Hello,

My goal is to design electronic stethoscope (the mos simple one) and i have big problem. I was thinking about using about microphone connected to circuit with audio amplifier and on output i have headphones. It's gonna work? and could anyone give some hints how to design this circuit ?

Regards.

 

My goal is to design electronic stethoscope (the mos simple one) and i have big problem.

Hi. I think this one is almost that you need. I hope will be good for you. Regards from Argentina.

P.S.: the web is in spanish, but I think is easy to understand the circuit.
http://www.hispavila.com/3ds/tutores/estetoscopio.htm

 

ooo thanks so much btw. now i am on student's exchange in spain, so spanish is not a problem .. ;]

 

They copied my circuit but used noisy old opamps:

 

They copied my circuit but used noisy old opamps:

EDIT:
Their date is before mine but I never saw theirs until now. So we both fixed the horrible original project a www.aaroncake.net .
They made a few changes that I should have made.

 

hey, i have few questions concerning this design:
1. Why we use capacitor C1?
2. in amplifier U2 why we construct this feedback formed by C4 and R7?
3. what does C5 and R8 are doing ? (can i use as C5 two normal condensators - not eletrolytic?)
4.R11 and C7 form next filter?
5.what elements : C10, C9,R14, C11 are making ?

Thanks in advance for the answer

 

hey, i have few questions concerning this Spanish design:
1. Why we use capacitor C1?

It filters the +9V supply to prevent the +9V from jumping up and down when the LM386 power amplifier uses a fairly high current to feed a speaker or headphones.

2. in amplifier U2 why we construct this feedback formed by C4 and R7?

U2 is a 2nd-order inverting lowpass filter.

3. what does C5 and R8 are doing ? (can i use as C5 two normal condensators - not eletrolytic?)

C5 uses two 1.2uf polarized capacitors in series and back-to-back to make a non-polarized 0.6uf capacitor. The capacitors are not needed and can be removed.
U3, R8 and R10 do nothing and can also be removed. The volume control R11 should be 10k and C7 should be removed.

5.what elements : C10, C9,R14, C11 are making?

C11 blocks the DC at the output of the power amplifier U5 from burning out the speaker or headphones. The other parts prevent the LM386 power amplifier from oscillating at a high frequency as shown on its datasheet.

 

so we can simplify this SPANISH design to this form ?

and the last question i hope what is the job of C6 and C8 ?

 

so we can simplify this SPANISH design to this form?

You used two very noisy old LM324 quad opamps wasting 6 opamps and needing 28 pins.
I used a single low noise dual opamp that has only 8 pins.

Your opamps have a total gain of only 2, then you add C6 to the LM386 power amp to boost its gain 10 times so it amplifies its own noise plus the noise from your old opamps.
I used gain in both opamps so that the LM386 power amp did not need to have its gain boosted then the noise level is low.

I think the second opamp in the original Spanish circuit is wrong. Yours is a wrong copy since theirs inverts and yours is non-inverting. I have only made non-inverting Sallen and Key second-order Butterworth lowpass filters. I have never seen inverting second-order filters like the Spanish circuit. Yours has the values of its capacitors wrong (C4 should have double the value of C3) so your circuit might oscillate.

what is the job of C6 and C8 ?

They are explained in the datasheet for the LM386.

 

i read your comment, then traveled along internet to find some solutions and i changes my design to some kind of mix of both :

values in low-pass filter are calculated for cutoff frecuency of value 150 Hz , with this value my results will be satisfactory? or should i choose another value of frequency? And one more question is about total gain. Now it has 10 times higher value than in your design , isn't it a problem ?

in advance and as for now.. thank you very much for your help

 

Read my comments in the sketch:

 

i hope verything is corrected... and those two amplifiers will be TL074...

 

C6 causes the LM386 power amp to have 10 times more gain so it amplifies its own noise 10 times more. For less noise, remove C6 and add more gain to the first opamp (its gain now is only 3.1). If the value of R2 is reduced then the first opamp will have more AC and DC gain. You don't want it to amplify its own input offset voltage so R2 should have a capacitor in series to ground.

The first opamp will not work without a resistor from its input to ground so that its input is at 0VDC. Then it can have a coupling capacitor from the mic that needs to be powered.

The 100uf value of the output capacitor is too low if you are driving an 8 ohm speaker or 8 ohm headphones.

 

hey its a nice idea to build this circuit. but one thing you need to confirm is that you catch up all the sounds. I am not an electronics expert, but a medical professional. so you need to pick up all the sounds. it includes the beats and flow of blood through veins and arteries. moreover the scope is used to check lungs and back. see for uses of it, and all the medical aspects. yours would be a boon to the medical field. all d best.

 

The lowpass filter cuts off frequencies above 100Hz which are the blood flowing and respiratory sounds. If the cutoff frequency of the filter is increased 10 or 20 times then these sounds (and background sounds) will be heard.

 

i think i corrected design following your hints..

 

Your preamp opamp needs more gain and needs an input capacitor.
R14 is 10 ohms (not 3.9 ohms) on the datasheet for the LM386 power amplifier.
You are missing a 100uf positive supply bypass capacitor in parallel with C10.
The negative supply needs a 0.1uF supply bypass capacitor.

 

I really appreciate your hints and your help. thank you very much. Can you have a look on final design with all the elements?

 

The mic needs a resistor as a filter and the LM386 power amplifier has no power supply.

 

ahh yes now it's ok?