I'm currently working on an arduino based Laminator-control, and the relais ist driving me crazy!
My setup is as follows: I use the analog 0 Pin set to pinmode output, and as a test I turn it on/off every second. Attached is a 2n3904 to base with a 19k resistor. Across C/E I feed 12V with a 220R resistor in line, to reduce current for the relais. Grounds are connected.
I orignally thought that 19k is to high to fully open the transistor, but this is obviously not the case, since even when the resistor is omitted, the relay won't trigger!
Appreciate your help, this is driving me crazy since I used the very same resistor in my ambilight project to switch 12V LEDs.