It won't light up at 5v. It says the operating range is 3.5~4.5v. When I first got it I hooked it to 3AAA batteries in series (4.5v) and the laser was real dim. I dropped back to 2 AAA batteries (3v) and it got real bright. I used as many resistors as I could find to cut the 5v phone charger down to get it as close to 3v as possible.