Electronic Stethoscope

[t_ang4]

I am very interested to build the electronic stethoscope found in this website under 'Scientific'. However, I have quite a number of difficulties.

1. Is the connection of the U2 correct? I mean I was not sure if there should be a feedback from the output of U2(pin 6). Since all the op-amp has its own feedback loop, why isn't there a feeback loop at the output of op amp U2.

2. What is a Bi-color LED? I have bought it and it has 3 leads. How do I connect it ?

3. How do I connect a phone jack? Could I connect it to the speaker or ear phone? How to connect them properly?

Any help will be greatly appreciated. Thank you in advance.

Project Link: http://www.electronics-lab.com/projects/science/014/

[t_ang4]

This is the circuit:

[mixos]

1. I think the connection is ok. There is no need to use a feedback loop in every opamp. There are many types of connections that can be used with opamps.

2. A Bi-color LED is a led that lights two colors. There are two types: with three or two leads. Here you need one with 2 leads. When the current flows in one direction the led lights the first color and when the currents reverses the second color lights.

3. Insert in the female jack (connected to circuit) a male jack and with a contuinity tester (ohmmeter at lowest scale) measure the leads to see which one of the male jack connects with the females one. When this clears out.. you can connect the correct leads to the circuit so if you insert a ear phone it is connected to the circuit. You don't have to care much about polatiry.

You may use a speaker if the amplification is sufficient. Just give it a try.

When you finish it .. tell as your experiences here..

[aichaila]

This project is kind of interesting. But I would like to ask.

What is the formula for each op-amp?
What does each opo-amp function as?

I ask this for some of the configurations of  the op- amp, I have not seen before. And what is the LED for?

[t_ang4]

Thank you for the kind reply, Mixos. Since I could only construct this circuit weekly, I could only tell about the problems i faced weekly. I feel that the circuit is progressing slowly.

1. Thanks to Mixos guidance, I am able to get an audible output by fixing the phone jack correctly. I could hear the sound from my earphone! However, the sound that i get from my earphone is a little faint. And also there is a lot of background noise (like an out of tuned tv) For your information, I have replaced the 2.5k pot to a 5k pot as the former is not available in my home country. Is this the cause of the not-so-loud sound? (I haven't fixed the stethoscope yet, as it is hard to find here.)

2. May I know why R14 has a very small resistance value of 3.9 ohm ? What will happen if I increase it to a higher value?

I will definitely share my experiences here. Thanks in advance!

[mozikluv]

 :)hi t_ang4,

Question 1: formula for op amp
Ans: none due to lack of info like amp gain

Question 2: funtion of each op amp
Ans: A1 - microphone preamp
       A2 - acts as buffer amp or isolating amp (isolates the input signal to load)
       A3 - buffer amp or unity gain amp (notice there is no Rf only direct connection)
       A4 - LED driver
       A5 - power amp

Question 3: R14 - 3.9ohms, what will happen if you increase the value.
Ans. Would like to give you an answer, but would rather let you experiment and see what happens. As a hint R14 works together with C5 to stabilize the output stage and to prevent oscillation.

Question 4: whats the LED for
Ans: since this is a stet used for monitoring heartbeat, the 1st led indicates the thumping of the heart and the 2nd led indicates the thomping of the heart. notice the spelling of the word thump & thomp. got what i mean?

Question 5: you are hearing noise from your headphone
Ans: what you hear is probably background noises or rf noise

re Q5, if it is rf noise your line from the output jack could be very long which can act as antenna or your line from the mike to the board is also very long.

Am quite interested with the outcome of your project, coz i also have designed an electronic stet but my parts count are low and i use it to detect engine noise.

happy experiment

[aichaila]

I have already fixed this circuit onto a vero board. But I get some noise when I try it out. I followed the advice of mozikluz, and thought it might be the wire of the phone jack that might be too long or the wire of the mic. I have shorted the wires and I still get the noise.Could it be an internal noise?

And what is the purpose of the C1 capacitor?

I will tell you what I do as clearly as I can. When I connect the bateries to the circuit, the LEDs will flicker on and off so fast that it is like it is on all the time. Then when I listen to the output( the telephone jack pluged with an ear phone), all I get is noise. When I blow at the mic, there is a sudden change of noise that I can hear. Which maybe means that it is working, right?

This is my experince.

[Staigen]

I belive there is a little drawing error at U2. C3 and R7 shall be connected to pin 6 on U2.

[t_ang4]

Hey, achaila! That was exactly what I got also from your description of those problems.

Thanks also to mozikluv help that I managed to reduced the annoying external noise a little bit, by reducing the length of the wires connecting the MIC and phone jack. However there is still lots of background noise.

As a novice in electronics, I always doubt myself. Regarding op amp U2, I found it to be similar as a "second order low pass filter". Though I am not very sure because according to mozikluv post, he said that it is a buffer amp... I have no one to turn to because I am not an electronics student - I am just doing it as a hobby. I love the many interesting circuit in this site.

My question is almost similar to achaila's one : How could I further reduce the noise heard from my ear phone, and also increase the volume further (though volume is maximum, the signal to my ear phone is still a little faint)?

[aichaila]

Hey, guess what? I did some poking around in my circuit and I finally made the circuit work   Even though I do not understand why but  when I changed the polarity of the mic, the noise and the flicker was gone and now it works perfectly.

I would like to thank everybody that helped me with this and mostly to this wonderful site for giving me the circuit to begin with. Thanks everybody.

[mozikluv]

 :)hello aichaila & t_ang4 & Staigen

Aichaila, the C1 is your ripple filter for your power line

Staigen, there is nothing wrong with the circuit diagram concerning C3 & R7 & R8. I have a question for you to analyze, from U1 output, where do you think does the signal go, does it go to the non-inverting input or inverting input

T_ang4, nice observation, what do you think is the main function of the gadget? So what do you enhance?

What kind of microphones are you using, is it the electet condenser microphone (EM-60 or EM-80)

Aichaila, now that you have made your project work, where are you gonna use it, just curious As an added idea, you can tinker around with the input of U2, you can experiment with different values for R & C, see what you'll have

[Staigen]

Hi there Mozikluv, if C3/R7 is connected to pin 6 of U2 the signal goes to the +(noninverting) input of U2 and the amp acts as a second order lowpass filter (like it says i the text), and if C3/R7 is not connected to pin 6 then i belive its a allpass filter with a little dip at the resonance frequency and some reduced output in the high passband! There is also somthing else that i dont like with this schematic, regarding the U5 opamp, it is not properly biased.

[aichaila]

Hello mozikluv,  I actually do not know what to do with the circuit yet but I think I will find something to do with it   I am just doing it as a hobby. I am quite interested in electronics.

As for the U2, I did some tinkering on it when my circuit was not working. Like, I connected the C3 and R7 to pin 6. What I got was that there was no output sound in the phonejack but there was light coming out of the LED(when I made a sound to the stethoscope )  And I will try out you suggestion of changing the value of C and R.

Hello staigen, which one is not properly biased? Do you mean the pin 4? I have not tried to connect the pin 4 to -9 v yet, but when I do, I will post it here( just to know what happens when I do )

[t_ang4]

 :)Thank you very much to all who replied to this forum. Though I am very slow to complete this apparently simple project, I still failed to make it work. I have tried achaila's method of inverting the polarity of the MIC (I do not know what model it is... it is a tiny one around 5mm in diameter), but still it doesn't work well.

One thing I can say is that I have fixed all the components correctly. The only thing I replaced is the Bi-polar LED with a normal LED. and i dont think it will affect the circuit considerably.

Maybe it's the way I construct the circuit.... maybe it's wrong... but I couldn't find any mistakes... thanx a lot!

Anyway, I really appreciated all the help and advice. I've learnt many valuable things in the process.
and also I will be constructing many other circuits in this website!