Hi,
my supply voltage is 9v and i know i would need to put the diode between the battery and the 5v reg.
voltage drop isn't to much a problem for me
If the 9V comes from a PP3 (the little box shaped 9V batteries)
voltage drop does matter for you!A regular 7805 needs around 8V in and an LDO needs around 5.5V (at the very least).
A 9V battery has got 6 cells. A single cell alkaline is considered flat at 0.9V, so to use a 9V battery up properly, your circuit need to work down to 5.4V, but with an LDO you can use it almost completely (forget about using a regular 7805 though).
If you add a diode, you're starting to throw away good power. With 1N400x, you will drop 0.7V, if you go with a Schottky diode, you can get down to 0.3V, using a power MOSFET you can get down to some mV, but if you add a fuse from eg. the positive input and then mount the diode (after the fuse) with the cathode towards the fuse and the anode towards the negative side ("ground"), you avoid the voltage drop completely. In case of wrong polarity, replace the spent fuse (after removing power).
