## How does an electret mic work?

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Not so much as how does it work, but how do you measure the output? I have built this basic powering circuit but when I hook it up to a mutlimeter, the voltage just drops to 0 as the capacitor charges. What should the result look like? I want to make a sound trigger (triggered by something like a loud clap) for my AVR project. I think I just need a mic that outputs from 0 to Vcc based on the intensity of the sound and hook that up to the ADC, right? I've looked at other people asking similar questions on the forum, but they were all told to use some OPAMP and this seemed a lot simpler. Thanks for the help!

Math is cool.
jevinskie.com

if you need handling something from the loudness of your voice what you need firs is opamp as comparator to detected the level of your sound, then the output comparator connect to flip-flop to hold the signal, if you try to read the level of your voice using multimeter its never happened, because the multimeter read your Vrms of your voice, :)

You are going to need some sort of amplifier, the output from electret mics is typically a few mV. You should find plenty of examples with a web search.

Leon

Leon Heller G1HSM

Quote:
....I hook it up to a mutlimeter....What should the result look like...
You need an oscilloscope to see the acoustical waveform from the mic.

Quote:
I think I just need a mic that outputs from 0 to Vcc based on the intensity of the sound and hook that up to the ADC, right?

JChristoff
Illinois

Last Edited: Thu. Jan 6, 2011 - 06:39 AM

Hi,

do a biasing of the capacitor´s output.
connect a 20k to GND and a 20k to VRef.

If no sound: then you can read about 1/2 of ADCmax.

Maybe your ADC has differential input and and input amplifier:
then make an additional voltage devider from VRef to gnd bypassed with the same capacitor as the mic.

Klaus
********************************
Look at: www.megausb.de (German)
********************************

I use this schematic for mic . Search yours mic .pdf for R1 selection

## Attachment(s):

Hi

Never seen an opamp drawn that way.
Is there a new format for linear components that I am not aware of ?

Ken

Hi Ken!
Opamp drawn in russian standart named GOST.
Alexander

Looks a bit like those IEC logic symbols that no-one uses apart from TI.

Leon

Leon Heller G1HSM

Looks like the SOT23-5 footprint the I use in some TI OpAmps...

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

All GOST symbols are rectangular like that. Groups of related pins are often separated from other groups (e.g. address bus). I'm pretty positive they share the same symbol set with German DIN standard, at least DIN logic symbols in Eagle look very similar to what I'm used to since childhood.

Some of them, namely logic gate symbols, are much easier to read IMO. When I look at a rectangle with "&" in it, I know for sure it's an AND gate. I still can't tell AND from OR in "western" notation (and from what I gather, there are "European" and "American" symbols for those too) -- there's no logic in those symbols. Pun intended ;)

The Dark Boxes are coming.

I got this circuit working... sort of... I can probably get away with not having an OPAMP except for one problem: when I power the circuit, there is a certain DC voltage from the mic output that decreases as time goes on... it takes probably ten minutes for the voltage to drop to zero. At that point, loud noises are large enough to read on the ADC. Why does it take so long to stabalize the voltage? I don't think the capacitor would take so long to charge! Thanks

```                                  10 uF
+ | |
|                     |        | |
| (positive lead)     |
|                     +----------/\/\/\---------+
MIC                              10 kohm         |
|                                               | +
| (ground lead) (-)                          -------
|                                          9v  ---
|                                               |
|                                               |
+-----------------------------------------------+----> ground```

Math is cool.
jevinskie.com

The long stabilization time is the time it takes the 10 uf to charge through the very high input impedance of the ADC. Making the coupling time constant shorter will also result in a higher low frequncy cutoff

Here is a starting point, assuming you want the audio signal level to dither around 1/2 the power supply voltage. The low frequency cutoff will be about 45 Hz. Make a 2:1 voltage divider with two 50k resistors. One resistor to the power supply, and the other resistor to ground.

Change your 10 uf capacitor to a 0.1 uf capacitor and connect it and the ADC input to the junction of the two resistors. Your signal will swing above and below 1/2 the power supply voltage (or whichever reference voltage you choose). The settling time will be much, much quicker (about 0.0025% of what it was before - 25 K Ohm / 1 Gig Ohm.

The low frequency cutoff is 1/(6.28 RC), where R is the value you would get if the resistors in the voltage divider were put in parallel and the 10K load resistor was added in series with them.

To make the charging time shorter and decrease the settling time futher, decrease the 25k resistors in the voltage divider or make the 0.1 uf coupling capacitor smaller.

--
"Why am I so soft in the middle when the rest of my life is so hard?"
-Paul Simon