is transistor switching can make this???

[marisan]

good day,
i'm working on with two voltage supply, one higher from the other(i.e. 12V and 9V), how can i let the higher voltage pass then block the other. is transistor switching can make it?
i'd love reading your replies, please help me figure this out. thank you.

 

[ahmad1954]

you can use 2 diodes for that. you should apply your voltages to anode of diodes and connect cathodes to each other.
in this situations greater voltage passes from diodes.

 

[KerimF]

As you know, if we have one voltage supply, we can switch it on and off by a PNP transitor.

Emitter connected to Vcc
Collector connected to load
Base to control

If there is another voltage supply, we may add another PNP transistor.

Is this what you are asking for?