Most are familier with the National Semiconductor design using a MOSFET to
prevent a battery from being connected if installed backwords. This process
will connect the negative terminal only if the polarity is correct.

    I would like to use this same technique in a different light. Add a
momentary switch between the resister and the gate of the mosfet.  When the
switch is pressed the circuit ground and the battery ground is connected and
current flows. So far so good, with no problems. If I release the switch the
ground is disconnected, and power is removed. But, If a PIC is connected so
that after it has powered up, one I/O pin will go high which is connected to
the gate through a high value resister. The switch could then be released
and the mosfet would remain conducting as long as the I/O pin is high. I
then could bring this I/O pin low to remove the power when the
microcontroller is finished, and all would be dead until the switch is
pressed again.

Potential problems:
1: VCC on the PIC will always be high. The ground is removed, but there is a
current path through the I/O pin and the gate of the MOSFET.

2: Removing the ground from the PIC during operation. (even though I am
shutting down and there are no application in progress the PIC is still
functioning)

3: Some circuits (like a linear regulator) will become a series pass
transistor without ground. I am using a 1.5V single cell battery and a
switching regulator to step up to 5 Vdc. this is not a problem, but I am
also using a 3.3 Vdc linear regulator from the 5 Vdc bus for specialty
functions. This may be a problem.

Any sugestion or ideas on this circuit or application?

Gordon Varney