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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!!!  :-*



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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.

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hi audioguru!!

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......       
.....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-----  :) :)


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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.

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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!

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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


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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.

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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?

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Hi Dj,
Did you substitute any parts from the parts list?
Everything with the ground symbol on the schematic must be connected together.
Since the LED stays on for a while, it indicates a DC problem. Measure the DC voltage at the outputs of opamps U1a and U1b. It should be very close to the circuit's ground.
When the LEDs blink, it indicates a problem with the batteries. Are they new?

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Hi Dj,
The project is designed for a common 2-wire electret microphone that is available at nearly every electronic parts supplier on this planet. You can steal one from a child's toy, telephone or cassette recorder.
A "condensor" mic needs an external 48V supply. An electret mic has the high voltage built-in. A condensor mic won't have an output in this circuit.

Many cheap electret mics are called "condensor" by mistake. Some have 3 wires but this project uses one with 2 wires, its output and ground. ;D

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hi again. Audioguru

thanx for ur patience n help, i have able to get some noise . but it is not the input i think which i m getting , LED is now off, but some times it turns on n at that time i m not able to get n e thing apart from clicks, so n e thing further that i can do??

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yes i placed the new batteries, and i think i m using very small 2 legged microphone so it must be wut u were mentioning , i just picked it from the vendor, the voltages at 386 opamps is fine, but when it reach the tl072 it turns -ve , idont know y it is happening but i connected the TL072 through LM386,
i m not getting n e output voice.

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