Okay, maybe I am thinking too much here, but what I figured was, that there must be some finite lower limit of current which the transformer supplies, say 50mA, so say the Arduino takes in 10mA, the other 40mA (so as to say) will go through the resistor.
When you plug the wall wart in, with nothing connected to it, do you need to keep a bucket under to collect all the current that is falling out the end?
There is a finite lower limit. It's 0.
It says the inputs to the 74XX series are high impendance state and should be either grounded or tied to Vcc...So I figure no harm in connecting it directly to the digital output pins of the Arduino??
Since the digital pins are connected either to Vcc (when HIGH) or ground (when LOW), they are a perfect match for the IC7447.
It shouldn't matter if I declare the pins as OUTPUT type using DDRB/D but then use digitalWrite() to output the actual values, right??
Right.