Just connect the +5v regulator to the Ignition wire. That's what I did
to power my VFD in my car...

> Michael Rigby-Jones wrote:
>
>      Hi
>
>      I am trying to find the best way to make a high side switch to
>      control an
>      auxillary power source for my pic in an automotive application.
>      When ignition
>      switch voltage is present (13.8V or so) I want the pic to control
>      a switch
>      that will direct 12V battery power to the  5V regulator for
>      perifferals. When
>      ignition switch is off,  high side switch will cutoff bat supply
>      to 5V
>      regulator for perifferals.
>
> Umm, can't you just connect your 5v reg to the 12v ignition supply?
>
>      I would like a logic level signal from pic to control 12V source
>      to regulator.
>
>      My plan was to use a pnp mosfet. It said it was a logic level
>      hexfet, but it
>      did not work. I tried an npn logic level mosfet and it didn't
>      work either. It
>      appeared the gate logic was dependent on the batter voltage. I
>      got a  pnp
>      transistor to work when gate voltage was above or below the batt
>      voltage, but
>      it was the inverse of what I was looking for. I know I am missing
>      something
>      simple.
>
>      It seems so simple I have done it in the past using 5v relays,
>      but I thought
>      there must be a simplier SS solution .
>
>      Could you provide some suggestions on how I can make a logic
>      level high side
>      switch cheap?
>
>      Thanks
>
>      Jon
>
> You could use a PNP bipolar transistor.  You will need a resistor from
> base to emitter to keep it switched off, and then pull the base to
> ground (through a current limiting resistor) to switch it on.  You
> will probably need to do this with an NPN transistor rather than using
> the PIC's ports directly.  This gives the general idea.  The resistor
> values are of the top of my head, the 2k will be the most important
> depending the the beta of the pnp transistor and the amount of current
> you want to draw, basically it needs to be driven into saturation.
>
> 12v
> ____    _______ Switched 12v
>  | e\> /c
> +-+  ---  (pnp)
> | |   |b
> | |10k|
> +-+   |
>  |____|
>       |
>      +-+
>      | |
>      | |2k
>      +-+
>      c|        4k7
> (npn)  \|_____+---+____From PIC pin
>       </|b |  +---+
>      e|   +-+
>       |   | |
>       |   | |10k
>       |   +-+
>       |    |
> --------------GND

--
Jason
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Website: http://www.execpc.com/~milsumai
E-Mail:  milsumai@execpc.com
ICQ # :  12978762
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