Stepping down from 5v to 3-4.5v

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Originally Posted by No1_sonuk

<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.

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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.