led question

[ptrx]

can a function led act as a polarity protection diode??

[Mike Burgundy]

it *is* a diode, so it does work, but it does have several drawbacks. A high threshold is one of them: you lose a LOT of voltage if it's in series. They're also more vulnerable, cannot hold back as much reverse-voltage (I think) as some diodes (400x series)and are not as cheap.
Parallel protection diodes aren't really useful either: they tend to fry when confronted with reversed polarity (and a wall-wart that can deliver), and then the circuit still gets hit by the reversed voltage.
Try these:
http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm
http://www.geofex.com/Article_Folders/cheapgoodprot.htm