microphone and oscilloscope!

Discussion in 'General Electronics Chat' started by Eric007, Jan 30, 2012.

  1. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    Hi All!

    Attached is my microphone circuit that I have built on breadboard...but I have not included the LPH in my breadboard...

    I just borrowed an oscilloscope (EZ Oscilloscope OS-5040A 40MHZ) so I can test my microphone...but I'm Not too sure how to set up this oscilloscope and test this mic!

    I don't know but I think I should expect some kind of waveform from the scope when I speak a word, right?

    the other attachement is the mic datasheet I think...
    BTW, I did some soldering on the mic so I can be able to breadboard it...

    Any help?

    Thanks for all your comments!
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    Make sure you connect the microphone the right way round. You may need capacitors in between the op amp outputs and the 1K resistors to remove DC offset. You should be able to see a waveform all along the signal path with the oscilloscope.
     
  3. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    I just breadboarded it just like in the attachement...
    Can u make a simulation of this circuit? Is it compulsory to put a cap between the output and the 1K? If yes, which value?

    As for the last op amp output, I've connected it straight to the ADC without that LPF, is it ok?

    Thanks!
     
  4. Markd77

    Senior Member

    Sep 7, 2009
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    Which op amps are you using and what is their power supply?
    What is the maximum voltage that the ADC input can take?
    Have a look here for an oscilloscope tutorial:
    http://www.doctronics.co.uk/scope.htm
    You should be able to tell with the scope if you need the capacitors. If the midpoint of the waveform becomes shifted too far away from 2.5V then you are going to need some.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The first capacitor after the microphone is needed, and the value 1uF is correct.
    If you used a buffer after the HPF you wouldn´t need such high value.

    Total gain of 1000 seems a little high to me. Try probing before and after the first opamp.
    Set your oscilloscope to DC coupling , 1V/div and timebase 1ms and check if it tapping the microphone produces any change.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Also, what opamp do you use? Is it powered from +5V or some other source?
     
  7. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    I'm using LM358N opamp powered to +5V
    About maximum voltage ADC can take, I will have to check to datasheet
    I have a 10uF cap for now...will try getting 1uF tomorrow (actually in a few hours)
    Also the +ve sign of the cap should face toward the mic or opamp?

    Thx!
     
  8. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    With Kubeek setup,

    When I blow to the mic with probe on mic output (before 1st opamp) there's changes on the scope...I can see some waveforms BUT when I probe the 1st, 2ndn 3rd output of opamp...nothing happens!

    I tried mesuring voltage on mic output (1.60V) and output of 3rd opamp (3.70V)

    What You guyz think?
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What is the voltage on the positive negative inputs of the first opamp?
    The bigger cap doesn´t matter, the + end should go to the opamp.
    The voltage on the mic ouptut should be around 4V if you use the 1K resistor, are you sure you have it connected the right way?
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    A gain of 1000 is a bit too much. Try a gain of 300.
    What type of microphone is it? Are you sure it is an electret microphone?
     
  11. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    I will do a measurement soon and will let you know...

    I've just got my 1uF cap so will put it...the +ve sign of the cap was connected to the mic...I was doing a mistake...I will post another schematic as I'm planning to build my new circuit...

    Thanks!
     
  12. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    The mic I'm using looks like the ones in the attachement...
    As you guyz saying my gain is too high well I have the following resistor values with me: 330K, 390K, 27K, 56K, 10K

    How about making a gain of 390? well i can make different gain with values above...

    Can I make that gain with a single opam as attached? or better with 2 opamp? also what do I do with that 1K at the output? leave it or remove it?

    I added a '+' to the cap to show how ima build it...hope i got Kubeek right?

    Thanks for comments!
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    The bias on the non-inverting input is wrong. You are asking for 2.5V x 390 = 1000V!
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I sugeest you start with one opamp stage with a gain of ~10 to get you going first.
    The polarity of the cap depends on the circuit. Theoreticaly if everything is working right, the voltage on the now-plus side of the cap should be 2.5V and you said that voltage on the mic side is 1.6 which seems a bit low to me but probably is allright.

    On the photo of the mic, the most left microphone has clearly visible which pin is ground and which is the supply/signal pin. The bottom pin that has a small connection to the metal outer shell, so that is the ground pin.

    Try the 390K/56K combination in the first gain stage and leave the rest unconnected. Disconnect the cap and check if you have 2.5V on the 10k/10k divider midpoint, the positive input pin, the negative input pin and the output pin of the opamp as well. If that is right, add the cap into the circuit and check if the voltages didn't change.

    Then try probing with the scope and tapping the microphone. You should see some signal before the cap, after the cap and on the ouput, but the voltages on negative and positive inputs of the opamp should remain almost unchanged.

    Post anything that doesn't match this :)
     
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  15. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I think the cap should take care of the DC bias, as the circuit is a unity gain follower for DC, but still OP should start with much lower gain.
    The input offset error is 2mV, so the DC error would be 1V at most, but still... High gain is better achieved through multiple lower gain stages.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    The output of the 3rd opamp is saturated at +3.7V which is as high as it can go.
    The opamps are missing coupling capacitors between them so they amplify the DC input offset voltage (7mV max) of each opamp.

    The LM358 is too noisy (hiss) to be a mic preamp. A TL071 single, TL072 dual or TL074 quad are low noise and have a wider bandwidth but need a supply that is at least +7V.

    I added the coupling capacitors and increased the value of all resistors.
    I added a power supply filter capacitor and added an RC filter for the microphone's power.

    The opamps have a very low input current so I made one filtered bias voltage for all of them.

    With your +5V supply and if the opamps have coupling capacitors then the average DC output voltage is +2.5V.
     
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  17. Markd77

    Senior Member

    Sep 7, 2009
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    That should work OK. If you look at the amplitude of the waveform at the microphone you should be able to work out what gain you need.
    Because the LM358 will only output 0-3.5V with a 5V supply, it would be better to adjust the voltage divider formed by the two 10K resistors so that the noninverting input is at 1.75V
     
  18. Markd77

    Senior Member

    Sep 7, 2009
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    Lots of replies since I started typing. Multiple op amps connected with the capacitors as Audioguru has drawn them work better than a single one.
     
  19. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    Thanks for all your comments...I'll reply to them one at a time...
     
  20. Eric007

    Thread Starter Senior Member

    Aug 5, 2011
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    Sorry! My electronics is pretty bad I think...I'll have to stop playing with microcontroller a bit and REVISE my electronics from scratch...
     
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