Speaker used as microphone for sound sensor

[pete.mcgo]

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.

 

[crutschow]

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.

 

[chuckey]

AND a high value resistor (10M?) across the input terminals, the piezio is also a capacitor.
Frank

 

[pplus]

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

 

[pete.mcgo]

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.

 

[pplus]

Show me the latest version of your circuit.

 

[pete.mcgo]

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

 

[pplus]

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.