I have tried two 2N3904 transistors with the schematic shown in this tutorial:
The only difference is that my supply voltage to the relay coil is 12v (base voltage is still 5v). I cannot get the relay to close. I am using this relay: http://www.radioshack.com/product/index.jsp?productId=2062482 which only takes 30ma across the coil to close.
I keep thinking I need to use a different base resistor, but I cannot figure out what to use... I'm not even sure how it was determined to use a 1k resistor in the original tutorial. It would have been easier if I had found a 5v relay here in town, but I could not.
I'm not sure this is a valid test with a meter, but when I supply 5v to the collector side of the transistor I can measure the voltage going from 0v to 5v on the emitter side of the transistor (measuring between emitter and ground). When I do the same test with 12v at the collector it only changes by about .5 volts.
Any help would be great :-)