Stepping down from 5v to 3-4.5v

[irving2008]

<noob mode> What's the laser for? </noob mode>

Better to use 2 diodes in series - they can give a different voltage drop dependent on current flow.

If you don't need to run the laser for a long duration (assuming it's for alignment of some kind) the batteries/relay idea is safer.

WHile technically that is true, for the 1N4001 series the forward voltage is not less than 0.6v and never more than 1v up to 1A. So even 1 diode is guaranteed to give below 4.5v from a 5v supply.