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Electronic Stethoscope


t_ang4
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the voltages at 386 opamps is fine

It has only one LM386 power amp IC which is not an opamp.

but when it reach the tl072 it turns -ve

The output voltages of the TL072 opamps must be zero volts since their input is zero volts.
C2 is shorted or backwards, or the positive power supply pin 8 isn't connected to +ve.

i connected the TL072 through LM386

The schematic shows the mic feeding the opamps which feed the volume control pot which feeds the LM386 power amp. How did you connect the opamps "through" the power amp?

Did you install the LM386 where the TL072 should be? ???
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Hi you all:
Just wanted to tell you that weve tryied the circuit (second version) and it works quite well. We have been able to hear through a speaker the heartbeats. It sounds quite well although it is very noisy so we have to stay still and quiet in order for the circuit to work well... ;D

Ive got a doubt about the circuit...
¿what are the reasons for having a High Pass Filter in U1a of 97 Hz and then a Low Pass Filter (Sallen -Key configuration) of 645 Hz.? wouldn't be better to just put low pass filters? or are trying to do a "band pass" filter?
thats all, thanks for the help in your posts and the time to post a new improved version  ;)

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Hi Fibrauk,
Welcome to our forum. I'm glad that my modfied project works well for you. ;D
The microphone holder called a "head" must be a solid material so it doesn't vibrate nor resonate. The microphone should be mounted to it in a rubber grommet to provide isolation from conducted sound.

The highpass filter made with R1 and R2 in series and C2 has C2 just as a DC-blocking capacitor and have a cutoff frequency of only 2.8Hz to pass the very low frequency heartbeat sounds.
I don't know how you figure 97Hz.

The Sallen-Key Butterworth lowpass filter made with U1b has a cutoff frequency of 103Hz to pass low frequency heartbeat sounds and to reduce background noises. Adding an additional U1b filter would reduce background noises better.
I don't know how you figure 645Hz.

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hi audioguru!!!

just few questions...

the ground and the negative can be shorted when only one battery is supplied or if an adaptor is used..

i want to use an AC adaptor of 9volts to go with the two 9volts batteries.. when the 9volts adaptor supplies, then  the Battery will automatically off or will not supply..

so can you give me please a circuit of it to use? this for the reason to have a back-up supply in case of battery voltage loss in my electronic stethoscope...

tnxxx...
---xoy---

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Hello all, I am new to this forum.  I am in a group doing this project for our electronics class.  Our question is, what values should we expect when we hook up the circuit to the power supply and measure output voltages and gains?

We want to make sure we are doing it correctly to receive the highest grade possible.  Thanks.

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Hi UH,
Welcome to our forum. ;D
We have two Electronic Stethoscope projects, the 1st one doesn't work and the 2nd one does. Which one are you building?

In the 2nd one, U1a has its input capacitively coupled so becomes a DC follower of its reference DC voltage on pin 3. Therefore the DC voltage of its output pin 6 is zero volts.
U1b has the DC voltage of its input from U1a which is zero volts, and has a gain of only 1.6, so the DC voltage of its output pin 6 should also be zero volts.

U4 has a gain of about 71 and will amplify the small offset voltage from U1b and its own offset voltage. Therefore its DC output voltage could be anywhere from about negative 0.7V to positive 0.7V.

U5 is an audio power amp with built-in biasing so its output voltage should be a little less than half the positive supply voltage. ;D 

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We are building the first one, but we connected U5 to -9 [V] instead of ground, and we replaced the potentiometer with a fixed 2000 [Ohm] resistor value. It works, but only when we use speakers for the mic and phone jack. But, when we hook up the mic to it instead, it doesn't work. What isn't supposed to work on the first Stethoscope?  ???

Thanks!

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we connected U5 to -9 [V] instead of ground

Maybe it is damaged. I recommend replacing it.

We replaced the potentiometer with a fixed 2000 [Ohm] resistor value.

Then how can you adjust the volume to match the sensitivity of your microphone and earphones?

It works, but only when we use speakers for the mic and phone jack. But, when we hook up the mic to it instead, it doesn't work.

The circuit is designed for an electret microphone. It is a condenser microphone with a built-in high voltage charge and a FET transistor. R1 in the circuit powers the FET and is its drain's load.
Maybe you are using a 3-wire electret microphone that needs a different circuit or a 2-wire one that is connected backwards.
The circuit might work with a dynamic (coil and magnet) microphone if R1, R16 and C1 removed. Many dynamic microphones sharply cutoff very low frequencies that are heard from a heartbeat.

What isn't supposed to work on the first Stethoscope?

Read this long thread especially the 1st two pages. I explain its problems in my quotes #25 and #27 on page 2. ;D
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The circuit is designed for an electret microphone. It is a condenser microphone with a built-in high voltage charge and a FET transistor.


I've never seen an eletret microphone with a high voltage charge before.
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The metalized film diaphragm of an electet mic is charged to about from 50V to 100V.
As it vibrates by sound, its capacitance changes from a fixed plate and a FET transistor buffers the resulting AC voltage caused by capacitive voltage divider action.

I said it was a high voltage because a condenser mic without a charge needs a power supply of about 48V. ;D

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Hi again,
  I was just curious what kind of voltages are output from the microphone from a heart beat.  I ask because I haven't bought a microphone yet and I just wanted to input a sine wave to make sure the circuit works.  I'd like to know what voltage to input into the circuit.  Thanks for any help you can provide.

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The metalized film diaphragm of an electet mic is charged to about from 50V to 100V.
As it vibrates by sound, its capacitance changes from a fixed plate and a FET transistor buffers the resulting AC voltage caused by capacitive voltage divider action.

I said it was a high voltage because a condenser mic without a charge needs a power supply of about 48V. ;D


No audioguru, this isn't true, I've cut open up one of these devices and it consists of a metal plate electrode with a very thisn film aluminium diaphram (cathode) placed in very close proximity to it, it's separated by a very thin plastic hoop. The diaphram moves under the sound preasure causing the electrode to be exposed to a changing electric feild this induces a small voltage on it which is amplified by the FET.

Don't believe me?
I'll get my digital camera and show you, we've got loads of these devices at work that we'll never use. ;D
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No audioguru, this isn't true

Hi Alun,
What isn't true?
Many articles in Google say that a voltage charge is placed in an electret material in the diaphragm, back plate or front plate. You can't measure the voltage charge because your meter will drain it away.
I have also looked inside them and used the tiny FET transistor in other circuits. ;D
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Don't you mean that the foil is polarised? This is totally differant, when you were talking about a high voltage charge I thought you were meant a miniture inverter inside the mic. ;D

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Yes that's polarization, the molecules line up so the negitive poles  and positive all point in the same direction, this is quite similar to the aluminium oxide molacules on an electrolytic capacitor.

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Hi Xoy,
Sorry, but your circuit won't work.
It has only a 9V positive supply. The project also needs a 9V negative supply. Its opamps won't work with their negative supply pin shorted to ground.

Why do you need an AC adapter?
If the circuit doesn't have the 741 and LEDs, the TL072's negative supply current is only 2.8mA. A little 9V alkaline battery will power it continuously for more than 900 hours, which is 37.5 days.

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  • 2 weeks later...

thanks Alun and Audioguru!!!!!

i have here a very small mic from ericsson cellphone, which is about 3/4 cenitimeter in diameter... i dont know to which unit it belongs.. but it has a long lead and a short one,, which one of them is positive?

thanks...


---xoy----

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Hi again,
  I was just curious what the sensitivity and signal to noise ratio of your microphone was.  I have the circuit built, but I'm having a hard time distinguising between possible heart beats and random noise.  Is there anything I might do to help with this?  I was also wondering if the gains of the circuit are enough to magnify the signal enough to be audible.  From my calculations given a microphone with a sensitivity of -35 dB, I find that heart sounds would only produce outputs at the microphone of around 10 microvolts.  To magnify this to the audible range, it seems like a total gain of 1000 or more would be necessary.  Thanks for any help you can give me.

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