The problem is, the led is always ON, and when i am testing the transistor with a multimeter in diode mode, i find 0,5v between the collector and emmiter. So i kill it and i don't know why =(
I need some help to understand where is my mistake.
you're not blowing them, they work still no doubt, collect D3 of Arduino to the Base Pin (middle/2 pin), Emitter to GND, Collector to your LED's GND (short lead) and the Anode + side goes straight to your 5v rail.
cjdelphi:
you're not blowing them, they work still no doubt, collect D3 of Arduino to the Base Pin (middle/2 pin), Emitter to GND, Collector to your LED's GND (short lead) and the Anode + side goes straight to your 5v rail.
According to the datasheet, the base pin is the pin 1, right ?
fair enough, so what resistors you using? to connect the transistor? i'd use a 1k for the base, and then 300 ohms to limit the LED current, also while you're at it, move it from 3v to 5v, it's possible if you're not
using a red led, it will be dimly lit, even less so with the voltage drop through the transistor.
different LED's require different voltages, Red being the brightest with the least (something to do with the band gap, wavelengths and things I never understood)
White for example will light dimly from a 3v cell coin, but a Red will shine really brightly... so if you used a white led, combine it with a voltage drop 3.3v, you're at the limit's a 5v would be better that's all.
Are you certain that is a TIP120 (NPN), and not a TIP125 (PNP)? Under these conditions, the TIP125 would keep the LED lit no matter the voltage on the base. Look at the polarity of the collector-emitter diode in the product schematic. Left is NPN, right is PNP.