Audioguru's Electronic Stethoscope Help

[jessbussert]

Hi folks,

I'm a half deaf nursing student with a smidgen of electronics knowledge.  I just breadboarded Audioguru's Electronic Stethoscope project and am having a few problems with it.  I originally found the schematic here:

    http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=5923

My problem is that, while I'm getting output, the gain is far too low to be usable.  I've rechecked my connections and components and am pretty sure I've got things all in the right places.  I originally questioned whether or not I was using the right kind of mic, but I think I've answered that question as well.

Can anyone look over the schematic and suggest components that I can replace in order to boost the gain?  Or, alternatively, offer some points on the circuit to test for voltages, resistances, and whatnot to help confirm that I've built the thing properly?

Thanks in advance for the help!
Jessica

 

[audioguru2]

The circuit should have more than enough output level.
Try connecting a 10uF capacitor between pin 1 and pin 8 of the LM386 to increase its gain 10 times. The positive wire of the capacitor to pin 1.

 

[jessbussert]

I had the chance to try your proposed fix last night.  Unfortunately, the addition of the cap only produced a crackly buzz and no other audio.  When I removed the cap, the quiet operation from before resumed.

Any ideas?  (I hope I'm not doing something stupid here...)

Thanks,
Jessica

 

[audioguru2]

If your circuit produces a buzz then maybe it has unshielded long wires on a breadboard that pick up mains hum.
The circuit should be built compactly on a pcb or on stripboard and shielded audio cable must be used to connect to the microphone.

Try the LM386 amplifier with the new 10uF capacitor and the volume control disconnected.
It should not produce a buzz but might produce a little hiss.

Then add the volume control without opamps U1.

Then add opamps U1 without the microphone.

Then you will see where the buzz comes from. 

 

[jessbussert]

When I pull the mic, I still get the crackle.  At first, there is no noise.  Then, after about 3 seconds, the crackle slowly starts back up.  When I pull the pot and/or U1, I get nothing whatsoever.

When I assemble this for the final, I was probably gonna build it dead-bug style just to compact the size as much as possible.  After, I'd probably dip it in epoxy to seal everything up nice and tight.  Any problems with building it like this?

The 10uf cap that you proposed above, if I increase or decrease that component, will I get more amplitude still?  Which direction do I want to go?

Thanks,
Jessica

 

[audioguru2]

If the capacitor added between pin 1 and pin 8 of the LM386 is increased to 100uf then the very low frequency heartbeat sounds will be amplified more  than if 10uf is used.
If 0.1uF is used then the gain is 200 above 1kHz and drops at lower frequencies.

 

[jessbussert]

Following that logic, if I shorted pins 1-8 the output would be the greatest?

On a lark, I tried this.  The gain is ok but not great.

Can you suggest a mod that could bump low volume signals significantly while clipping high volume signals such as when you bump the stethoscope against the table?  Am I asking for too much?

Thanks,
Jessica

 

[audioguru2]

Shorting pins 1 and 8 on the LM386 messes up the DC bias. You are supposed to connect a capacitor between pin 1 and pin 8 to increase the AC gain but not mess up the DC bias.

Then the gain is 200 in the amplifier IC plus the gain of the preamp which is a lot of gain.

You can make an audio compressor by looking for its circuit in google. Usually a FET is used to contol the amount of gain.

 

[harsh0702]

hello........ 
i tried the circuit diagram for the electronic stethoscope and its working too.  the only problem which i am facing is , it is not sensitive enough to detect the heart pulse. only when i tap the stethoscope its sound can be heard in loudspeaker , but it is not able to detect the heart beat.  how to increase its sensitivity? 

anyone pls reply me...

 

[audioguru2]

It is designed to drive headphones, the kind that completely cover your ears.
If you use a loudspeaker then it needs to have more gain which will cause acoustical feedback howling because the microphone will hear the speaker and the sound will go around and around.

EDIT: Where will you find a loudspeaker that can produce the extremely low frequency of a heatbeat?
Good quality headphones sound fine.