Electronic Stethoscope

Discussion in 'Homework Help' started by Crossroad, Nov 4, 2011.

  1. Crossroad

    Thread Starter New Member

    Nov 4, 2011
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    I am building an electronic stereoscopic and having problems getting a "lub" "dub" out put signal. I am getting some signal when the stereoscopic is put nect to my chest, but it looks like noise, not not a distinguished big peak, smaller lub-dub signal(without any filers or gain, just with powering it with dc). I was told the frequency for the heart was around 15-12Hz which is what I have built my filter for. I having problems specifically with my low pass filter, since sounds below 15hz is not that substantial. I have inputted an ac signal along my circuit, correct gain, but the rate of attenuation is too slow and the voltage only zeros at 1000+ hz. Also, I get no signal with my filters and op amp gains. Any tips and suggestions?
    Here is a quick doodle of my schematics.
    [​IMG]
     
  2. MrChips

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    Oct 2, 2009
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    You need to state what opamp you are using.
    Also your schematic shows 5V but no -5V supply.
    You need to show power supply decoupling capacitors too.
     
  3. Crossroad

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    Nov 4, 2011
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    They are 741's and my mistake, forgot the negatives on the amps. Oh, power supply decoupling capacitors? I don't have them on the circuit. I'm majoring in BME, so circuits are not exactly my forte. A quick google search revealed they reduce noise caused by other components in the circuit. How do I incorporate them in my circuit?
     
  4. bertus

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  5. Adjuster

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    Even if the vibrations generated by the heart do contain contain substantial energy in the region of 12Hz-15Hz, these frequencies are at or below the lower limit of the normal human hearing range which is generally accepted to be around 20Hz. Commonly obtainable electret microphones (also earphones, and particularly loudspeakers) will usually also show a falling response below a few tens of Hz.

    If therefore your aim is to produce a result that someone can hear, you should aim to cover a wider frequency range. This could be from perhaps 10Hz (in practice, possibly a few tens of Hz) to a few hundreds of Hz or the low kHz. This advertising material for a well known marque of commercial stethoscopes suggests responses of this order. http://www.rehaboutlet.com/stethoscopes.htm

    On the other hand, you may be attempting to study the sub-audio frequency content, to measure heart rate or some more technical aspect of heart action. In that case my remarks about audibility do not apply: it would then however be important to use a transducer (a microphone or accelerometer head) with adequate response at the low frequencies required.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    The lm741 is most happy at +/- 15 volt as supply voltage. Will you be able to increase your supply voltage. Say at least +/- 9 volt.
     
  7. Audioguru

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    Dec 20, 2007
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    1) The electret mic should be mounted in a stethoscope head. I used a plastic jar lid about 7.5cm in diameter which provided a sealed air space between the mic and the chest. It worked well.
    2) A lousy old 741 opamp is too noisy (rumble and hiss) to be used as a mic preamp. Use a modern low noise opamp instead. I used a TL072 dual opamp. One opamp was the high input impedance non-inverting preamp and the other opamp was the active lowpass filter.
    3) The 2.2k resistor that powers the electret mic has a value so low that it attenuates the signal from the mic.
    4) The first opamp is inverting with a very low input impedance of only 2.2k that also attenuates the signal from the mic.
    5) The filters are extremely simple with a very gradual slope. Use an active 2nd-order Sallen and Key Butterworth lowpass filter.
    6) There is no headphones power amplifier. An opamp can drive minimum 2k ohms, not low impedance headphones. I used an LM386 little power amplifier to drive my 32 ohms headphones which are 16 ohms when in mono.
     
  8. Crossroad

    Thread Starter New Member

    Nov 4, 2011
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    1.check
    2.I am limited to my lab's supplies, will a tlc2272 dual op amp do?
    3./4.So I should use a higher value resistor?
    5.What do suggest for the values of capacitors?
    6.I do not need it to connect to headphones, I just need a decent lub-dub signal right now, which later i can connect to a led bar graph.
     
  9. Crossroad

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    Nov 4, 2011
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    Also, bear in mind that I can only use 4.5-5V max for voltage supply because I have to demo this in class.
     
  10. MrChips

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    You could investigate using an LM324 with single 5V supply.
     
  11. Audioguru

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    Good.
    Yes, it is a dual, it is low noise and it will work with a single 5V supply.
    Use a non-inverting preamp opamp and a non-inverting lowpass filter opamp.
    calculate then for the low frequency response you want.
    Of course you must use headphones. A speaker will not produce the very low frequency sounds and it will produce acoustical feedback howling.
    You need a little power amp like an LM386 to drive the headphones.

    DO NOT use a lousy old noisy LM324 quad or its dual sister the LM358.
     
  12. t06afre

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    I Also think the TLC2272 will be an good choice for your project. But do not create problems for your self by only using a +5 volt supply. If you have access to +/- 5 volt use this as your supply voltage
     
  13. Audioguru

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    A single 5V supply for the TLC2272 will be fine. The opamp inputs can easily be biased at +2.5V with a voltage divider made with two series resistors and a filter capacitor.
    An LM386 little power amp and an LM3915 bar graph also work well with a single 5V supply.
     
  14. bertus

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  15. Audioguru

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    Can you believe that Texas Instruments shows the schematic Completely Wrong?
     
  16. Crossroad

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    Nov 4, 2011
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    Thank you all, heading to the lab at school later today, on a weekend blah! As for the power supply, I have been given a circuit that gives me V+ and V-, but since the tlc 2272 does not necessarily require V-, i will try that first.
     
  17. Adjuster

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    Note that if you have only a single supply, a half-supply bias point must be established in order for audio amplification to be possible. This is not optional.

    Edit: There are other ways of saying this, and the optimum voltage may not be exactly half-way in some cases, but I thought to try to get a comment in quickly before you went to the lab to try a single supply design on one supply only, with disappointing results.
     
  18. t06afre

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    You can not just only +5 volt and then just use a design that is designed for V+, and V- supply voltage. You have to redesign. Why go to all this trouble. I do not know but it is your call. Put your effort into plan that is more likely to succeed:rolleyes:
     
  19. Crossroad

    Thread Starter New Member

    Nov 4, 2011
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    Hi all, thanks for the help, I have a damn good signal now. The thing is, I have it passing an amplifier with gain 100, and all is well, but when I pass it to a second amplifier, the signal is gone!!! When I tap on it to check, that noise is still there, amplified! A simple amp is acting like a filter! What do? Also, suggestion for an envelope detector. Mine is kinda working but the max amplitude decrease from .7V at my 100x amplifier, to .2 V at the end of the envelope detector...
     
  20. Adjuster

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    If you are using a single supply, have you remembered to arrange for a correct half-supply operating point for each amplifier?

    Edit: If this requires more than one connection to a Vcc/2 source, prefer to make up more than one divider. Alternatively, make an active bias source using an amplifier. Sharing a single divider between stages may cause instability. In principle this can be decoupled with a capacitor but this needs to be very big if you want 10Hz response.

    It may also be advisable to minimise the DC gain of the amplifiers (the circuit in Audioguru's post #15 shows this). AC coupling between amplifiers may also be required. If this is not done, DC offsets may build up along the amplifier chain until an amplifier is driven to an extreme voltage.
     
    Last edited: Nov 7, 2011
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