Well I have my arduino powering 5 mechanical relays via a transistor, and it works great.
But
When I connect the same circuit to the parallel port of my computer, the relays have a very faint click. It is like there is more resistence in the transistor, when less voltage (2v?) is applied to it via the computers parallel port, rather than 5v from arduino?
(The external power source that powers the relays stays the same - a 9v battery)