First Stage; Second Stage - sparkfun SEN-12642 Hook-Up Manual

Finally, the red line is the gate output. This output is low when conditions are quiet and goes high when
sound is detected.
How It Works
Having examined the outputs, lets also take a quick walk through the schematic, to gain an understanding of how
each stage works.

First Stage

The first section of the circuit is an electret microphone capsule. This portion of the circuit borrows from the
Electret Microphone breakout board.
The capsule is biased by the supply voltage through R1, and it outputs an AC voltage that is riding a DC offset of
approximately ½ the supply voltage.
The output from the capsule is an extremely small voltage, so the signal from the capsule is amplified by IC1G1,
an operational amplifier stage. By default, the preamplifier has an arithmetic gain of 100 (20 dB), and the gain can
be adjusted by populating R17 (which we'll examine in detail on the next page).
The output is DC coupled, riding one half the supply voltage, so it can be directly connected to the ADC of
audio
a microcontroller. In perfectly quiet conditions, it will ideally read ½ full scale, or 512 on a 10-bit converter.

Second Stage

The second stage of the circuit is an envelope follower.
IC1G3 forms an opamp-based precision rectifier. This stage implements the equation
Microphone and Preamplifier
Envelope Follower