic switches like Cmos transmission gates, or an audio mixer circuit.

can the volume control be after the lm386 ?
So you can merge the chipcorder output and the lm386 output and after that, control volume ?

In order to keep the LM386 and a volume control from overloading, the volume control is always ahead of the amplifier.

 

[f3x]

In order to keep the LM386 and a volume control from overloading, the volume control is always ahead of the amplifier.

I undestand an opamp can overload... but how can a potentiometer overload ? does it have to do with headphone impedance ? Can i use a preset voltage divider before the lm386 then a potentiometer to reduce a bit the gain ?

Cmos transmission gates

What are those ? is this a way to setup transistor ?


I've seen audio mixer .. those are opamp used in addition mode rigth ?
I really do not want to go in that route... the current setup is already complicated.
I'd rather use K.I.S.S. philosophy.
All i want is preventing mylself from damaging the expensive chipcorder or the computer soundcard.

[edit] Just seen this Fet mixer project
http://www.electronics-lab.com/projects/audio/034/index.html

so basicly all i need is to add 2N3819 Junction FET
and one capacitor / resistance ?

 

[audioguru2]

A volume cotrol rated at 1/2W might be turned down only 10%. Then 1/10th of it would have the output current in it which could easily burn it.

A Cmos transmission gate is an electronic switch. There are many different types.

You could use that FET audio mixer if you want.

 

[f3x]

Just seen at digikey that this particular model is in sold in package of 2000+ !!
anything else to recomand ?

 

[audioguru2]

Just seen at digikey that this particular model is in sold in package of 2000+ !!
anything else to recomand ?

I guess Digikey don't sell many jFETs.

Newarkinone sell the 2N3819 in single quantities for only $.166US today.