I think it is more important to emphasis that a voltage regulator output (like the 3.3V output) should notbe connected to a power source (like the battery). There is no reason why it is still connnected.Whether or not the GND is connected is almost beside the point because a battery shouldn't be connected to a regulator output. I think it is easier for a NOOB to remember that than it is to determine whether there is a current loop between the battery and the regulator based on whether the Vcc & GND are BOTH connected to the battery.
"Because I was unraveling the OPs given circuit "Now that does make a lot of sense. Notice she/he has modified the their schematic in post #1."When they do run into problems"The operative word is "When" "I still wouldn't call that a rule of thumb. Its a common practice. "I said it's 'my' rule of thumb.And I finished with "best practices".I think we scared off the OP
I think we scared off the OP
[Edit]This is revision 1 of the schematic that I've modified according to the inputs given so far:In the original schematics the emitter of the transistor was connected to VCC instead of GND.This revision uses an NDP6020P logic level MOSFET instead of an FQP47P06.There is now a 10kΩ pull-down resistor at the Arduino pin D4.R1 is now 260kΩ and R2 33kΩ, which must be verified.The circuit is part of a battery powered IOT application where a minimal power consumption and voltage drop are crucial. After some research I came to the above solution to switch all unused components off while an Atmega328P is in deep sleep.The LED is just an example of a consumer, in reality this would be at least an HC-12 433 MHz HF module and one or more sensors which together will probably use 1 A max.[End Edit]
I think that whole referencing/connecting issue would be best be eliminated by using an opto-isolatorto drive the mosfet that switches the battery to the HC-12 module. That way there is complete isolationbetween the Pro-Mini and the circuit it is switching on and off.
Removing images from a post makes subsequent responses hard to follow.