Newsgroups: comp.robotics
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From: jfox@netcom.com (Jeff Fox)
Subject: sound i/o circuit question
Message-ID: <jfoxD5yspn.EBp@netcom.com>
Sender: jfox@netcom20.netcom.com
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date: Fri, 24 Mar 1995 21:51:23 GMT
Lines: 40

I have question about a circuit.  It should be easy for
someone with experience with this.  I want to use a 1450 op
amp (or some other cheap 8 pin readily available at radio shack)
and a condensor mic and an earphone to make a speech (sound)
input output device on a small embedded design.

I want to use one of the two amps (in the 8 pin dip) to amplify
the output of the mic.  I want to use the other side as a comparitor
to compare the amplified mic output to a digital output from the
computer which is going into an earphone or speaker.  The earphone
should act like an integrator so essentialy I am trying to make a
simple differential pulse width modulation/demodulation circiut.

The computer will read the output of the comparitor (comparing
the amplfied mic to the speaker output, and toggle an output bit
up or down.  I should be able to record or play back sound samples
this way.  I think it should look something like this:

                                                     /|
        digital data from computer >----+-------+--/  |
+5 --+                                  |       | sprk| ) ) )
     \                                  |       +--\  |
     / R1                               |        GND \|
     \                                  |   |\
     /             .1 uf   |\           +---|+ \
     +-------+------) |----|+ \             |    >----> data to computer
     \       |             |    >-+---------|- /
     / R2    O mic   +--+--|- /   |         |/
     \       |       \  |  |/     |
     /       |    R3 /  |         |
     |       |       \  +/\/\-----+
     |       |       /   R4
GND -+-------+-------+

Any ideas or suggestions about this approach?  Also any suggestions for
the values for the resistors, or additions or corrections?  The software
will be trivial, and it should not require an analog input to the
computer this way.

Jeff Fox
