You can use either P-channel MOSFET or PNP transistor to do this. With
MOSFET your voltage loss on the 3.6V is going to be the Rds of MOSFET and
the drain current. With transistor this is going to be Ic, Ib and hfe.

Here we go with circuits;
                             +3.6V
                            |
                            |
                         |--|S
        +5V--------|-----|<-|
                   |     |--|D
                   \        |
                   /        |-------+5V
                   \R 100k  L
                   /        O
                   \        A
                0V          D

                           ------3.6V
                          /
                        |/E      P
               --------B|        N
                \       |\C      P
                /         \
                \R 1k     |
                /          |------5.0V          |
                |0V        L
                           O
                           A
                           D

With the transistor circuit, when 5V is supplied, the diode between
collector-base (PN junction) will take base voltage to 4.3V and turn OFF
the transistor. When 5V is absent, base voltage will drop to 2.9 and
transistor will conduct. the Vce can be little as 0.05V for up to 500mA
(ZTX705,MPSA77P)


Regards,

Jay


At 17:56 26/07/99 -0400, you wrote:
>Does someone have a simple switch to switch out a 3.6 volt battery when a 5
>volt voltage source is present?
>
>I have a card I'm making using a PIC to keep time.  The battery is 3.6
>volts, but when the card is installed, I would rather use the systems 5
>volts instead.
>
>Thanks,
>
>Tony
>
>
>