Hi,
Had to rush last night, so today did better version of yesterday's switch circuit.
Also, for fun and in case of use, 2nd picture is to show a BJT AND gate with a positive and negative input for enabling and disabling 3.3V supply when both it and 5V supplies are present but enabling 5V with an NPN.
T1 and T3 work as the AND gate. T4 enables 5V supply.
When 3.3V is present and 5V is not, 3.3V supply is enabled.
When 3.3V is present and 5V is present, 3.3V supply is disabled.
When only 5V supply is present, that is enabled.
Not great, but functional circuits for such a basic function if comparators and logic gates are not available. As can be seen, bulk capacitance doesn't hold up Vout as well as desired and current to load drops quickly, and BC547/557 vce is pretty high and 'steals' voltage from the load, plus are only capable of 100mA.
If not what you wanted, sorry, tried...
Guess
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Picture 1: pnp 'power path' switch
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Picture 2: BJT AND and NPN 'power path' switch separated
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Picture 3: BJT AND and NPN 'power path' switch together
I'm not so good at this stuff and still have a lot to learn...