250mW LASER stuck on

[HankMcSpank]

I reckon you need to establish what the cutoff voltage for your FET really is (as it's being help 'on' all the time)....like I say the 'actual' can differ somewhat from the 'spec'.

here's what I'd do...

('scuse the scribble ....back of an envelope - literally, & while cookinng the family bacon sarnies with my other hand!!)

...you'll need a variable resistor** (can be any value really)...just adjust the pot's wiper until the LED turns off & then measure your voltage at the gate. I reckon you'll find your fet needs more than +5V to turn it off. If so, you'll need a FET with a low cutoff voltage.


** if you don't have a variable resistor you could use a resistor chain to get different voltage presented at the gate. Eg a 5k + 5K resistor chain will give you 6V at the gate etc.