Electronic Stethoscope

[audioguru2]

Hi Aaron,
It is great to hear that you got it working. ;D ;D
But it is too bad that it passes friction noises.
In my test setup, I used a soft, thick rubber grommet around my electret microphone to isolate it from friction and handling noise conducted from the stethoscope head. I used a very flexible shielded cable to stop noise from being conducted down the cable.
I didn't use a proper head, instead I used a plastic jar lid that made many noises because it wasn't heavy. With a solid heavy head that is used with some lube, you shouldn't hear many friction noises.

The circuit's lowpass filter passes 25Hz to 30Hz heartbeat sounds and their lower harmonics up to 103Hz, and reduces the level of higher frequencies caused by noises. The circuit's cuoff frequency is 103Hz if C3 and C4 are 47nF. If you use 3.3uF then the circuit will have a cutoff frequency of only 1.5Hz and the important heartbeat sounds would barely be heard and lower harmonics would not be heard at all.

Be certain that your headphones can reproduce the very low frequency of heartbeat sounds. Some headphones have a boost at 100Hz to give a false effect of bass, but cutoff lower frequencies.

 

[xoy]

hi again audioguru!!!

i know what you suggested to me seems really reliable and very useful... but a kind of person like me, cant really fully understand it.. specially those latching and other terms, which i was not familiar..
i again presented another circuit here.. please post corrections here,... also on what should be the placing of pins of cd4511 and cd4518 on the counter.. hope you wont get tired of helping me .. i really thank you!!!  :-*

----XOY----

View attachment 37546

 

[xoy]

add..

View attachment 37547

 

[audioguru2]

Hi Xoy,
1) Your transistor is connected to ground instead of to the signal.
2) You are trying to use the transistor as a gate, but it doesn't isolate the timer from the signal. When the 555 timer circuit has a logic high output then a high current flows beween the signal and the output of the 555 through the forward biased base-emitter diode of the transistor. The transistor needs a current-limiting resistor in series with its base.
3) The voltage at the LEDs is too low to drive logic ICs in the counter.
4) The voltage at the LEDs (and output of U4) swings negative which might damage the logic ICs.
5) The voltage at the LEDs (and output of U4) is not a single pulse for each heartbeat. Frequently it is 25 to 30 pulses because something vibrates like a drum. It needs to be filtered to produce a single pulse, and the single pulse will need a Schmitt-trigger circuit to speed-up its edges.
6) I have never used a CD4511, but if I was going to learn about it and use it then I would study its datasheet and circuits that use it.
7) Before making a heartbeat counter circuit, perhaps you should learn about how counters work and understand about why the display driver should be latched and why the counter should be reset for each counting period.
8) You should plan on using a big battery or power supply to power the high current of the 14 LEDs in the counter circuit. A little 9V alkaline battery would last not much longer than an hour if the display shows 88.
 

 

[xoy]

hi audioguru!!
tnx....

1.) you have told me before (page 13, reply #170 and #172) to add another Ub1 stage to detect only the exact heartbeat pulses... so maybe the problem about the numerous pulses is out...

2)  i have here again the circuit... i add the schmitt trigger that turns the analog signal into digital signal. .(http://www.play-hookey.com/digital/experiments/rtl_schmitt.html)

3.)  also include the circuit from the timer......       
                (http://www.electronics-lab.com/projects/oscillators_timers/015/)
.....to transform it into a counter,  problem is..  ??? ??? i dont know in where pins of cd4518 should be the input signal, be placed.
***if i add another display... how??

please post corrections on the circuit... if there is..

tnx again!!!
----XOY----- 

View attachment 37565

 

[audioguru2]

Hi Xoy,
Again, you have the input of your circuit connected to ground instead of to a signal point in the electronic stethoscope circuit.

Your Schmitt trigger circuit is incomplete, because with its resistor values, it will never have a logic low output as explained in the article you posted. It needs the additional 1 or 2 transistors shown in the article.
A single Schmitt trigger inverter from a Cmos IC with 6 of them would be much simpler and cheaper.

As you know, the Schmitt trigger needs to be driven from an additional U1b lowpass filter circuit to pass only individual heartbeats.

As shown, your counter circuit will continuously count any input it gets, without stopping. I think it resets itself each 60 counts then starts counting again.
It needs to have your Schmitt trigger circuit connected to it somewhere to clock it, your 60 seconds 555 timer connected to it somewhere to gate it, and needs to have its "reset" and "hold" switches replaced by an inverter or something to allow them to be driven with the correct polarity from the 555.

I am sorry, but your counter project seems to be way over your head for your limited knowlege about counters and digital stuff.
I would like to help you more with your heartbeat counter project but it is off-topic for this electronic stethoscope discussion. Maybe another member can help you some more.

 

[xoy]

hi audioguru!!!!!

ok tnx...

i will post again,if  i have finalized it... tnx for all the patience...

tnx!!! your still my guru!!! 
---XOY---

 

[Staigen1]

Hey Xoy, maybee i can help you a little

But first you have to start up a new thread, you can call it Digital Stethoscope or something you like. Then we take it from there.

//Staigen

 

[rohansethi]

hi

i am also trying to make a digital Stethoscope. Plz can soemone help me out here...
I will post the circuit i have designed soon!!

thanx
rohan

 

[audioguru2]

On my old computer's broken hard-drive there is a link I've seen that has a digital display showing heartbeats per minute of an LED shining through a finger. That counter circuit could be driven from this acoustical stethoscope circuit. ;D

 

[ante1]

Hi Audioguru,

Here is one example using an IR LED:

View attachment 37603

 

[dj10]

to audioguru does the electronic stethoscope 2 circuit works fine on bread board bcoz i dont have time to fix it on vero board, so plz reply quickly as possible.

 

[audioguru2]

Its gain is high so the wiring of a breadboard would pick-up mains hum. A breadboard would work if you put it in a metal box connected to its ground. ;D

 

[dj10]

u r saying that just connect the ground to n e metal or specifically a metal box. :

 

[audioguru2]

The breadboard's wiring would be shielded if it is inside a metal box that is connected to the circuit's ground.
Without shielding, the wiring would pickup mains hum, static from appliances and local AM radio stations.

 

[dukedevil]

Dear all,
  I am just starting to work on this project for a design course.  I was trying to understand the uses of each of the components in the circuit diagram.  What are the uses of R1, C1, and R16?  Do they create a voltage divider?  If so, what is the output voltage supplied to the mic?  Thanks for all of your help!

 

[audioguru2]

Hi Duke,
Welcome to our forum. ;D
You must be talking about my Electronic Stethoscope 2 project, because the original project didn't have most of these important parts.
1) The electret microphone has a field-effect transistor impedance converter inside. R1 powers it and provides a load for it.
2) C1 and R16 filter the +9V battery supply to avoid motorboating sounds (putt, putt, putt etc) when the battery runs down and develops a high internal resistance. When the battery has a high internal resistance then its voltage jumps around due to load current changes (Ohm's Law). You don't want opamp U1a amplifying those voltage changes.

All electret mics are different but most get a supply DC voltage of about 3V from this circuit. ;D

View attachment 37664

 

[dukedevil]

Thanks a lot for a quick response!! Since I have to build this circuit for a class and justify why each component is a certain value, I was wondering if there is a certain reason for the gains in the circuit (for example, why does U1a have a gain of 3.9 and U1b have a gain of 1.6?)  Thanks again!

 

[audioguru2]

Hi Duke,
Recently, I discovered that the FET in an electret microphone doesn't have a very high output impedance but it is about 4.3k ohms. It is in parallel with the 10k bias/load resistor R1 and results in a total resistance into the opamp of 5.2k. Therefore the gain of U1a is 9.0 and is what is needed.

U1b is a Butterworth lowpass filter and needs to have a gain of 1.6 in order for its resistors R5 and R6 and its capacitors C3 and C4 to have equal values.

 

[dj10]

to audioguru

i need a help regarding E-sthetho 2 circuit , i have fixed the circuit on vero board but when i turn on the circuit all iget is just a CLICK and nothing else after certain period of time , led keeps on for some time and then blink,

plz help me , wut mught be the problem?