Chris,

A carbon mic contains loosely packed carbon grains, and depending on whether
they are compressed or released by the membrane, its internal resistance
changes.  They do not produce electricity like other types of mic's, that's
why you didn't see anything on the scope.  Try to connect it like shown
below, and place oscilloscope probes in parallel with the microphone:

  ,------------<  +5V
    |

    R 100
    |
    |
   mic
    |
    '------------<  GND

Even though they are much noisier than other types of microphones, when I
was about 12, I preferred them over electret because I could get amplitude
modulation by simply plugging the mic in series with FM transmitter.
Electrets required a separate amplifier.

Hope this helps,

Vitaliy


----- Original Message -----
From: Chris Eddy <ceddy@NB.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 02.20.2002 2:49 PM
Subject: [EE]: question about carbon microphones..


> I have a handset, identical to a regular telephone handset. It has one
> of those carbon electret microphones in it. I cannot get any sort of
> signal on my scope when I holler in the mic. I am used to the active
> mics, like the two pin device that takes a 1K pullup to power it. Does
> anyone know what sort of amplifier is necessary? Impedance of the mic
> element? Gain that is necessary?
>
> Thanks all
> Chris~
>
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