Transistor help

Hello

Im trying to use a transistor as a switch to power 5 LED in series. The 5v from the arduino isnt enough voltage, so im routing 12v from a wall wart. 12v is going to the collector through a 1k resistor, a digital pin from the arduino to the base through a 100 ohm resistor, and the emitter going to the LED's (which are hooked up to ground). When the base is high, the emiter-ground voltage is 5v, not 12v. Im guessing that this isn't enough to saturate the transistor (It woks with routing 12 to the base, by the way).

My question is is there a way to saturate the transistor to make this work? I see some circuits with a resistor going from collector to the base & emitter to base. Im not quite sure what this is for, and I don't want to blow anything up.

Here is a pic to demonstrate what i mean (and to my amazing artwork)
http://dl.dropbox.com/u/6919533/transistor.jpg

Any recommendations?

You have it wired all wrong.

Your +12vdc should wire to the 3 series connected LEDSs along with a series wired current limiting resistor to the collector of the NPN transistor. The emitter for the transistor should wire to both the 12vdc ground and the arduino ground. The arduino output pin should wire to a series current limiting resistor to the base of the transistor.

Lefty

To level-shift you cannot use an emitter-follower circuit, a common-emitter circuit is needed. Emitter-follower (as the name implies) has the emitter voltage track the base voltage (less one diode-drop).

This means no voltage amplification... You want to take 5V switching signal upto 12V, so you need voltage amplification - thus common-emitter circuit needed.

Try about 1k from Ardiuno pin to transistor base, emitter to GND, collector to LEDs in series with their current-limit resistor upto the 12V rail.

Too complicated - move the LEDs to the other side of the transistor, and lower the 1K.

(12V - Vforward - Vforward - Vce)/.02 = R needed.
If voltage drop across each LED is 1V, and 0.7 across the transistor, then:
(12-1-1-0.7)/.02 = 465 ohms.