Ok well I understand how to use PWM with an LED but I've run into a problem. I'm trying to do the same thing with a laser and controll the PWM rate with a pot. I'm using an NPN transistor to control the on off becuase the current regulator I made needs a 9v battery. BUT, instead of getting about 125mW out of the laser, its only going up to about 4-5 MAYBE. I'm thinking that the problem lies in the transistor. The reason I say this is becuase I've checked the pin on the arduino to make sure that wasnt the problem and the laser will work on its own. The transistor is rated at 1.8W power dissipation. I'm not sure but I think thats it but I'd like a second oppinion, that's why I'm asking lol. Aaaanyway, if anyone can help me out I'd appreciate it.
What is the normal operating voltage and current of the laser? Regular NPN transistors will have a voltage drop across them that may be significant at low voltages. You should be able to measure it with a multimeter. Also check the voltage drop across the laser. You may need to incease the supply voltage by the sum of these to get the desired current. MOSFET transistors usually have a lower voltage drop.
The operating current is 200-250mA and the Vf is just under 5v. I built the current regulator to work well with a 9v and I can get close to 140mW out of the diode. I haven't checked the voltage drops because I kind of ran out of play time haha, but I will next time I get a chance to work on it. I'm thinking though that it'll be much more efficient to just build a new current regulator with different resistor values because adding more voltage would just mean more room filled with batteries an I don't want that lol. I may just get some MOSFETs too that way I'll end up with an over all more efficient circuit. Thanks for your input zoomkat I appreciate your help.