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Speaker used as microphone for sound sensor

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Newbie level 3
Feb 13, 2013
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Hi guys.
I´m working on one project to school and i´ve got one big problem.
Usage of this kind of speakers is requested.
**broken link removed**
It is originnaly piezo alarm speaker, i have removed the sin generator.
Then i´ve measured voltage at the output while talking to it from distance about 10cm, it was about 15-20mV.
So i amplified it with operational amplifier 100 times, so now is at output 1.5 - 2V. When you scream to it it will gain about 5V.
I need at the output log 1 as 5V.
Also i need to controll the sensibility of the microphone.
So i designed this circuit with schmitt trigger.
The trigger may switch to log 1 at 4V output from mic and to log 0 at 3.5V.
But it is not working. I have always at output of first opamp about 8V i have tried TL081 OP07 NE5534 but still i can´t get it working properly.
Thanks for any help.

You need a resistor to ground at the (+) input of the second amp, since there is no dc path for the charge on C1 to dissipate. Without that it latches at the high level and stays there.

Yes. this is correct. Schmitt trigger also made ​​an error. In high state it will charge the capacitor C1 through R3.shmidt.png
P. Horowitz W. Hill "The art of electronics" 1994 p.231

Guys thanks for the replies.
I have tried all that you adviced but still not working. I have all the time -11.64V at the output.
Is there some other way how to solve this problem ? Like not using op amps but maybe som ic or... whatever. I would be very grateful.

Show me the latest version of your circuit.

Here it is. Amps are supplied by 4W +-15V supply.


  • mic_sensor.jpg
    21.3 KB · Views: 105

Schmitt trigger is not changed. I have already said that your version will not work. In my version R1/R2 set threshold, R3 - hysteresis. R4 is not needed.

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