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Electronic stethoscope

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microcare

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I have developed an electronic stethoscope using standard circuit given on :
https://www.aaroncake.net/circuits/steth.asp


Now, I am able to the heart pulses on microphone.My problem here is that ,I want to give these pulses to a micro controller AT89c51 for counting purpose. I don't know how to do it as the amplitude of pulses is very less and also it contains noise .

My goal is to achieve NOISE FREE pulses so that they can be fed to micro controller.

Kindly help me regarding this
you can also e-mail me on microcaretech@gmail.com or microcare@hotmail.com


Thanking you
 
noise free pulses....

so the job is to remove the noise.... if u know the frequeny content of that noise, u can build a filter circuit (passive or active) accordingly....

so determine the frequency content and construct a flter....
 
the main problem is

THE PULSES ARE VERY FEEBLE(LESS AMPLITUDE)

Hence, they cant be given directly to Micro controller since sufficient voltage level is not there.
It cannot be even given to transistorized amplifiers as the input is less than 0.7V .
 
the problem is when u amplify them, the noise part of the signal will also get amplified....

u can construct a non-inverting amplifier using high precision op-amps... anyway wats the amplitude level like?
 
I checked it on oscilloscope its less than 0.7v

AMPLIFICATION is required because Microcontroller needs to be fed with the signal
 
thats understood....

anyway with the voltage levels being around 0.7V, i guess by constructing a non-inverting amplifier with a good op-amp, u can amplify the signal to around 5V quite easily and then feed the same to a micro-controller...

if ur signal has a negative part also (like the one u c in sinusoidal signals), then u might have to add an offset....
 
u can also try using a normal microphone pre-amplifier.... construct by using normal bjt transistors and resistor....
 
Thanks for ur suggestion bro, but do u think any transistorized amplifier would work in this case?
I mean even non -inv amplifiers consist of OPAMPS which again employ transistors .Just a doubt !!!!


However, I will surely try ur solution tomorrow itself and let you know .
 
The circuit has a second-order Butterworth lowpass filter that cuts noise above 103Hz.
A lot of noise is caused by friction if the microphone moves.

The gain is low on the LM386 so it doesn't blow your ears off. Its gain can be increased 10 times if you connect a 10uF capacitor between pin 1 and pin 8 as shown on its datasheet. But then the output will be above and below 0V because of the output coupling capacitor. The negative part will destroy your micro-controller. If you connect the micro-controller directly to the output pin 5 of the LM386 then there is no negative voltage but the positive swing will be too high and an attenuator is needed to reduce the max swing to 5V.
 
thank u for your suggestion,the gain improved by 10mf between 1 & 8 of final lm386.But even though wave form appears to b of 5v, the microcontroller 89c2051 does see the change from 0 to 5v ?
slnt:
mov a, #031h ;dsp 1 on lcd
lcall txt ;till wav is low
jnb p3.0,slnt ;
lub:
mov a,#032h ;dsp 2 on lcd
lcall txt ;if thewa goes high
jb p3.0,lub

sjmp slnt

the executin abov dsplays 11111111......
and not 111112222221111 etc
thus proving that the rise is not seen by 89c2051 ?
I have resources to program ATMEL controller only/
Further i tried CD4093 for proper square wave ,the 35Mhz scope shows the square wave but MUC does not see
I am STUCK
 
The LM386 has an output bias voltage at rest of about 4.5V.
The input opamp inverts the signal.
So the output of the LM386 goes to 0.7V for each heartbeat but then might rebound up to 8.3V following each beat.
 
Also using your circuit

Hello,

I also built your circuit, but I didn't get it to work. I excluded the LED flashing section, and I made a modification such that a dpdt switch switches between 47nF and 4.7nF capacitors for easy change btwn heart and respiration sounds.

Is there any reason that the switch would screw up the filter function?

Also...

I didn't have a 2.2-10k log pot so I put a 2.2k in series with a linear 10k pot.
Only thing is I notice in your schematic that the pot is grounded. I haven't grounded mine. Is this necessary? I read 0-10k ohm resistance through the two wires I did use.

Cheers, MikeTheMike
 
I also built your circuit, but I didn't get it to work.

I didn't have a 2.2-10k log pot so I put a 2.2k in series with a linear 10k pot.
Only thing is I notice in your schematic that the pot is grounded. I haven't grounded mine. Is this necessary? I read 0-10k ohm resistance through the two wires I did use.
The volume control can be 2.2k, 5k or 10k. It is an adjustable voltage disvider and it does not need extra resistors. It should be logarithmic, not linear.
When the volume control is turned up to max then its slider connects to the input from U1b. when the volume control is at half then the slider has an attenuated signal from U1b.
When the slider is turned down to ground then there is no signal on the slider.
Of course one terminal of the volume control is grounded. The way yours is connected then the slider will have a very loud signal all the time.

Your switch must change both capacitors in the filter at the same time.
I don't know why your circuit doesn't work. It is a very simplw circuit.
 
Last edited:
I spent a little longer on it, and it works now, but I have a few additional questions.

What is the extent of the expected amplification? are there ways to further increase it?

I have already put the lm386 to 200x gain, and was wondering about the TL072.

Are there any ways which will increase the signal without increasing the noise too much?

Thanks for the fast reply, I appreciate the time you devote to discussing electronics with everyone.

Cheers, MikeTheMike
 
I think you might have something else connected wrong. Maybe the mic is connected backwards. Did you use a 2-wires electret mic? Its case should be connected to the circuit's ground.

The first opamp has a gain of about 10 and the second opamp has a gain of 1.6.
The LM386 power amp has a gain of 20. so the total amount of gain is 320 which is plenty to drive headphones.

If you increase the gain of the LM386 to 200 then it amplifies its own noise and the total gain is 3200 which is too much.

If the noise you hear is a rumbling hiss then use an LM4562 dual opamp instead of the TL072. It is 6.7 times quieter.
 
I must have something hooked up wrong...

I have managed to hear heart sounds with the one set of capacitors, and they disappear when I switch to the second set. So I assume my mod to employ caps for heart or respiration sounds is working.

I have the case of the electret sent to ground, and the signal goes to pin two of the TL072 through a 4.7uF electrolytic and a 2.2k resistor.

It's possible some of my components are junk because I rescue a lot of them from dead electronics.

The problem is that the signal is probably less than 10% above the noise, and with the 386 at 200x the noise is pretty high. I will try dropping it to 20x and see if it helps now.

Cheers, Mike
 
Switching the filter capacitors should not change the amount of sound of heatbeats. It just allows more high frequencies to be added.

What kind of noise do you have? If it is mains hum then the mic must be connected with shielded audio cable.

The frequency of heartbeats is very low. A cheap little speaker or cheap earphones woill not produce it well.
 
Would there be any value in increasing the gain on the second stage of the TL072? From what I've read, your configuration is 'unity gain' which to me sounds like there is no amplification occuring in the second stage, while if you stuck a resistor between 6 and 7 you could introduce gain in the second stage as well as the first.

Could you describe the advantages and disadvantages of introducing gain in the second stage of the TL072 in your circuit?

Bear with me, this is the first time I've actually done any reading about op amps beyond just cursory glances.
 
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