I'm using an Arduino in a project, but this is not an Arduino question! I'm able to switch 5+ volt high or low using a digital write pin. I need to use a 7812 voltage regulator to regulate voltage coming in to some other power transistors, but it does not need to be on at all times consuming power and creating heat. I'm wondering if I can use an NPN transistor to turn on/off the GND to the 7812. Is it just as simple as hooking up the NPN to the 7812? Also, will I need a high power NPN since the 7812 will be drawing upwards of 1.4 amps? Is there a much better way to switch the 7812 on and off?
I dont think there is really a clean safe way to do that, you could instead chose other regulators in the market which have a specific input pin to enable/disable the regulator like this one for example, http://www.linear.com/solutions/1703.
You could switch the voltage into or out of the 7812 using a MOSFET or BJT high-side switch. The latter choice means the regulator will continue to draw a few mA even when the output is off. Don't muck with the GND pin. You'd need a couple transistors (one power) a zener and few resistors.
If you want a single-chip solution there are very few choices in through-hole linear regulators with shutdown. The LT1764AET#06PBF is only good for 20V (abs max) input. If that's okay, it should work for you- the /SHUTDOWN input is compatible with your control signal. Be sure to pay attention to ALL the information on capacitor ESR, they are not as easy to work with as the 78xx regulators. You'll also need a couple of resistors to set the 12V output voltage.
If you don't mind a SMT TO-263 package, the TL1963AKTTR is about half the price and has similar limitations and requirements to the LT1764AET#06PBF. It's stable with low-ESR ceramic capacitors, without any series resistor so it's a bit more user-friendly than the LT product.
Again, pay attention to the capacitors, particularly the output capacitor, application information for these (and all LDOs and negative regulators). They're not like 78xx regulators.
The current through the ground pin of a 78xx regulator is under 10 mA, so you wouldn't need a high-current transistor there. However, disconnecting the ground pin probably won't turn off the 7812 output - I would expect the output voltage to rise to within a volt or two of the input (but I could be wrong).
What exactly do you mean by "turn off the 7812"?
I would use a power pnp transistor, or better yet a p channel mosfet to switch off the regulator.
By the way, 1.2 amp with a linear regulator would require a beefy heatsink.