I bought a 50mW 3vdc green laser from ebay. It has a spring for use with 2 AAA batteries or 2 wires for breadboard connection. I tried powering up the laser with 3.3v since that is the lowest voltage I can attain from the arduino uno. For some reason the laser didn't light. Is +0.3v going too much over the device rating? Could it be a faulty laser? How can I step down to 3.0v cheap? 
Have you try with 2 AAA batteries?
3.3V should be okay, have you connect both 3.3V and Gnd pins?
I trust that you realise that the metal housing of the laser is the positive supply terminal.
I seriously doubt the arduino 3.3V regulator can power a laser. I will be very surprised if it could. What is the current rating for the laser at 3.3V ?
50 mW = 3.3 V * 15 mA
Hardly a problem.
So a laser draws less than a red led at 5V. Wouldn't have expected that.
raschemmel:
So a laser draws less than a red led at 5V. Wouldn't have expected that.
Nor would it be even possible. 
A laser is rated in terms of output power, and would be no more than 10% efficient, if that, even less for a green laser using a harmonic multiplier.
Connect it to a meter and measure it, but I doubt it would be less than 200 mA.
... Verified ...
OK, the tiny red sub-milliwatt laser on the breadboard next to me draws some 20+ mA (and wouldn't light when I used the 200 mA scale on the multimeter - I had to use the 10A range) whilst the green laser I came across yesterday (Probably 50 mW) draws 320 mA.
Why is Jremington saying it's 15mA ?
raschemmel:
Why is Jremington saying it's 15mA ?
He was simply back-calculating what 50 mW would be at 3.3V. Not actually relevant to the discussion because of the misunderstanding regarding input and output.
Now regarding my observations, whilst the laser in question includes an actual current regulator chip (which is to say, it is a proper design, not to be compared with the tiny milliwatt red laser whose current regulator is a single resistor!), the laser diode is an infra-red device which can be dismounted from the harmonic multiplier, so it presumably runs at about 2V. I calculate 640 mW input to the diode and (the module has been separated from its test certificate) if it is indeed about 50 mW output, then efficiency would be less than 10%.
Now driving this at 5V, 320 mA is consuming 1.6 watts (and without additional heatsinking, these lasers are not rated for continuous operation). Even at 3.3V, 320 mA would I think be a bit much for the Arduino 3.3V regulator and a laser requires a minimum current to "lase". At a lesser current, it should glow but the harmonic multiplier will probably not activate. Note that these green lasers are noted to pass a considerable amount of the IR excitation beam through, so a digital camera might see something at below the laser threshold.
I found out the ebay seller from China wired black (-) and red (+) backwards. It works with 3v (2 x AAA batteries). Is there a cheaper way to step down from 3.3v to 3.0v without buying the step-down voltage chip? I don't want to over work this laser.
Gimme a Break!!
50mW greenie e-bay laser = DPSS, prob > 50mW = prob 200mW or better IR pump diode.
No wonder the Arduino 3.3V supply couldn't get it to threshold current.......
@123Splat,
When I posted this,
I seriously doubt the arduino 3.3V regulator can power a laser. I will be very surprised if it could. What is the current rating for the laser at 3.3V ?
I was thinking this: (but worded my post conservatively because I wasn't really sure)
Gimme a Break!!
50mW greenie e-bay laser = DPSS, prob > 50mW = prob 200mW or better IR pump diode.
No wonder the Arduino 3.3V supply couldn't get it to threshold current.......
How can I switch this 3v laser on/off using the blink example? I'm curious because there is no communication wire from the lazer. 
jremington:
50 mW = 3.3 V * 15 mA
Hardly a problem.
50 milliwatts is the optical output power. Lasers are very inefficient and the INPUT power is much higher. I suspect the OP's laser module draws 1/2 to 3/4 ampere at 3.3 volts......
In general the 3.3volt current capacity of an arduino boards has a historical problem that we still live with today. The older arduino boards that use the FTDI USB chip used the chips internal 3.3vdc regulator to wire to the shield 3V3 pin and was limited to 50ma or so, and if more was drawn could and would destroy the FTDI chip. Later with Uno board development they installed a proper on-board 3.3vdc voltage regulator and probably can deliver up to 100 ma or better. However the trap is that if any 3rd party shield developer designed their shield board to draw more then 40ma or so and a user installed it on their older FTDI based board, it could damage the users arduino board. Therefore to this day even the current Uno board carries this information on their product description about the shield 3V3 pin:
3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
encryptor:
I found out the ebay seller from China wired black (-) and red (+) backwards. It works with 3v (2 x AAA batteries). Is there a cheaper way to step down from 3.3v to 3.0v without buying the step-down voltage chip? I don't want to over work this laser.
You don't need to step down the power supply. Laser diode modules have built in current regulators. That module would probably run just fine on 5 volts (but don't try it because if I'm wrong you would fry the diode in a nanosecond!).
I would suggest getting a stand alone 3.3 volt regulator (maybe an LM-317) and drive the laser diode that way.
Now picture the LM-317 circuit and add a small NPN transistor (say a 2N2222) to the LM317 control pin (collector) and ground (emitter) and drive the base through a 470 ohm or so resistor. When the Arduino port is high, the transistor is saturated and the LM-317 control pin pulled to ground (causing it to output only 1.25 volts). The laser diode goes off. Turn off the Arduino port. The transistor goes open and the LM-317 output returns to 3.3 volts and the laser turns on.
Bingo... voltage regulated power supply and on/off control all in one! 
Krupski gets the cigar on this one! (those green dpss modules will handle 6volts from four AA's just fine).
I think I would try a MOSFET, but he's got it.
encryptor:
How can I switch this 3v laser on/off using the blink example? I'm curious because there is no communication wire from the lazer.
The MCU pin can drive around 10 mA or so. That is not enough for your LED/Laser. You can use a transistor as a switch. For the 3.3 V you can use either the Arduino VCC (although 50 mA might still not be enough) or a battery. Remember to connect the battery GND with the Arduino GND to have the same reference.
I wouldn't worry about the 3.3V -> 3V transition, but if you care that much you can make a small voltage divider. Use R1 as a small value (10% of the internal sensor's resistance ; ignoring the transistor's resistance in conduction) and R2 a big value ;50 Kohm should be fine.
Good luck
New Bitmap Image (2).bmp (147 KB)
Attached is the circuit that I was talking about. It provides regulated 3.3 volts (3.28 actually) and the ability to turn the laser on and off with the Arduino.
Ok, I'll give it a try. Thanks for the information! 8)