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. :D I've learnt many valuable things in the process. and also I will be constructing many other circuits in this website!

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)?

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!

This is the circuit:

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/