Powering the UNO

Hi, I've using the Nano powered with regulated 5V into pin 27.

Now, in the UNO specs it says

"5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it."

So can't I supply the UNO with regulated 5V into this pin?

The definition pin 27 is a bit vague if you talk about a Arduino. Most of the time that’s used to refer to a digital pin.

But if you mean power the Arduino via the Vcc/5V pin that’s absolutely fine. It’s even better then to rely on the onboard 5v regulator if you actually want to drive some stuff.

I understand that the 5V pin should be used as an output pin for powering (small load) devices. Now, can those devices be powered by an external regulated 5V source, like a 7805 regulator? Provided the ground is common, will it work?

I need to control a motor depending on a couple of IR sensors. I think that I should take the 12V from the transformer for powering the H bridge, a 7805 for a 5V line to power the sensors, and a 7808 for powering the UNO with 8V so it uses its internal regulator. Is the only requisite that the ground be common, while the sensors can have their own 5V source? The sensor works by closing a phototransistor collector so sending the pin to +5V, but then again, shouldn't that 5V source be the same of the boards? This is puzzling me.

Connect all Gnds. External devices can have external power sources. Don't connect anything that will bring Arduino IO pins above 5V. Post a schematic of your planned design for better comments.

OK, I will post a schematic.

Buit, if I set a pin as INPUT then it will be HIGH if I connect a wire to it and touch the 5V pin. But what if I touch 5V from another source, even if the gnd is common? Will it work? That is basically my scenario.

Yes, 3V to 5V from another source will be read as a High. Gnd Must be common.