Hello to all again,
I have attached my schematic of a project in development. i am using 2 X 5V power banks with a booster on each to bring up the voltage to 9 volts for my arduino and 12 volts for my buzzer and LED assemblies. I will be using transistors controlled by the arduino to switch on the LED assemblies and the buzzer. I do not know how to connect the grounds .
should the transistor emmiter go to the 12 volt ground from the booster or the ground from the pin from the arduino?
also , should I connect power bank grounds together? I’m at a loss at understanding this side of my project.

any help will be greatly appreciated, dermot

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All of the grounds should connect together.

The Arduino can run from 5V (into the 5V pin) so you shouldn't need two power supplies.

The 12V booster ground must connect to the Arduino ground so that the outputs that drive the transistor bases have a ground reference. So all of the grounds should be connected together, preferably at one place (star ground).

Thanks for the replies guys, I was going to use a 5v supply for the arduino but when I read up on it I found that the preferred voltage is between 7 - 12 volts , that is to get a more stable program execution.

Thanks again for the info.

When you supply power through the power plug the voltage must be dropped by the regulator. Vin - 5V * current (supplied by regulator) Watts must be dissipated. And the current available is limited depending on Vin. The higher Vin the less current can be drawn before the regulator gets hot and shuts down. Seems wasteful especially since you are running on batteries. Supplying 5V via the 5V input bypasses the regulator.

that is to get a more stable program execution.

I don't understand that.

sorry about the delay in getting back to this reply , groundFungus. i've been away and i'm just getting back into this thread ! Anyway , i read on a previous thread that it would be better to use a 7 - 12 v supply and let the regulator do its thing. as opposed to putting a 5 v battery supply which would probably end up not lasting too long and this would lead to an unstable program execution. by applying 9v from a 5 v power bank via DC - DC BOOSTER step up module i would get a more stable set up .

Supplying 5v is fine - you can't do it through the barrel jack (that needs 7v - 9v - if you put 5v on the barrel jack or Vin pin, you get out 3.something), but you can do it through the USB connector (this is what I recommend). The 5v pin can also be used BUT you can damage the board if it's powered through the 5v pin and you plug it into a computer (or other power source) via the USB port (that's why I recommend the USB cable method).

What you're saying about a 5v battery setup not lasting or leading to unstable execution is nonsense. Either the person you heard it from didn't know his stuff, or you misunderstood him. Maybe he was telling you what would happen if you powered it through the barrel jack? Which of course won't work as noted above.

Powering off a power bank through the USB connector would be how I would power an Arduino board off a power bank.

Cheers DrAzzy, this stuff is still new to me and I'm getting up to speed slowly but surely! i'm using a nano board so I'll just put the power bank 5 volts through the USB. I hadn't thought of the danger of powering it through the 5v pin and using a computer through the USB at the same time. Thanks again.

I have just charged up my power bank and it has 5.2 volts. Is this ok to power through the USB?