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Electronic Stethoscope


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This is only one problem of many with the original project.
There is no way a 741 opamp can work in this circuit without a negative supply for its pin 4 that some builders added after hearing about it.
There is also no way a 741 can provide enough current to drive an 8 ohm earphone or two paralleled ones.
There is also no way this 741 can amplify audio properly without any negative feedback. Its gain would be 5000 times too much.


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Hi Dazza,
It is wonderful to hear that you are going to be a daddy!  :o  ;D

The LM386 1/2W amp has been very popular over here. It is used in nearly every small radio or tape player, and wireless baby monitor that I have ever seen. I don't know what brands you can buy down under, but Philips and Motorola (now their consumer analog ICs are made by ON Semi) have suitable small bridged amps. Bridged amps don't need that big ugly output coupling cap:

Its gain is fixed at 100 but can be reduced.
Its gain is determined by two external resistors.

Motorola (ON Semi) Its gain is also determined by two external resistors.

The TL072 dual low-noise opamp is also very popular over here and is used in almost every consumer product that has high quality audio, and nearly everything I make. There must be many alternatives but about the only one I think about uses triple the supply current and is also made by many companies (Philips invented it but don't make it anymore):

Happy thumping!  ;D

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Hi audioguru,

I have a listing in my Jaycar catalogue for a (LM386N-1) low voltage 1 watt amplifier.  And I just found in my junk box an (TL072) :D.

While I am away, I'm hoping to get the chants to visit a Jaycar electronics shop, there usually very knowledgeable,  unlike Dick Smith >:(, so hopefully they can offer me some solutions/alternative components for both my projects :).

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Hi Bugzy,
Welcome to our forum.  ;D
Are you making the original Electronic Stethoscope project or the corrected and improved Electronic Stethoscope-2 project?
Most electret microphones have very good frequency response of from about 30Hz to about 15KHz and the response is fairly flat.
Whoever makes your EM-80 mic should have its spec's on their website.  ;D

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I've completed this project, electronic Stethoscope 2 :).  Either I'm expecting too much from this project :(, or more than likely I've made an error some where :-\.

I can hear a "very vary" faint but clear heartbeat, when I adjust the 10K LOG potentiometer to around its minimum, say less than a quarter of its wipe,any more than that and I get crackling coming through the headphones, also when I first applied power I heard nothing coming through the headphones until I adjusted the potentiometer to about half and then I heard motor boating through the headphones, I applied power again about an hour later and this time it worked.  I have experimented with different stethoscope heads and mikes, with very little improvement :(.

I use this PCB that someone posted and found that R8 and R7 were incorrect and needed to be swaped around, also I discovered that the positive and negative terminal connections are a dead short, so I connected them to negative of C8 and positive of C7, of course not before I flattened two 9V batteries ::) ;D, hopefully I haven't damaged one of the ICs, could this be why it's not working as well as I expected ???.

Now that I'm low on 9V batteries ;D, how do I power this project from my power supply, can I use two 9V regulators, how do I configure them ???.


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Hi Dazza,
I'm sorry I didn't make a pcb for this project.  :'(
It's too bad the pcb shorts flattened 2 batteries and I see where its R7 and R8 are swapped.
I don't think the IC's are damaged by the batteries being shorted.

I think that the LM386 as U5 is oscillating when you turn up the volume. Its ground pin 4 and the load's ground are on the same pcb path as the input ground. U5 and the output ground should be on one path, and the input, preamp parts and the volume control should have their own path, to the junction of C7 and C8.

Also, U5's unused gain pin 8 is very close to its output pin 5's pcb path and R14. When not used, pin 8 can be considered as a positive AC input and at very high frequencies, around and around she goes. Try lifting pin 8 away from the board.

Did you use a ceramic disc cap for C5? It is important because the LM386 has a frequency response that extends out to 300KHz. At such a high frequency, the inductance of other types of caps, headphones or a speaker cause them to be a very high impedance.

You can use a power supply that doesn't need regulation, that is somewhere around positive and negative 6V to 12V.

The you can hear your unborn child, congrat's!  ;D

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Thanks for your reply audioguru :),

Yes, I did use a ceramic disc cap for C5. I had an idea that track placement would be important for this project, that's why I used the PCB pattern that a member posted, as I had no idea on correct track placement to design my own PCB, maybe I should give it a go, I will have to start by redrawing the schematics as I find it difficult to follow the way it is drawn, then I will need to post it for inspection to be sure I got it right, before having a go at designing a PCB, Mr Audioguru Buddy old Powell ;D, can you inspect my redrawn schematics once it is done and then the many attempts that I'm likely to make, at designing a decent PCB for this project ???.

Can you tell me which track placement are important, as I said I have no idea but I'm willing to have a go :), I'm sure others would appreciate a PCB for this project :).

This is the first time my partner has taken an interest in one of my electronic projects, she spent ages with it trying to hear our unborn child's heartbeat, even though it was barely working ;D.  At our last checkup at the hospital they used a small handheld electronic stethoscope to listen to the babies heartbeat, it was just amazing to hear our babies heartbeat and that was what made me keen on making my own electronic stethoscope :D.

This will be our second child, our first is nine now our daughter, and it really is no less exciting then the first 8).

My partner had some very serious medical problems, not long after our daughter was born, as a result we didn't believe it possible to have another baby and in fact it is somewhat risky, for both mother and child and the medication that my partner must take ads to the risk :(, I think it would be very reassuring to be able to hear our babies heartbeat throughout the pregnancy :) :).

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Hi Dazza,
Sure I will help you on this project.  ;D
Please don't give-up on the other member's pcb yet. I am only guessing that U5 is oscillating, maybe something else is wrong.
1) Measure the DC voltage at the microphone. With a 9V supply, it should be between +2V and +7V.
2) Measure the DC voltage at pin 7 of U1B. It should be less than about plus or minus 30mV.
3) Measure the DC voltage at pin 5 of U5 with the volume control turned down. With a +9V supply, it should be between +4V and +5V. Turn up the volume and see if it changes.

Cut the ground trace at the edge of the board between the volume control and U5. Then connect the ground of the mic to the junction of C7 and C8. Maybe that's all it needs to fix it.  ;D 

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Hi, audioguru and Thanks for your help,

(1) DC voltage at the microphone    (4.80V)
(2) DC voltage at pin 7 of U1B      (  03.9mV ) 
(3)    the DC voltage at pin 5 of U5 with the volume control turned down (4.19V) and with volume control turned fully on, it continuously changes between about (4.07V) and (4.22V)                                                                                             

I made the changes you suggested and it improved maybe a little.

The crackling only happens when I hold the stethoscope head to my skin, and when I hold the stethoscope head in the air I can hear the sound of the TV in the background, and the strength of the sound does rise and fall with the volume control.  I did use 10V caps for C6 and C7 could this have cause any problems? I was at first convinced that it was the stethoscope head I was using, so I did try several different ones with no improvement :-\.

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Hi Audioguru :),

I've attached a sound file of exactly what I am hearing while listening to my heartbeat, while adjusting the volume control from minimum to maximum 8). As I adjust the volume control from its minimum I can just make out but barely hear my heartbeat it is very vary faint, with no distortion crackling or friction noises, if I adjust the volume control any more than about less than quarter, the distortion and crackling cuts in :(.  I've tried using large plastic jar lids and metal lids of all different shapes and sizes with the same results, the stethoscope head I used for the recording is a very small metal jar lid with a small hole drilled in to it, to pass through the shielded cable, then I used a thick amount of hot glue to secure the Mike to the inside of the lid, then I coded the entire lid with a thick layer of hot glue including the rim that would come in contact with my skin, then came the fun part ;D ;D, I smeared some olive oil on my chest and still it didn't make any difference ???.


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Hi Dazza,
I can hear your heartbeat!  ;D
It sounds pretty good at about 1/4 volume but the crackling with more volume seems to be the LM386 clipping at its max output. It also seems to be motorboating at about 8 or 10Hz. I wish I had 'scope software on my computer to see it.
During my tests I could never turn-up the volume, it would be way too loud in my 32 ohms (16 ohms when paralleled) Walkman headphones. And besides, it would make terrible howling acoustical feedback when turned-up too high.
Are your headphones any good for the very low frequencies?  ???
Is your mic any good to pickup the very low frequencies?  ???

I think that you need a softer and thicker isolation grommet between the mic and the jar lid.
My mic has a dense foam sleeve about 1mm thick around it. I pushed it into the hole in the jar lid with the cable hanging out the back. I could hear my hands touching the cable.
Maybe your skin is too loose and is flapping around!  ;D ;D
You could try hot-gluing the thingy directly to your chest!  ;D

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Hi, audioguru, Alun

Sorry Audioguru for sending you on a wild goose chase  :(, it seems as though my electronic stethoscope is working the way it should, and it is I that doesn't understand that it is :-[.

The electronic stethoscope that they used at the hospital sounded much different, I think that the sound that we were hearing, was a higher frequency representation of the actual heartbeat, so basically it worked the same way as this project filtering out all the sounds except for heartbeat sounds, but then modified the heartbeat sounds so that it could be clearly played at high volume on a small speaker.

So what I would like to try to do is take the signal from pin 7of UB1 then modify the signal to a higher frequency or in a way so that it can be fedback to be amplified and played at high volume on a speaker without distortion,

I guess the add-on circuit would work in a similar way as a voice changer?, if you were to say hello in a deep voice into it, it would come out the other end, clearly as (hello) but sounding like a chipmunk.

Now the big question is how do I go about achieving this ;D, how do I take a low frequency sound and turn it into a higher frequency sound ???.

Alun, that's a good idea I'll have to try that :D.

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Hi audioguru,

HAY if I can't get it to work well as a stethoscope, it sure does make a great AM radio :D ;D.

I connected one leg of a 0.1uF ceramic disc capacitor to pin 3 of U5, and the other leg touching my soldering iron, then I adjusted the volume control to tune into 1332AM 8).  The capacitor was heating up a little but I had to keep it their, because there was a heated discussion about Dr Jayant Patel, who was the head surgeon here in Bundaberg and has fled the country because he was illegally performing surgery, including surgery on my partner Paula :o >:(.  He was working here illegally using forged documents because he had been struck off the medical register in America.  This guy is bad news, he is responsible for many deaths and botched surgerys :'(.

O I've gotten off track here a bit.  I'm still trying yet more different stethoscope heads hoping to make a difference, as well as different microphones from my junk box. I tried connecting it to my stereo and I was able to turn the volume control up to nearly half, without any distraughtion or crackling noises, then I increased the volume using the stereo volume control, "WoW" it was loud and those little subwoofers were fairly thumping ;D.  Even when I turn the stereo volume control write-down low there was very little noise but the speakers were still moving quite a bit.

Well my main goal with this project is to be able to here my unborn Childs heartbeat, so it doesn't really matter what it sounds like, as long as you can be certain that it is actually the heartbeat of the baby that you can here ;).

Hang on hold the phone, actually I don't think it is even the heartbeat that we were hearing at the hospital, I'm pretty certain it was the blood rushing through the heart that we could here, so would it be just a matter of isolating that frequency ???

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Hi Dazza,
I am sorry to hear about the "surgeon". I hope that everthing is alright.

You have a great idea!  ;D
Of course the baby's heartbeat will be hard to hear, but maybe the bloodflow is better heard.
Try decreasing the value of C3 and C4 for a 1KHz low-pass cutoff frequency, and decreasing the value of C2 to 0.47uF for a 160Hz high-pass cutoff frequency. You could even try eliminating C3 and C4 for wideband sound. Turn-up the volume slowly so you can avoid acoustical feedback.  ;D 

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Hi audioguru,

These changes are giving very good results :D, I can hear heartbeat sounds very clearly with increased volume without distortion, I changed C2 to 0.47uF as you suggested and for C3 and C4 I used to 3.3uF, unfortunately I don't have much in the way of MKT capacitors of many different values to try, but come Thursday I shall get a few to try.

We tried listening for the babies heartbeat or four at least the sound of the blood rushing through the placenta, which sounds much like a heartbeat, which was what we heard when they used the electronic stethoscope at the hospital, but we could not hear any think.

I'm beginning to think that this project will not be able to do what I want it to do, but still it has been a very successful project performing the way it was meant to, and will be very useful 8).

Audioguru, this might be a completely stupid idea ::), but what would be the results of adding an extra 2 Mike's in parallel, then replicating every think from C2 to just before R11, so basically you would have three separate parallel TL072 stages feeding U5, would this give increase sensitivity and allow for a combination of filtering, basically allow for more flexibility of what sound you want and don't want and the amount of that sound to be allowed through to the final stage ???.

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Hi Dazza,
It is great to hear that it is working better.  ;D
You probably used 3.3nF (not uF) for C3 and C4 for a low-pass cutoff frequency of about 1.4KHz.

Changing the equal values of R5 and R6 changes the low-pass cutoff frequency the same as changing C3 and C4. You could try a 100K stereo volume control for R5 and R6 with each part wired as a rheostat and in series with a 3.3K resistor, with 4.7nF caps for C3 and C4, for variable control of the low-pass cutoff frequency fom 330Hz to 10KHz.

I don't think adding extra mics is a good idea.
The sensitivity of the circuit is controlled by its volume control and the value of R4.  ;D 

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Thanks Audioguru, that's exactly what I want to be able to do :), have as much control as possible to experiment with,  while actually using the device.

Now I need to be able to do one more thing ;D, before I make these changes.

Can I replace R4 with a variable resistor, will doing this allow me to adjust its gain, the reason why I want to do this is because the way that I have made the stethoscope head.

I'll post more sound files as I progress 8).

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