Voltage source switching with FETs

[3cdb]

Hi all!

I figured I'd ask on this forum about possible fixes/recommendations/ideas for what I'm trying to do.

I've been trying to make a small circuit which switches its output either from a regulated +5Vdc source, or from a +4.2Vdc source (from an 18650 cell).

The idea is that when the 5V is connected, the output is 5V but when the 5V is disconnected, the output is 4.2V instead.

I have come up with a circuit so far, but after building and testing, it does not seem to work 

I have attached the circuit I have come up with so far.

Anyone have any ideas of what I can do?

[Kim Christensen]

Maybe have a look at this page to see exactly how complex you need to go. (I think a version of example 2 will work for you)

[Peabody]

This circuit should work.  If both supplies are connected, the one with the higher voltage will be selected.

[inse]

Two Schottky diodes would not do the job?

[Peabody]

Yes, two diodes would certainly work, but then you have a 0.3V voltage drop in the battery line which reduces the effective battery life.  The mosfet replaces the diode in that line, and the drop is very low when it's turned on.