Sound Responsive Circuit

An easy way to make any circuit sound responsive.  

You can easily make any circuit you're working on sound responsive. I will be showing how to do this with an electret microphone.  Some people choose to use a 1/8" input (headphone jack) but I prefer a microphone so the entire system is stand alone and doesn't require running wires too it.  You can adjust for your needs.

The schematic is fairly simple.  The electret microphone inside is essentially a variable capacitor and a FET.  We use a circuit to DC bias the FET.  Then, as acoustic waves are sent into the microphone, they vibrate the "Sheets" of capacitive material.  This creates the "variable" part of the capacitor.  The output from the microphone will then be essentially an AC voltage varying over time. 

You can see in the second image from wikipedia how the microphone appears to a circuit, and the standard circuit for biasing the FET.

The output from the microphone is very small in terms of our circuit power, so we send the output from the microphone through an operational amplifier to boost the signal to a peak to peak voltage we can use.  We also use that op-amp circuit as a filter to block the DC offset from the biasing circuit.

We will then send that output to a comparator.  Sometimes I'll attach a potentiometer to the other terminal of the comparator so that i can adjust the "sensitivity" of the sound reaction.

You then have the output of the comparator.  Usually my application calls for attaching an LED to this out, which can just be sent through a 1k Ohm resistor.  But from here you can send the output to anything your heart desires.

So as the microphone picks up audio, it is sent through the amplifier/filter stage and then to our comparator.  The potentiometer will allow us to adjust whether we want the audio to be loud before it starts flashing the LED, or if we want it to respond on even quieter noises.

This can be applied to a number of different circuits and enhanced to become more accurate and more useful, but here is the basic start to making any circuit audio responsive.

I'm constantly adjusting these circuits, and your results are definitely going to be different, so you will need to adjust some of the values and play around with it until your get it working perfect.