Hi Andrew, i'm not sure if someone here already
posted this solution to the list but there is a
simple circuit, 2 resistors and one SCR, that does
exactly what you want with zero power in the "off"
state and compatible with many 7805 voltage
regulators etc.

I just put the circuit diagram and description up
on this web page:
http://centauri.ezy.net.au/~fastvid/self_swi.htm
-Roman


Andrew Kieran wrote:
>=20
> I=92m no EE so I=92m sure that someone out there has a very simple
> solution to my problem.  I want to momentarily press a
> pushbutton to deliver power to a PIC/LCD circuit.  On startup,
> the program will turn on an output pin that will keep the power
> on until, when finished its tasks, the program turns off the
> power to the PIC/LCD circuit. The circuit consumes about 250 -
> 300ma.
>=20
> This is easily done with a relay, but I want to keep it solid
> state.  I tried using a transistor to do this, but the voltage
> drop across the transistor meant that the circuit got less than
> 5V.  Next, I tried putting the transistor in front of the
> voltage regulator and this works.  But is seems inelegant and
> it may be susceptable to problems as the battery voltage
> drops.  Is there a better solution?  If possible, please be as
> specific as you can, or perhaps point me to a good reference
> site.  Thanks for your help.

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