Transistors as a switch

So, im doing this new project where im basically using arduino and processing to control 4 led strips in order to make them flash according to the "beat" of the music. I have all the code from a tutorial online, however i just cant get these transistors to work properly. I require NPN transistor for this and because so am using some TIP122G, because i was thinking since the LED strips work on max current of 2 amps id need a fairly big transistor.

Here is the schematic:

http://cdn.instructables.com/FXF/CNGS/FT7PU9SN/FXFCNGSFT7PU9SN.LARGE.jpg

however it is a bit different as im not using individual LED's but rather a waterproof strip i got off ebay. and also not using any resistors. So instead the positive from power supply runs to positive of strip and negative from strip runs to collector of transistor

Problem is i have connected the LED strip to the collector leg and the negative terminal of the power supply + arduino ground to the emitter side, however the LED strip still lights up without the base being plugged into the arduino PIN. Its not full brightness but rather dimmed.

Any help on this?? the tutorial im "trying" to achieve is LED Dance Room : 7 Steps - Instructables

Have you tried connecting the base to the emitter - this should reduce the off-state
leakage to a minimum, but the issue is that this device has a leakage rating of 0.5mA
(although that’s at 30V, but I suspect at 12V the leakage is on the order of 0.1mA).

Since the eye’s sensitivity to light is logarithmic we can detect a brightness range
of many thousands to one, so even this little current will produce visible illumination
in LEDs.

You can cure this crudely by adding a resistor between collector and 12V supply, in
parallel with the LED strip(s), try something like 10k, if that doesn’t work try 2k2 or
so… This should take all the leakage current away from the LED strips.

Seems i got it wrong. The led strip needs to be attached to the emitter, and the collecter is ground (negative power supply). Problem is, every leg now turns the LED strip on fully. As in we i attach the negative power supply to the collector with the base disconnected the led turns on...... full. WHY?

Your first schematic was correct, why change it?
Draw a new schematic.
It you change from positive voltage to negativ, normaly you can not use NPN transistors and how do you connect the uC?

Pelle

agwhiteb0i:
Seems i got it wrong. The led strip needs to be attached to the emitter, and the collecter is ground (negative power supply). Problem is, every leg now turns the LED strip on fully. As in we i attach the negative power supply to the collector with the base disconnected the led turns on...... full. WHY?

You may have just exceed the Vbe abs max of your transistor. Its NPN, emitter to ground,
collector to load, other end of load to +ve supply. Never take the base of an NPN more than
a volt or two more negative than the emitter, that's a very easy way to destroy a transistor.