Let me see if I can remember how I did this...  it's been a while, but I
built a parallel printer tester back in about '92 or so, using an 8748.

Power was derived from a 9V battery or *any* wall-wart up to about 24V AC
or DC.  I used a bridge rectifier, followed by a P-MOSFET with the gate
tied high with a 100K resistor, as I recall.  Of course a voltage
regulator followed.  Power ON was by plugging into a printer cable with a
printer attached, which grounded the gate on the MOSFET -- of course this
could just as easily have been a pushbutton, but my design was completely
switchless.  One could very easily use a PIC output pin, set it LOW on
power up, tri-state it (make it an input) to shut down completely.  In my
printer tester, it just stayed turned on while plugged into a printer.
Pretty nice, you could throw it into a tool box without having to worry
about whether it was still on, or would get turned on by accident.

Hope this helps, and hope I didn't mis-remember any of the details.

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
          - Arnold Edinborough


On Mon, 5 Aug 2002, Roman Black wrote:

> There is a design for self switched PIC here:
> http://www.romanblack.com/self_swi.htm
>
> uses one cheap tiny (TO-92) SCR, and 2 resistors,
> will work with a 7805 reg or just with the battery
> and the PIC. :o)
> -Roman
>
>
>
>
> A.J. Tufgar wrote:
> >
> > Edson,
> >       Have the second pole of all your buttons goto an 2 input or gate.
> > and the output of the or goto the pic power pin (you'll have to make
> > sure it handle you pic's current load or you'll have to put some sort
> > of driver behind it).  The other input goes to a pic i/o pin.
> >
> > When a button is pressed it will turn your pic on via the or gate.
> >  Your pic then changes the line going to the or gate high, so if the
> > user releases the button before your pic is done it keeps power to
> > itself.  When it's done set the line low and it will lose power.
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>

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