Noise Switch

I was a looking for a simple electronic device that will produce a logic "1" when noise inside a room is more than a threshold. I found some but was too expensive for me, so I decided to built one on my own.
The logic is easy, break down a pc microphone, connect it to power, amplify the signal (with a variable resistance) and after that smooth out the noise with capacitors.... Thats it!!! Now lets built it...

An old pc microphone will do the job.

pc microphone

And now we have the microphone lets design the circuit:

noise switch

There is nothing special with this design (the circuit is similar to current switch). First we supply the microphone with voltage through a variable resistor (R7), this potensiometer will increase or decrease the sensitivity of microphone. Next a high pass filter will cut all the dc voltage, the high pass filter is nothing more than a capacitor and a big resistor, you can find more here). 

Now we are ready to amplify the input signal using an opamp, the application will be 100 times the input signal (gain = R9/R1 = 100/1). Then a pull down resistor with a decoupling capacitor will smooth and somehow add all the frequencies into a dc signal. 

Finally, as I always do, I use an optocoupler to isolate the circuit from other devices, and I am connecting the transistor in common emitter mode, so the output will be non-inverting (when sound "appears"  in the input logic "1" will appear at the output.

Thats it.... Simple ha?