> > OK, Let's say I put an NPN transistor (say BD139) in the earth leg of
> > the 3pin 7808 & switch it on/off with a 4N25. No worries, that kills
> > the 7812 ... BUT the o/p voltage then floats to supply, 12V in this
> > case.
> >
> > So what you need is some simple way to switch off the regulator and
> > have its o/p volts go to zero when it's OFF = when the npn tarnsistor
> > is OFF.
> >
> > Any ideas?


Hi, there's a circuit here:
http://centauri.ezy.net.au/~fastvid/self_swi.htm
that might do what you want. :o)
-Roman

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