At 07:16 PM 10/13/00 -0400, you wrote:
>Elementary electronics question here.
>
>How can I hook a mike to a PIC input so it triggers at
>a certian level?  I am guessing one could do it with a
>transistor and pot for adjusting the trigger level, but
>I have been unable to rig anything up that works.  Ack.

The easy way is with an op-amp.  Some mics require a DC voltage to operate,
but you can get that easily.

Make a divider to get the mic voltage you want, from two resistors, connect
the center to the op-amp + pin. For most mics, from a 5V supply, a pair of
4.7k resistors will be fine.

Now connect the mic to the - input through a similar resistor (R)

Adjust the gain, by connecting a resistor >>R from the output to the - input.
The gain figure is the ratio of this gain resistor, to the resistor from
the mic to the - input.

The output of this circuit will sit at 2.5V, and have excursions up and
down as the sound goes. With a gain of 100 or more, you should be able to
get it across the logic thresholds.  Putting an hc14 gate between it and
the pic will help, then the pic will only see 1 or 0 on the inputs.

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