3 volt output + 3 volt battery on same circuit

[pd2]

So I've forgotten almost the electronics I did 30 years ago and need help please.
I am using the Raspi 3 volt output to power a small simple transmitter that loses its settings after power loss. There is no program interface to the transmitter so I cannot reload settings. The original transmitter circuit is powered by a 3 volt button cell so hardly any current and only for 2 seconds.
I seem to remember that I could use both supplies simultaneously as long as I had some diodes in there to prevent reverse current flow to both supplies. Can anyone advise what components I should purchase for this voltage? Thanks.

[pcmanbob]

So what exactly are you trying to do ?

Do you just want to use the pi to press the button on the remote ?

[boyoh]

So I've forgotten almost the electronics I did 30 years ago and need help please.
I am using the Raspi 3 volt output to power a small simple transmitter that loses its settings after power loss. There is no program interface to the transmitter so I cannot reload settings. The original transmitter circuit is powered by a 3 volt button cell so hardly any current and only for 2 seconds.
I seem to remember that I could use both supplies simultaneously as long as I had some diodes in there to prevent reverse current flow to both supplies. Can anyone advise what components I should purchase for this voltage? Thanks.

BoyOh
Paralleling two power supply's and using , Diodes to stop feed back. Both power supply's must be the same voltage and out/ put impedance , Each feed back diode will need 0.7 volt to forward bias it, so this must be in your voltage calculation .
Is the button cell to keep the Transmitter program flashed . Now you want to use the Pi to operate your project, You will have to
compensate for the voltage loss with a voltage leveling stage

Regards BoyOH

[pd2]

Thanks BoyOh yes it's to keep the transmitter settings alive. Thanks.