> Find a op-amp that will work on as little as 1.1V (if possible) and
> power this off the pattery directly, then run the output to the inverted
> SHDN line.  When a pressure change is sensed the logic board would turn
> on and the pic would be able to hold shdn high until it wanted the
> device to sleep.  I've looked for low voltage op-amp and comparators but
> nothing comes close to 1.1V.  Any sources?

You'll need some form of wakeup interrupt when there's a pressure change.  I
can think of one way of doing this with one opamp, but it will only sense a
decrease in output voltage.  To trigger off of both rising and falling
voltages, you'd need a window comparator.

The single opamp method can esily be extended to a window comparator
technique, and as I'm drawing in ASCII, I'll leave that to you.


                                |\
Vin o--+------------------------|+\
       |                        |  \
       |   R1                   |   >---o Int
       +-/\/\/\/---+---+        |  /
                   |   |    +---|-/
                   |   >    |   |/
                   |  <  R2 |
                   |   >    |
                   |  <     |
                   |   >    |
                   |  <     |
                   |   |    |
                C ---  +----+
                  ---  |
                   |   >
                   |  <  R3
                   |   >
                   |  <
                   |   >
                   |  <
                   |   |
                   +-+-+
                     |
                   -----
                  / / /

Now, R1, with C controls how fast the opamp (in comparator mode) will adapt
to changes without triggering.  You will probably want a large value there
if you want to detect slow changes.  The input to this circuit should really
be buffered with either a unity gain follower, or a small gain stage.

R2 and R3 control how large of a change is required to trigger the opamp,
and provide some noise immunity and resistance to triggering on small
changes.  You will likely want R3 to be large with respect to R2.

As for the opamp, you will likely want one with FET inputs.

If you want to use a window comparator configuration, add a diode to the
ouput of each of the two opamps (with the anode towards the opamp), and a
load resistor off the output (after the diode) to ground.  The first opamp
will be connected the same way as the circuit I drew above, but the
additional opamp will will need its own input circuit, which will have to
have the divider on the buffered line, rather than off of the capacitor. so
that when the voltage rises sharply, the capacitor stays at the same
voltage, and the opamp triggers after the divider voltage gets above the
capacitor voltage.

Hope this helps you.

--Brendan

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