Hi, Picdude.
The internal protection diodes are keeping the transistor on. Even with the 
I/O floating, the voltage at the pin will be 5.7V, what keeps the PNP 
transistor ON. You can move R2 to the base of the transistor, and calculate 
the resistor divider to switch on the transistor only with less than 5V on 
the PIC I/O.
Fabricio.


----- Original Message ----- 
From: "PicDude" <picdude2@avn-tech.com>
To: <piclist@mit.edu>
Sent: Wednesday, June 11, 2008 1:53 PM
Subject: [PIC] +12V high-side transistor switch?


>
> I need to switch on/off a 12V load, high-side, from a PIC running on 5V, 
> and
> tried this circuit, but the output is always on.  With the PIC I/O 
> floating
> (set as input), the load should be switched off, and with the I/O low, the
> load should be switched on.  I'm using 1k for R1 and 10K for R2.  The load
> will be up to ~250mA, btw.
>
> Any idea what I'm doing wrong here?  The circuit...
> http://www.narwani.net/neil/electronics/PIC-HS12VSwitch-01.gif
>
> -- 

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