Why is transistor constantly open with 12V power adapter?
Having nearly everything done on my latest project I've run into an issue which leaves me clueless. When using a transistor as a switch it's all good when ran in a test mode of single LED powered from arduino, however, when I try using the 12V power supply I have (220->12V converter of 24W) the transistor is always open. No matter if I try to control the strip I will need or a single LED, no matter that I even tried an arduino code which sets the pin connected to transistor base to low, it's still on. Slight blinking can be seen on the LED strip. I've tried both TIP120 and 3904 transistors (didn't exceed the amperage on that), result's the same. I've already tried 2 different power supplies as well. When the arduino is off power - transistor is closed and LEDs are off.
I'm simply not able to connect the dots in this case as of now, maybe someone has dealt with a similar issue?
TL;DR Same setup with transistor works perfect on arduino power, but always on when fed 12V && arduino is on.
Thanks in advance! Going to sleep on it now, maybe will have ideas myself in the morning as well.