You can simplify the circuit a bit by eliminating Q1 and
connecting the left end of R3 to the base of Q2.  Also, put in
that 10K base-to-ground resistor Spehro suggested so the circuit
doesn't turn on due to transistor leakage.

I've largely abandoned 7805 voltage regulators for my hobby
stuff, especially for battery operation.  They waste too much
current and require several volts of input headroom.  I now use
the LP2951 low-dropout regulator from National Semiconductor; it
draws only about a hundred microamps for itself (vs. several
milliamps for a 7805), will regulate well with an input only a
few hundred mV above the output, and it also has a handy ERR
output which you can use as a PIC reset input.  You can set it to
5 volts just by strapping a couple of pins together, or program
it to a different voltage with a resistor voltage divider.  It
comes in an 8-pin DIP.  IMO, it's the ideal hobby voltage
regulator.

HTH a bit...

Dave D.

Chris wrote...

>Any comments on the circuit at http://www.nosreme.org/tmp/selfpwr.png?
>
>The intention is to run a PIC-based circuit (16F628) from a 9V battery,
>but reducing the power consumption to (virtually) zero when not in use.
>I also want to be able to read the status of the push-to-make switch
>while running.
>
>The switch turns on the MOSFET via NPN Q2, and the PIC can then keep the
>power on through Q1.  It can read the status of the switch on another
>pin through R6.
>
>This is a one-off hobby project, and I already have these parts around,
>which is why I haven't just got a regulator with shutdown option.
>
>I have seen Roman Black's circuit at
>http://centauri.ezy.net.au/~fastvid/self_swi.htm, but as this circuit is
>likely to be running for a couple of hours at a time, I'd rather not
>have to sink 5-10ma the whole time.

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