Help with LED project

Hello, I'm trying to control some LEDs with an Arduino Mega... I've put together a circuit as shown here:

I'm using 12v instead of 5v and the resistor between the pin and the transistor is a 2.2K. I'm also using 3 LEDs in serie with a 120 ohm resistor (LED: 3.2 Volt, 20 mA). When the pin is set on HIGH the LEDs are fully lit as expected, but when it's set on LOW the LEDs still a bit lit, and I don't understand why... can anyone help me? thank you very much :)

msm

Sorry I forgot to post the link :P

http://www.arduino.cc/playground/uploads/Learning/multiple_leds2.jpg

It would be better to post a schematic of how your circuit is actually connected.

Hello, here is the schematic of my circuit: http://host.presenze.com/immagini/102010/20101018080442multipleledsjpg.jpg

Most LEDs have a reverse voltage limit - I have found that even with 2 LEDs spanning 12V they will turn on some, you need 3 in series to get them fully off. Can you put just 5V across the LED instead?

Hmm, I looked at your 2nd design after I posted - I have the same circuit in a fencing scoring machine, but at lower current - 270R in series with 3 LEDs, and 1kR into the base, did some trial & error to end up with the current thru the LEDs I wanted while making sure the arduino wasn't driving too much current out. Not sure what my transister is, some NPN that I had a little drawer full of. Are you sure you have just 12V going in? Some supplies, like inexpensive "unregulated" wall-warts, may say 12V but actually out more like 14 when lightly loaded.

Thank you CrossRoads, I just checked again the current, it's between 11.9V and 12V! Maybe I can try with different Resistor on the transistor's base... What was the Voltage on your project?

I have +12V going across 5 LEDs +resister in 4 places, and +12V across 3 LEDs + resister in other places. Where I wanted 2 LEDs, I had to drop down to +5V instead, like you I coulnd't keep the 1 and 2 LEDs off with 12V+resister. Robert