Electronic Stethoscope 2

[loveguru]

hi audioguru,
i have completed the circuit of e scop- 2. but it is not working.
led indicating only one light at a time.
one problem created that the ic tl072 has gain 21(which i have purchesed) but it is giving arrond 70.is it fault?
what shuold be actual gain?
i have reconnected ckt but still it is not working?
according to your advice what i have to do?
so that i can make ckt working...
plz reply fast bcoz i have only 5 days to complete it.

 

[audioguru2]

Hi Loveguru,
Does the LED change slowly from red to green back and forth? Then you probably have 47uF for C3 and C4 instead of 47nF, so they are 1000 times too much capacitance. 47nF is also 0.047uF.
Measure the DC output voltage of U1a and U1b, they should be very close (less than 20mV) to zero volts.

When you purchase a TL072, its opamps each have a gain of about 200,000 (1/5 of a million!).
1) The negative feedback in the circuit reduces U1a's gain to about 3.85 (47K divided by [2.2K plus 10K]). The gain would be much higher if the FET in the microphone had a low output impedance. If you use a low impedance generator instead of the microphone, the gain is 21.4 (47K divided by 2.2K) at frequencies higher than about 30Hz.
2) The gain of U1b is 1.59 (1 plus [33K divided by 56K]) at frequencies lower than about 103Hz.
3) The gain of U4 is 71.2 (1 plus [330K divided by 4.7K]).
4) The gain of U5 is 20 when nothing is connected to its pins 1 and 8.

I hope you get it working soon.

 

[Emad1]

Hi Audioguru
sorry for being back so late
I want to ask about modifing a circut working on 3 volt and used as an hearing aid as a stethoscope?? this circuit is present in this site
http://www.electronicsforu.com/EFYLinux/circuit/aug2003/CI-2-hearing-aid.pdf
Shoud it do well if a filter circuit is added between transistor 1 and 2 ?????
Many thanx

 

[audioguru2]

Hi Doctor Emad,
It is good to hear from you.

I played with circuits like this Low-Cost Hearing Aid circuit about 42 years ago! I am sorry but I don't like it:
1) Although it operates with a nice low supply voltage of 3V so it needs only two battery cells (or just a single 3V lithium cell), it eats batteries if you could get one to work
2) The transistors listed have a very wide range of current gain from 110 to 800, and together with a lack of feedback for T2, T3 and T4 will result in a random operating point for them. Most likely their operating point will be with them saturated, drawing a high battery current and no amplification. Don't touch it.

If I designed an electronic stethoscope for use with a small battery, I would use one or two headphone amplifier ICs of which there are many available. Some operate with a supply voltage as low as 0.9V. However they are all surface-mount which is difficult to work with, and some are tiny and don't even have leads, just bumps underneath. How would you solder those kind by hand?

There is only one IC that could be used in a 2V to 3V electronic stethoscope that I have used before. Two would be needed. It is the MC34119, available today at Digikey for $1.53US or at Newarkinone (Farnell) for $1.05US each in a quantity of one to ten. Its features:
1) 8 pins DIL for easy circuit building by hobbiests.
2) Operates with a supply voltage down to only 2.0V.
3) Low supply current.
4) Bridged output that can drive headphones without an output coupling capacitor.
However, it is not new and might be discontinued soon.
Its datasheet is here:
http://www.oselectronics.com/downloads/mc34119.pdf

 

[yerima]

Hi guys
I am a new member from nigeria. First of all my regards to all that have contributed to this forum to become one of the best learning centre in the world, so much has been archieved by people who have no knowledge about electronic circuit, how the components function etc. This is due to the healthy debates and selfless services from people like Audioguru,CDAK etc.
I was able to build the electronic stethoscope 2 which worked very well. I use a very cheap 32 ohms empedance head phone to listen to my heart beat clearly, the only problem is that it is not very loud. I then added a 10uf capacitor b/w pin 1 and 8 of the power amplifier (lm 386) which increased the gain from 20 to 200, i can now listen to my heat beat clearly with the lungs auscultation.
the ony problem i have is it picks up outside conversations and cell phones ringing, any body with an idea how it can be eliminated.
also how can it be adopted to serve both as a noise detector in circuits and as an electronic stethoscope.
Finally what is the input voltage to the pre-amp, how is it determine (calculated), what is the input/out put voltage to and from the power amp how is it calculated.
It would be of tremendous help to me , as a final year project for the award of post HND in elect./telecomms.
Thanks Aminu yerima.

 

[audioguru2]

Hi Aminu,
Welcome to our forum and greetings to Nigeria.
I am glad that your project works very well.
Its maximum volume is determined by the impedance of the headphones and their quality. 8 ohm headphones draw 4 times the power of 32 ohm ones, and "rare-earth" neo.....um magnets are more efficient. I recommend using cover-the-ear headphones that block background sound and acoustical feedback.

Background sounds can be reduced mechanically and electronically.
Mechanical:
1) Use a solid and dense "head". I tried a plastic jar lid and background sound went right through it.
2) Seal the microphone to the head with a tight rubber grommet or silicone sealant. Some people mounted the microphone in the plastic tube connected to the head and the background sound went through the sides of the tube.
3) Seal the back of the microphone where its wires connect.

Electronic:
1) The project has a precision 2-pole Sallen and Key Butterworth low-pass filter that cuts-off frequencies above 103 Hz. Heartbeat sounds are passed and background sounds are reduced. The filter has only 2 poles so is not perfect.
You can add a second filter stage like the project's U1b by using a TL071 opamp. You will need to duplicate R5 to R8 and C3 and C4. Connect the new 2nd filter stage between the output of U1b and the volume control. Please see my attached graph showing the cascaded filter's improvement.
2) You can reduce background sounds even more by reducing the cutoff frequency of the filters. Try changing the values of C3 and C4 to 100nF for a 48Hz cutoff. If a 2nd filter stage is added, also change its capacitors.
3) If higher frequency respiratory sounds need to be heard, the filters can have their cutoff frequency increased to 1KHz by changing the value of its capacitors C3 and C4 to 4.7nF. Also change them in the 2nd filter stage if it is added. Because the bandwidth is increased, background sound pickup is also increased.

It is difficult to know the gain of the preamp because it is determined by the ratio of its negative feedback resistor R4 and the series combination of its input resistor R2, C2 and the parallel combination of the microphone and its power-feed resistor R1.
Also it is difficult to know the output voltage of the microphone because it is loaded down by the low impedance of R2.

I recently tested my electret microphone in this circuit. The microphone's current and impedance changed as it warmed-up in 30 seconds! The microphone's impedance in parallel with R1 measured 3.1K ohms. Therefore the gain of U1a with it connected is 8.87 at very low frequencies.
Microphones are made differently by many manufacturers, so their impedance is all different and therefore the gain of the preamp will be different.

View attachment 36569

 

[Emad1]

Hi audioguru

many thaaaaanx
Realy I dont know how to thank u. Forgive me my english is so bad to do that

 

[loveguru]

hi audioguru,
i gone through your advice but still not getting responce.
what should be the fault i cant understand.
i go through each stage stape by stape everything is ok.
but not getting ckt to be run. ???
only 2 days for me.
i have connected 47 microf insted of 47 nf but led still glowing only in one light.
u r saying that our o/p shuld be nearly zero so how can we get gain 3.8?
what is your advice.
plz help us

 

[audioguru2]

Hi Con,
You can make your 10K linear trimpot volume control similar to an audio taper one by connecting a 3.3K resistor from its slider to ground. ;D

 

[loveguru]

hi audioguru,
it is somewhat good news that when i connected 47 microf capa. instead of 47nf when i speak to mice(whose voice is very high compared to our heartbeat), led alternating indicating red and green light.
but still i m not hearing any type of sound from my ear phone.
is there any spetial type of mice there which is to be very sencitive in our case bcoz i have used simple mice?
what should be the problem?
plz reply fast as possible as early.
thanks for help............

 

[audioguru2]

Hi Loveguru,
Your circuit doesn't work so let's start from the beginning.
1) The microphone must be an electret type. They are used in tape recorders, telephones and cell phones. It should have only 2 pins or connections. One pin connects to its metal case and should be connected to the shield of its cable which connects to the circuit's ground. The other microphone pin connects to the inner wire of its shielded cable and connects to C2 and R1.
2) All ground connections shown on the schematic should be connected together and to the junction of the two 9V batteries.
3) The positive terminal of the upper 9V battery on the schematic connects through one pole of the switch to all 5 places on the schematic marked "+9V".
4) The negative terminal of the lower 9V battery on the schematic connects through the other pole on the switch to all 3 places on the schematic marked "-9V".
5) Make the circuit exactly like the schematic with 47nF (or 0.047uF or 473) capacitors as C3 and C4.
6) Turn on the switch and connect the black terminal (-) of your DC voltage meter to the circuit ground.
7) Measure the voltages with the meter's red terminal (+) at the locations as follows:
a) Positive battery. It should be between +7V and +9V.
b) Negative battery. It should be between -7V and -9V.
c) Pin 1 of U1. It should be 0.01V or less.
d) Pin 7 of U1. It should be 0.02V or less.
e) Pin 6 of U4. It should be 1.4V or less.
f) Pin 5 of U5. It should be about +3.2V to +5.0V.

Since your LED was glowing on one side, some of these voltages must be wrong and we can fix it when you tell me the voltages.

 

[loveguru]

HI...........AUDIO GURU........
I HAVE NOT CHEKED THE VOLTAGES AS U TOLD......
I HAVE USED MY WALKMAN RECORDING MIC.
I HAVE MADE MY CIRCUIT RUNNING .BUT.........I CAN'T LISTEN MY HEARBEATS FROM IT........
IF I SPEAK IN MIC.I CAN LISTEN IT FROM THE EAR-PHONE...........
EVEN IF WE BLOW.I MEAN DOING LIKE THUMPING AND THOMPING BY OUR SELF-THE BI-COLOR LED CHANGES IT COLOR.........
BUT I CAN'T LISTEN HEART-BEATS.
PLEASE TELL ME .WHAT SHOULD I DO?
IF THERE ANY MIC. WHICH IS MORE SENSITIVE THEN THIS ELECTER ONE?
PLEASE TELL ME ITS NAME.I HAVE TO DO IT VERY SOON.
AND 1 THING IS ALSO.I WANT TO ASK THAT I HEAR SOME BACK-NOICE IN HEAD-PHONE.WHAT SHOULD I DO TO REMOVE IT?
PLEASE TELL ME ABT IT................................
PLEASE REPLY ME SOON?????????????
HEY I GOT AN IDEA .CAN I USE ONE AMPLIFIER BEFORE MIC.?
HEY.I HAVE ALSO TRIED THAT REAL STE.'S HEAD,4 MORE SENSITIVITY BUT NOT GETTING HEAR-BEATS............
I HAVE NOT MEASERED THE VOLTAGES U TOLD BUT I WILL TELL U SOON.
SO
SO.................
PLEASE GIVE ME ADVICE.
OK.
BYE.


LOVEGURU.

 

[audioguru2]

Hi Loveguru,
1) WHEN YOU SPEAK, he, he, please don't shout, I mean, when you speak into the microphone at a normal voice level and the microphone is about 1/2m from your mouth, with the volume control at maximum can you hear your voice loud and clear in the earphone? If it is distorted turn the volume down a little then is it loud and clear?
2) Is you earphone made to play full-range music with bass, and not just for telephone sounds? Connect a speaker with 5m cable instead of your earphone, can you hear heatbeats from the speaker?
3) Does your Walkman recording microphone have electronics or its own battery with it? Can you seal it to a stethoscope head?
4) Did you use a 4.7uF (four point seven microfarads) capacitor for C2?
5) Did you use a 1000uF (one thousand microfarads) capacitor for C6?
6) Did you use an LM386 for U5?
I am waiting for your reply in this topic.

 

[loveguru]

HI.AUDIOGURU..........
I FORGOT TO TELL THAT WHEN I REMOVEV MY R11 POT......THEN I GOT MY OUTPUT........I DON'T KNOW IS IT CO-INCIDENTALY OR NOT?
RECENTLY I HAVE NOT USED THAT POT BECOZ I THOUGHT THAT IS NOT WORKING PROPERLY.
I HAVE USED ALL THREE COMPONENTS AS U ASKED IN LAST 3 Q.
MY MIC. HAS ITS OWN BIASING OR NOT THAT I DON'T KNOW BUT IF I USE IT AS INPUT I CAN HEAR MY VOICE.
YES.I MADE THE CONNECTION B'W MY MIC. AND STE. HEAD THROUGH THE RUBBER TUBE.BUT NOT GETTING THE DESIRED RESULT.
HEY...........IF I SPEAK AT THE DISTANCE AS U TOLD-1/2 M,THEN I CAN'T GET MY OUTPUT.
RECENTLY I HAVE TO SPEAK IN MIC. FROM 1/2 CM.
AT 4-5 CM .ONLY LED GLOWS BUT NOT COMING ANY SOUND.
PLEASE TELL ME THE WAY TO HEAR HEARBEATS.
DON'T U KNOW ANY SENSITIVE MIC.TAHT CAN DETCT OUR HEART-BEATS?
IF U KNOW THEN TELL ME.
OK
BYE.
LOVEGURU.

 

[audioguru2]

Hi Loveguru,
Your microphone is not working properly in this circuit. Maybe it is dynamic type (coil and magnet) instead of electret (condenser with built-in high voltage charge, and FET impedance converter). Get an electronic part electret microphone or take one from a tape recorder, telephone, answering machine or toy.
Also get a new pot.

Before you said you heard "back-noice". People talking? Hiss? Rumbling? Please describe the noise.

 

[trigger]

Also some microphone frequency response will be poor at low frequency.

OT:.... better not to use all capital letters..... that just like shouting and rude....

 

[loveguru]

Hi................audioguru............
i will try for taht mic.
tha back-noice about which i was talkin is like:"shshshshshhhhhhhh"
if we don't speak in mic.then it also created in head-phone.
what should i do?
Loveguru.

and i want to know about you.
what is your name?
form which country u r?
what r u?
ok.
bye.

 

[audioguru2]

Hi Loveguru,
I hope you find an electret mic that works.
The noise you call :"shshshshshhhhhhhh", we call "hiss".
If you replaced your defective volume control and turn it to zero, if you still hear hiss then it is coming from the LM386. They are cheap but not too bad. Many people use them for driving their headphones from CD players. Maybe yours is bad or a counterfeit.
You could add a simple filter like this:

View attachment 36588

 

[loveguru]

hi audioguru and all others persons which are helping me,
my circuit is working but not satisfactor.i can hear big sound(when i slightly speak to the mice) from my headphone but it is not able to catch heartbeat sound.
actually till now i have tried on bread board in which connection bi default may be disconnected internally.so that now i m trying it built on pcd in which connection are done by shouldering.
the connection 100% right on that
can i replace 10k pot by 47k pot or not.
i have sony(company) sterio mice.
is it well for us?
plz reply soon
very very thanks for giving repling

 

[audioguru2]

Hi Loveguru,
Sure you can use a 47K volume control. If it's linear instead of audio-taper, connect a 22K resistor between pin 3 of U5 and ground. You might have to use a shielded cable for its output to U5, because its higher resistance might pickup hum in the wire.

You have 3 problems:
1) Your Sony mic (like the pic attached?) has built-in electronics to cutoff frequencies below 50Hz, to avoid picking-up low frequencies like trucks and heartbeats! Again and again I say, get a cheap electret mic capsule.
2) He, he. Sorry, but you should be using a microphone, not a computer mouse (or two for stereo, one for each hand) nor real live mice! ;D
3) He, he. Sorry again, but you should solder the wires. That's what I do. I only shoulder soccer balls! ;D

View attachment 36595