After much thinking I am going to use the BC337. It supports 800 mA.Usually it's per 3 serially connected LEDs - have been in all the LED strings I've worked with.
12V to each 3 LEDs (plus resistor).
So 18 LEDs will take 120mA in my book and a BC337 would be a good choice. ~2.5mA to the base (you need to saturate the transistor to get the C-E drop low).
(5-0.7)/0.0025 = ~1k8 and that's the base resistor, no further resistors needed.
I thought it was a long string, a couple of meters at least and in that case, I'd have suggested a MOSFET like this
https://www.sparkfun.com/products/10213
with a gate resistor of 22..47 Ohm.
Also your statement is correct, I found out that every 3 LEDs have a 150 ohm resistor so if they are all connected in parallel and assuming each LEDs take three volts, the current through each patch is 20mA so the whole strip of 18 LEDs would take 120 mA
Now the Power generated is P=VI so where I= 120mA and V would be 0.7 (max collector -emitter saturation voltage) so P =84mW that is less than the 625mW that is the limit (Total device dissipation)
I ask this because my supervisor mentioned something about requiring a heatsink and gave me another transistor (I dont recall the number now) , but I dont think in these conditions is necessary. What do you think?
EDIT: the transistor I got was the 2SK2313 (a FET)
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