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