You miss the point on the barrel jack. If his circuit is designed for 5 volts and he plugs in 12 volts then it could cause excessive current and burn out his circuit.
If Vin is powered (and greater than 6.6 volts the USB power is blocked by the Arduino.
If you use Vin as a voltage source you are drawing current from the inputs of three devices.
The relavent parts of the schematic are here:
So, you say, the 5+ pin is the most safe solution for tapping 5V from the Arduino?
Yes.
That is exactly my point. He’s not powering it using the barrel jack. And no matter which pin he chooses, he is drawing current from other devices. Except with the 5v pin it is even FARTHER down the chain past the regulator.
He’s good either way if his current requirement is low and it sounds like less than 100ma anyway, depending on how old the LCD is and whether there is a resistor incorporated on the backlight.
Yes, not now. But in a year if he decided to hook his Arduino to an E-stop via the barrel jack, do you think he is going to remember that his other circuit is limited to 5 volts?
Absolutely not. If you look at the schematic (and remember that he is powered by the USB port) then the +5 volt pin is closer to the source and doesn’t have to go backward through the regulator or backwards through the voltage divider op amp to get to Vin.
The voltage regulator in this part of the schematic is not used if power comes from the USB cable.
The lcd is powered from the Arduino Mini Pro. It has it’s own power down step from the 24VDC.
The only card drawing power from the Uno 5+ pin will be the OptoSmall card. This card will keep the Uno power from messing with the Mini Pro.
You’re right, he should put a sticker on his barrel jack: “NO.” Or maybe just a skull and crossbones. Or fill it with hot glue.
Or get 5V from another USB port.
This device pulls about 10mAmps per channel, so, with four channels you’ve got 40mAmps. The Arduino/gShield draws about 50mAmps for a total of 90mAmps without anything else connected.
You have 500mAmps available so you are only going to be using about 18 percent of the available current.
Nothing to worry about.
Yeah, with my luck the sticker would fall off. 
