Watt's the wattage when used on a PIC?

[Charlie]

If the input is only for a trigger (e.g.. turns on with the ignition), why don't you use a 5V reg (7805)?
I've used them and it works perfectly.

Badger

And it only cost about 100x as much as a resistor, and needs filtering so add another 10x, and is slow to respond, but sure a regulator and many other complex circuits will work to sense a level that doesn't change very fast. Grahamg has the cheapest solution with a single high value resistor, if the 5V VDD supply is capable of both sourcing and sinking current. A resistor divider pair will work if the supply can source only. No need for zeners or anything else.