# 5v to 3v

I just got a 3v input laser pointer head and I was wondering if there is a way to drop voltage from 5v to 3v to accommodate the laser without burning it. Is there is a solution using only a handful of resistors? I've heard the term voltage divider used before but I am new to this whole math with electricity thing.

EDIT: I was thinking maybe a led in series with a 2v drop?

There may be a single-resistor solution. It requires knowing the amount of current drawn by the laser, then using Ohm's law to calculate the resistance required to drop two volts.

A voltage regulator is another approach, e.g. Smart | Connected | Secure | Microchip Technology

A third possibility, similar to the resistor solution, is to use three silicon diodes (e.g. 1N4001) in series, which will drop about 2V or so.

All I know is the datasheet says <40mA.

I’m not sure how to interpret that.

Use a low dropout voltage regulator, or use either a string of 3 diodes or a yellow LED. Red only drops out about 1.5V, so you'd want a diode in series with it if you use red.

tripphippie:
All I know is the datasheet says <40mA.

I’m not sure how to interpret that.

Yeah, I’d probably measure it. With the voltage regulator solution, knowing the exact current isn’t required.

OTOH, calculating a resistor value for a 40mA current is easy enough. R = V / I = 2V / 0.04A = 50?. 47? and 51? are common standard values. Using the resistor, measure the voltage across the laser, and if it’s not fairly close to 3V, use a different resistor value. Given the spec (< 40mA) you may find that a larger resistor is required if your unit only draws, for example, 30mA. So maybe start high and work down, i.e. try 100?, 82?, 68?, etc.

Would this resistor be a pull down or in series? An example circuit would be stellar.

tripphippie:
Would this resistor be a pull down or in series?

series

However personally I would go with the voltage regulator option, it is much safer. Powering a Laser is more complex than you might think. See:-
http://www.repairfaq.org/sam/laserdps.htm

Grumpy_Mike:
However personally I would go with the voltage regulator option, it is much safer.

Agree, that is probably the best technical solution. What I can't tell is whether it's just a bare diode. I think some of these modules have built-in driver circuitry.

This is probably one of them, driving a laser diode is not straightforward.
http://www.teamwavelength.com/info/laserdiodedrivers.php
With basis in this any voltage <3V will do, regulation might increase output.

The Uno has a 150mA 3.3V regulator. You can source the laser+ from that, control an NPN transisitor on the laser- to turn it off & on.
Use a transitor with 300mV Vce

Sam's Laser FAQ is incredibly helpful. Thank you grumpy Mike.

I think I'm just going to go with a 100o resister for now. Maybe I will pick up a voltage regulator next time I'm at radio shack or ordering online.

CrossRoads is right. Just use the 3.3V regulated output.

What if current draw is more or less than expected? If it draws less, that 100 ohm resistor won't drop as much voltage.